NUREG/CR-4513, Rev. 1

 ANL-93/22

Estimation of Fracture Toughness

of Cast Stainless Steels During

Thermal Aging in LWR Systems

Argonne National Laboratory

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Washington, DC 20555-0001
 

 NUREG/CR-4513, Rev. 1

 ANL-93/22

Estimation of Fracture Toughness

of Cast Stainless Steels During

Thermal Aging in LWR Systems

Manuscript Completed: May 1994

Date Published: May 1994

Prepared by

O. K. Chopra

Argonne National Laboratory

9700 South Cass Avenue

Argonne, IL 60439

Prepared for

Division of Engineering 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC FIN A2243
i

Previous Documents in Series

Initial Assessment of the Mechanisms and Significance of Low–Temperature Embrittlement of

Cast Stainless Steels in LWR Systems, NUREG/CR–5385, ANL–89/17 (August 1990).

Estimation of Fracture Toughness of Cast Stainless Steels During Thermal Aging in LWR Systems,

NUREG/CR–4513, ANL–90/42 (June 1991).
i

Estimation of Fracture Toughness of Cast Stainless Steels during Thermal Aging in

LWR Systems – Revision 1

by

O. K. Chopra

Abstract

This report presents a revision of the procedure and correlations presented earlier in

NUREG/CR–4513, ANL–90/42 (June 1991) for predicting the change in mechanical properties

of cast stainless steel components due to thermal aging during service in light water reactors at

280–330°C (535–625°F). The correlations presented in this report are based on an expanded

data base and have been optimized with mechanical–property data on cast stainless steels

aged up to ≈58,000 h at 290–350°C (554–633°F). The correlations for estimating the change in

tensile stress, including the Ramberg/Osgood parameters for strain hardening, are also

described. The fracture toughness J–R curve, tensile stress, and Charpy–impact energy of

aged cast stainless steels are estimated from known material information. Mechanical

properties of a specific cast stainless steel are estimated from the extent and kinetics of

thermal embrittlement. Embrittlement of cast stainless steels is characterized in terms of

room–temperature Charpy–impact energy. The extent or degree of thermal embrittlement at

“saturation,” i.e., the minimum impact energy that can be achieved for a material after

long–term aging, is determined from the chemical composition of the steel. Charpy–impact

energy as a function of time and temperature of reactor service is estimated from the kinetics

of thermal embrittlement, which are also determined from the chemical composition. The

initial impact energy of the unaged steel is required for these estimations. Initial tensile flow

stress is needed for estimating the flow stress of the aged material. The fracture toughness

J–R curve for the material is then obtained by correlating room–temperature Charpy–impact

energy with fracture toughness parameters. The values of JIC are determined from the

estimated J–R curve and flow stress. A common “predicted lower–bound” J–R curve for cast

stainless steels of unknown chemical composition is also defined for a given grade of steel,

range of ferrite content, and temperature. Examples of estimating mechanical properties of

cast stainless steel components during reactor service are presented.
i
v

Contents

Nomenclature................................................................................................................ ix

Executive Summary....................................................................................................... xi

Acknowledgments.......................................................................................................... xiii

1Introduction................................................................................................................ 1

2Mechanism of Thermal Embrittlement .......................................................................... 3

3Assessment of Thermal Embrittlement.......................................................................... 5

3.1 Estimate for Steels of Unknown Composition: Lower–Bound Values .................. 6

3.1.1 Ferrite Content >15%.......................................................................... 7

3.1.2 Ferrite Content 10–15% ...................................................................... 10

3.1.3 Ferrite Content <10%.......................................................................... 12

3.2 Estimate for Steels of Known Composition and Unknown Service History:

Saturation Values .......................................................................................... 14

3.2.1 Charpy–Impact Energy........................................................................ 14

3.2.2 Fracture Toughness J–R Curve............................................................ 19

3.3 Estimate for Steels of Known Composition and Service History:

Service Time Values ....................................................................................... 29

3.3.1 Charpy–Impact Energy........................................................................ 29

3.3.2 Fracture Toughness J–R Curve............................................................ 36

3.4 Estimation of Tensile Properties ...................................................................... 38

3.4.1 Tensile Strength ................................................................................. 38

3.4.2 Ramberg–Osgood Parameters .............................................................. 46

4Conclusions ................................................................................................................ 48

References..................................................................................................................... 51

Appendix: Estimation of Fracture Properties of Cast SSs .................................................. A–1
v

List of Figures

1. Decrease in Charpy–impact energy for various heats of cast stainless steels aged

at 400°C................................................................................................................ 3

2. Flow diagram for estimating mechanical properties of aged cast stainless steels in

LWR systems......................................................................................................... 6

3. Predicted lower–bound J–R curves at RT and 290°C for static–cast SSs with ferrite

contents >15, 10–15, or <10% ................................................................................ 8

4. Predicted lower–bound J–R curves at RT and 290°C for centrifugally cast SSs with

ferrite contents >15, 10–15, or <10%....................................................................... 9

5. Measured and calculated ferrite content of various heats of cast stainless steel ......... 14

6. Correlation between RT Charpy–impact energy at saturation and the material

parameter φ for CF–3, CF–8, and CF–8M steels ........................................................ 17

7. Experimental values of saturation RT impact energy and those estimated in terms

of the material parameter and composition alone..................................................... 18

8. Experimental and lower of the two estimated values of saturation RT impact

energy................................................................................................................... 18

9. Experimental values of saturation RT impact energy and those estimated from old

and new correlations.............................................................................................. 18

10. Estimates of ferrite content obtained from Hull's equivalent factors and ASTM

A 800.................................................................................................................... 18

11. Correlation between RT Charpy–impact energy and coefficient C for cast stainless

steel at 290–320°C and at RT ................................................................................. 19

12. Correlation between RT impact energy and exponent n of the power–law J–R curve

at RT and 290°C for cast stainless steels ................................................................. 21

13. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of centrifugally cast CF–3, CF–8, and CF–8M pipes, and

determined experimentally ..................................................................................... 24

14. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–3 and CF–8 steels, and determined

experimentally....................................................................................................... 25

15. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–8M steels, and determined experimentally..... 26
v

16. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–3 and CF–8M steels, and determined

experimentally....................................................................................................... 27

17. Fracture toughness J–R curve for unaged cast stainless steels at 290°C and

wrought stainless steels at various temperatures..................................................... 28

18. Observed and estimated activation energy for thermal embrittlement of cast

stainless steels ...................................................................................................... 30

19. RT Charpy–impact energy for 16 ANL heats and 1 EPRI heat, observed

experimentally and estimated from the composition and initial impact energy of

the materials ......................................................................................................... 31

20. RT Charpy–impact energy for the GF, FRA and EdF heats, observed experimentally

and estimated from the composition and initial impact energy of the materials .......... 34

21. Fracture toughness J–R curve at RT and 290°C, estimated from the chemical

composition and initial Charpy–impact energy and determined experimentally .......... 39

22. Fracture toughness J–R curves at RT and 290°C, estimated from the chemical

composition and initial Charpy–impact energy and determined experimentally .......... 40

23. Flow stress ratio of aged cast stainless steels at RT and 290°C as a function of the

normalized aging parameter ................................................................................... 43

24. Experimental and estimated flow stress of aged cast stainless steel at 290°C and

RT ........................................................................................................................ 44

25. Experimental and estimated values of JIC for aged cast stainless steels..................... 45

26. Experimental and estimated yield stress of aged cast stainless steel at 290°C and

RT ........................................................................................................................ 46

27. Correlation between the Ramberg/Osgood parameter α1 and flow stress at RT and

290°C for cast stainless steels ................................................................................ 47

28. Engineering stress–vs.–strain curve at RT and 290°C, estimated from the chemical

composition and initial tensile strength and determined experimentally for

thermally aged CF–3, CF–8, and CF–8M steels......................................................... 49

A–1. Estimation scheme for Example 1........................................................................... A–1

A–2. Estimation scheme for Example 2........................................................................... A–3

A–3. Estimation scheme for Example 3........................................................................... A–5

A–4. Estimation scheme for Example 4........................................................................... A–7
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A–5. Estimation scheme for Example 5...........................................................................A–10

List of Tables

1. Chemical composition, ferrite content, and kinetics of thermal embrittlement for

various heats of cast stainless steels....................................................................... 16
i

Nomenclature

C Coefficient of the power–law J–R curve.

Creq Chromium equivalent for a material (wt.%).

CV Room–temperature “normalized” Charpy–impact energy, i.e., Charpy–impact energy

per unit fracture area, at any given service and aging time (J/cm2). The fracture

area for a standard Charpy V–notch specimen (ASTM Specification E 23) is 0.8 cm2.

Divide the value of impact energy in J by 0.8 to obtain “normalized” impact energy.

CVint Initial room–temperature “normalized” Charpy–impact energy of a material, i.e.,

unaged material (J/cm2).

CVsat Room–temperature “normalized” Charpy–impact energy of a material at saturation,

i.e., the minimum impact energy that would be achieved for the material after

long–term service (J/cm2).

CMTR Certified material test record.

Jd Deformation J per ASTM Specification E 813-85 or E 1152–87 (kJ/m2).

n Exponent of the power–law J–R curve.

n1 Ramberg/Osgood parameter.

Nieq Nickel equivalent for a material (wt.%).

P Aging parameter, i.e., the log of the time of aging at 400°C.

Q Activation energy for the process of thermal embrittlement (kJ/mole).

Rf Ratio of tensile flow stress of aged σfaged and unaged σfunaged cast stainless steel.

Ry Ratio of tensile yield stress of aged σyaged and unaged σyunaged cast stainless steel.

t Service or aging time (h).

Ts Service or aging temperature (°C).

α Shape factor of the curve for the change in room–temperature Charpy–impact energy

with time and temperature of aging.

α1 Ramberg/Osgood parameter.

β Half the maximum change in room–temperature Charpy–impact energy.

δc Ferrite content calculated from the chemical composition of a material (%).

∆a Crack extension (mm).

Φ Material parameter.

θ Aging behavior at 400°C, i.e., the log of the time to achieve β reduction in impact

energy at 400°C.
x

In this report, all values of impact energy are considered to be for a standard

Charpy–V–notch specimen per ASTM Specification E 23, i.e., 10 x 10–mm cross section and

2–mm V notch. Impact energies obtained on subsize specimens should be normalized with

respect to the actual cross–sectional area and appropriate correction factors should be applied

to account for size effects. Similarly, impact energy from other standards, e.g., U–notch

specimen, should be converted to a Charpy–V–notch value by appropriate correlations.

SI units of measurements have been used in this report. Conversion factors for measurements

in British units are as follows:

To convert from to multiply by

in. mm 25.4

J* ft·lb 0.7376

kJ/m2 in.–lb/in.2 5.71015

kJ/mole kcal/mole 0.239

 

* When impact energy is expressed in J/cm2, first multiply by 0.8 to obtain impact energy of a standard Charpy

V–notch specimen in J.
x

Executive Summary

Cast stainless steels used in valve bodies, pump casings, piping, and other components in

coolant systems of light water reactors (LWRs) suffer a loss in fracture toughness due to thermal aging after many years of service at temperatures in the range of 280–320°C (≈535–610°F).

Thermal aging of cast stainless steels at these temperatures causes an increase in hardness

and tensile strength and a decrease in ductility, impact strength, and fracture toughness of the

material. The Charpy transition curve shifts to higher temperatures. Therefore, an assessment

of mechanical–property degradation due to thermal embrittlement is required to evaluate the

performance of cast stainless steel components during prolonged exposure to service temperatures.

Investigations at Argonne National Laboratory (ANL) and elsewhere have shown that thermal embrittlement of cast stainless steel components can occur during the reactor design lifetime of 40 yr. Various grades and heats of cast stainless steel exhibit varying degrees of thermal embrittlement. In general, the low–C CF–3 steels are the most resistant to thermal embrittlement, and the Mo–bearing, high–C CF–8M steels are the least resistant.

Embrittlement of cast stainless steels results in brittle fracture associated with either

cleavage of the ferrite or separation of the ferrite/austenite phase boundary. Thermal aging of

cast stainless steels at temperatures <500°C (<932°F) leads to precipitation of additional

phases in the ferrite, e.g., formation of a Cr–rich α' phase by spinodal decomposition; nucleation and growth of α'; precipitation of a Ni– and Si–rich G phase, M23C6, and γ2 (austenite);

and additional precipitation and/or growth of existing carbides at the ferrite/austenite phase

boundaries. Formation of the α' phase provides the strengthening mechanisms that increase

strain hardening and local tensile stress. Consequently, the critical stress level for brittle

fracture is attained at higher temperatures. Predominantly brittle failure occurs when either

the ferrite phase is continuous (e.g., in cast material with a large ferrite content) or the ferrite/austenite phase boundary provides an easy path for crack propagation (e.g., in high–C

grades of cast steel with large phase–boundary carbides). Consequently, the amount, size, and

distribution of the ferrite phase in the duplex structure, and the presence of phase–boundary

carbides are important parameters in controlling the degree or extent of thermal embrittlement.

A procedure and correlations have been developed at ANL for estimating the Charpy–impact energy and fracture toughness J–R curve of cast stainless steel components under LWR

operating conditions from material information readily available in certified material test

records (CMTRs). The procedure and correlations were published in NUREG/CR–4513,

ANL–90/42 (June 1991). This report presents a revision of the procedure and correlations presented earlier. All the correlations have been optimized to provide more accurate estimates.

The correlations are based on an expanded data base and mechanical–property results on cast

stainless steels that were aged up to ≈58,000 h at 290–350°C (554–662°F). The procedure for

estimating the change in tensile stress, including the Ramberg/Osgood parameters for strain

hardening, is also presented.

Fracture toughness of a specific cast stainless steel is estimated from the extent and kinetics of thermal embrittlement. The extent of thermal embrittlement is characterized by the

room–temperature (RT) Charpy–impact energy. A correlation for the extent of thermal embrittlement at “saturation,” i.e., the minimum impact energy that would be achieved for the mate-
x

rial after long–term aging, is given in terms of the chemical composition. The extent of thermal

embrittlement as a function of time and temperature of reactor service is estimated from the

extent of embrittlement at saturation and from the correlations describing the kinetics of embrittlement, which are also given in terms of chemical composition. The fracture toughness

J–R curve for the material is then obtained from the correlation between the fracture toughness

parameters and the RT Charpy–impact energy used to characterize the extent of thermal embrittlement. Tensile yield and flow stresses and Ramberg/Osgood parameters for tensile strain

hardening are estimated from the flow stress of the unaged material and the kinetics of embrittlement. Fracture toughness JIC and tearing modulus can then be determined from the estimated J–R curve and tensile flow stress. A common lower–bound J–R curve for cast materials

of unknown chemical composition is also defined for a given material specification, ferrite content, and temperature. Examples of estimating mechanical properties of cast stainless steel

components during reactor service are presented.
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Acknowledgments

This work was supported by the Office of Nuclear Regulatory Research, U.S. Nuclear

Regulatory Commission (NRC), under FIN A2243, Program Manager: Joe Muscara. The author

thanks W. J. Shack and T. F. Kassner for their helpful discussions.
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1

1 Introduction

Cast duplex stainless steels used in light water reactor (LWR) systems for primary pressure–boundary components, such as valve bodies, pump casings, and primary coolant piping,

are susceptible to thermal embrittlement at reactor operating temperatures, i.e., 280–320°C

(536–608°F). Thermal aging of cast stainless steels (i.e., ASTM Specification A–351 for Grades*

CF–3, CF–3A, CF–8, CF–8A, and CF–8M) at these temperatures causes an increase in hardness

and tensile strength and a decrease in ductility, impact strength, and fracture toughness of the

material and the Charpy transition curve shifts to higher temperatures. Investigations at

Argonne National Laboratory (ANL)1–14 and elsewhere15–22 have shown that thermal embrittlement of cast stainless steel components occurs during the reactor design lifetime of 40 yr.

Various grades and heats of cast stainless steel exhibit varying degrees of thermal embrittlement. In general, the low–C CF–3 steels are the most resistant to thermal embrittlement, and

the Mo–bearing, high–C CF–8M steels are the least resistant.

An assessment of mechanical–property degradation due to thermal embrittlement is

therefore required to evaluate the performance of cast stainless steel components during prolonged exposure to service temperatures, because rupture of the primary pressure boundary

could lead to a loss–of–coolant accident and possible exposure of the public to radiation. A

procedure and correlations have been developed at ANL for estimating Charpy–impact energy

and fracture toughness J–R curve of cast stainless steel components under LWR operating

conditions from material information readily available in certified material test records

(CMTRs). The procedure and correlations were published previously.9

Fracture toughness of a specific cast stainless steel is estimated from the extent and kinetics of thermal embrittlement. The extent of thermal embrittlement is characterized by the

room–temperature (RT) “normalized” Charpy–impact energy (Charpy–impact energy per unit

fracture area). A correlation for the extent of thermal embrittlement at “saturation,” i.e., the

minimum Charpy–impact energy that would be achieved for the material after long–term aging,

is given in terms of chemical composition. The extent of thermal embrittlement as a function

of time and temperature of reactor service is estimated from the extent of embrittlement at

saturation and from the correlations describing the kinetics of embrittlement, which are also

given in terms of chemical composition. The fracture toughness J–R curve for the material is

then obtained from the correlation between fracture toughness parameters and RT

Charpy–impact energy used to characterize the extent of thermal embrittlement. A common

lower–bound J–R curve for cast materials of unknown chemical composition is defined for a

given material specification, ferrite content, and temperature.

These correlations have been updated and optimized with an expanded data base and mechanical–property results on cast stainless steels that were aged up to ≈58,000 h at 290–350°C

(554–662°F).10,11 Correlations have also been developed for estimating changes in tensile

stress and Ramberg/Osgood parameters for tensile strain hardening.23 This report presents a

revision of the procedure and correlations presented earlier.9 The correlations for estimating

the change in tensile stress, including the Ramberg/Osgood parameters, are also described.

 

* In this report, Grades CF–3A and CF–8A are considered equivalent to CF–3 and CF–8, respectively. The

A designation represents high tensile strength. The chemical composition of CF–3A and CF–8A is further

restricted within the composition limits of CF–3 and CF–8, respectively, to obtain a ferrite/austenite ratio

that results in higher ultimate and yield strengths.
2

Examples of estimating mechanical properties of cast stainless steel components during reactor service are presented. The differences between the correlations described in this report and

those presented earlier are as follows:

• The correlations have been optimized by using a larger data base (e.g., ≈80 compositions of

cast SS) and mechanical–property results on materials that were aged up to ≈58,000 h at

290–350°C (554–662°F). The earlier correlations were based on ≈45 compositions of cast

SS and aging times up to 30,000 h. For the 80 compositions of cast SSs used in this

study, estimates based on the old correlations yield conservative values of fracture toughness.

• The saturation RT impact energy CVsat is estimated from two different correlations. For

most heats, the two methods give comparable estimates. For a few heats, however, one or

the other set of expressions gives more accurate estimates. It is likely that minor differences in the composition and microstructure of the ferrite caused by differences in

production heat treatment and possibly the casting process influence CVsat values. These

factors cannot be quantified from the present data base. To ensure that the estimates are

either accurate or conservative for all heats of cast SS within ASTM Specification A 351,

the lower of the two estimated values is used for estimating the fracture properties.

• Different correlations are used to estimate the saturation RT impact energy CVsat for

CF–8M steels with <10 and ≥10 wt.% Ni.

• Separate correlations are given for estimating fracture toughness J–R curves for static–

and centrifugally cast SSs.

• The correlation for estimating exponent n of the power–law J–R curve has been modified.

• For CF–3 and CF–8 steels, the expression for estimating the activation energy for thermal

embrittlement has been modified. The effect of Mo and Mn content has been omitted and

the effect of C content has been added in the updated expression.

• Correlations are included for estimating the yield and flow stresses and Ramberg/Osgood

parameters of aged cast stainless steels from the initial tensile properties and the kinetics

of thermal embrittlement.

The correlations presented in this report are valid for static– and centrifugally cast CF–3,

CF–3A, CF–8, CF–8A, and CF–8M steels defined by ASTM Specification A 351. The criteria

used in developing these correlations ensure that the estimated mechanical properties are adequately conservative for compositions of cast stainless steel within ASTM A 351. However, the

correlations for estimating fracture toughness J–R curve are not applicable to those

compositions of static–cast CF–8M steel for which the estimated value of CVsat is <25 J/cm2

(<15 ft·lb). These compositions contain >25% ferrite. On the other hand, the correlations may

not encompass all metallurgical factors that can arise from differences in production heat

treatment or casting processes and, therefore, may be overly conservative for some steels.

Mechanical properties are expressed in SI units (see Nomenclature for units of measure and for

conversion factors for British units).
3

2 Mechanism of Thermal Embrittlement

Thermal embrittlement of cast duplex stainless steels results in brittle fracture associated

with either cleavage of ferrite or separation of the ferrite/austenite phase boundary. Thermal

aging of cast stainless steels at temperatures <500°C (932°F) leads to precipitation of additional

phases in the ferrite, e.g., formation of a Cr–rich α' phase by spinodal decomposition; nucleation and growth of α'; precipitation of a Ni– and Si–rich G phase, M23C6 carbide, and γ2

austenite; and additional precipitation and/or growth of existing carbides at the ferrite/austenite phase boundaries.10–14,24–29 Thermal aging has little or no effect on the

austenite phase. The formation of Cr–rich α' phase by spinodal decomposition of ferrite is the

primary mechanism for thermal embrittlement. The α' phase strengthens the ferrite matrix,

i.e., it increases strain hardening and local tensile stress, so that the critical stress level for

brittle fracture is achieved at higher temperatures.

The degree or extent of thermal embrittlement is controlled by the amount of brittle fracture, which depends on both material and aging conditions. In some cast stainless steels, a

fraction of the material may fail in brittle fashion but the surrounding austenite provides

ductility and toughness. Such steels have adequate impact strength even after long–term

aging. Predominantly brittle failure occurs when either the ferrite phase is continuous, e.g., in

cast material with a large ferrite content, or the ferrite/austenite phase boundary provides an

easy path for crack propagation, e.g., in high–C or high–N steels that contain phase–boundary

carbides or nitrides. Consequently, the amount, size, and distribution of ferrite in the duplex

structure and phase–boundary precipitates are important parameters that control the extent of

thermal embrittlement. The decrease in RT Charpy–impact energy during thermal aging at

400°C (752°F) of various heats of cast stainless steel4–6,15,19,21 is shown in Fig. 1. The results

indicate that all the materials reach a “saturation” RT impact energy, i.e., a minimum value

that would be achieved by the material after long–term aging. The actual value of saturation

RT impact energy for a specific cast stainless steel is independent of aging temperature but depends strongly on the chemical composition of the steel; it is lower for the Mo–bearing CF–8M

0

100

200

300

400

101 102 103 104 105

Aged at 400°C

Cast SS

Impact Energy, C

v (ft·lb)

0

Aging Time (h)

50

100

150

200

Impact Energy, C

v (J/cm 2 )

GF

EPRI

FRA

ANL

Figure 1. Decrease in Charpy–impact energy for various heats of cast

stainless steels aged at 400°C
4

steels than for the Mo–free CF–3 or CF–8 steels, and decreases with an increase in ferrite content or the concentration of C or N in the steel.

The time to reach saturation, i.e., the kinetics of thermal embrittlement, depends on both

material and aging parameters. Figure 1 indicates that the time for aging at 400°C for a given

decrease in impact energy varies by more than two orders of magnitude for the various heats,

e.g., <1,000 h for the Electric Power Research Institute (EPRI) heat, 1,000–10,000 h for the

ANL and Framatome (FRA) heats, and 10,000–30,000 h for the Georg Fischer Co. (GF) heats.

Activation energies of thermal embrittlement range from 65 to 230 kJ/mole (15 to

55 kcal/mole).1–6,13–15,17–19,30 These values are well below the 202 kJ/mole (48 kcal/mole)

value associated with Cr bulk diffusion in the Fe–28Cr alloy.31 Small changes in the constituent elements of the material can cause the kinetics of thermal embrittlement to vary

significantly. The logarithm of the aging time at 400°C for a 50% reduction in RT

Charpy–impact energy has been shown to be an important parameter for characterizing the

kinetics of thermal embrittlement.9

Activation energy is high for steels that show fast embrittlement at 400°C and low for

those that show slow embrittlement at 400°C. Also, materials with the same chemical composition but different heat treatment show different kinetics of embrittlement.15 Microstructural

examination of aged cast stainless steels suggests that slow embrittlement at 400°C and low

activation energy are associated with clusters of Ni–Si, Mo–Si, and Ni–Si–Mo in the ferrite matrix.12–14 These clusters are considered precursors of G–phase nucleation and precipitation.

Cast stainless steels with low activation energy and slow embrittlement at 400°C show

G–phase precipitation after aging, and steels with high activation energy and fast

embrittlement at 400°C do not contain a G phase.12–14,24,25 The presence of Ni–Si–Mo

clusters in the ferrite matrix of an unaged material may be considered a signature of steels that

are potentially sensitive to thermal embrittlement, i.e., steels with Ni–Si–Mo clusters in the

ferrite matrix show low activation energy for thermal embrittlement but take longer to embrittle

at 400°C.

The kinetics of thermal embrittlement of cast stainless steels are controlled primarily by

the kinetics of ferrite strengthening, e.g., activation energy determined from ferrite hardness

measurements shows very good agreement with that obtained from the Charpy–impact

data.17,18,30 Microstructural characterization and annealing studies on thermally aged cast

stainless steel show that strengthening of ferrite is caused primarily by spinodal decomposition

of ferrite to form Cr–rich α' phase.4,12,13 Consequently, the kinetics of thermal embrittlement

should be controlled by the amplitude and frequency of Cr fluctuations produced by spinodal

decomposition, i.e., by the size and spacing of the α' phase. The low activation energies of

thermal embrittlement are most likely caused by variations in the spacing of Cr fluctuations;

atom probe field–ion microscopy studies indicate that the spacing between Cr fluctuations decreases with decreasing temperature.24,28 During thermal aging, production heat treatment

and possibly the casting process, both of which affect ferrite composition and microstructure of

unaged material, can influence microstructural evolution and, therefore, the kinetics of embrittlement.

For most materials, the kinetics of thermal embrittlement vary with aging temperature.

For a specific heat of cast stainless steel, activation energy of thermal embrittlement is not

constant over the temperature range of 290–450°C (554–842°F), but increases with decreasing

temperature.1–3,17 The increase is particularly significant between 400 and 450°C (752 and
5

842°F). In addition, materials aged at 450°C show precipitation of phase–boundary carbides

(also nitrides in high–N steels) and a decrease in ferrite content of the steel.1,2 At reactor temperatures, such processes either do not occur or their kinetics are extremely slow.

Consequently, data obtained after 450°C aging do not reflect the mechanisms that are active

under reactor operating conditions, and extrapolation of the 450°C data to predict the extent of

thermal embrittlement at reactor temperatures is not valid. The activation energy for thermal

embrittlement may be represented by an average value in the temperature range of 290–400°C

(554–752°F).

3 Assessment of Thermal Embrittlement

A flow diagram for estimating mechanical properties of cast stainless steels during reactor

service is shown in Fig. 2. The estimation scheme is divided into three sections on the basis of

available material information. In Section A of the flow diagram, “predicted lower–bound”

fracture toughness is defined for CF–3, CF–8, and CF–8M steels of unknown composition.

When the ferrite content of the steel is known, a different lower–bound fracture toughness and

impact energy are defined for steels containing <10%, 10–15%, or >15% ferrite.

Sections B and C of the flow diagram present procedures for estimating mechanical properties when a CMTR is available, i.e., the chemical composition of the steel is known. Section

B describes the estimation of “saturation” impact energy and fracture toughness J–R curve.

The only information needed for these estimations is the chemical composition of the material.

The correlations presented here account for the degradation of mechanical properties due to

thermal aging. They do not explicitly consider the initial fracture properties of the unaged

material. It is possible that the estimations of saturation fracture toughness based on

chemical composition are higher than the fracture toughness of the unaged material. Some

cast stainless steels are inherently weak and may have poor fracture properties in the unaged

condition. When information is available on either the initial fracture toughness of the

material or the initial RT Charpy–impact energy for estimating fracture toughness, and when

the J–R curve estimated from the chemical composition is higher than the initial fracture

toughness of unaged material, the latter is used as the saturation J–R curve of the material.

Such cast stainless steels are relatively insensitive to thermal aging and the fracture toughness

of the material would not change during reactor service. Furthermore, when no information is

available on the initial fracture toughness of a material, the minimum fracture toughness of

unaged cast stainless steels is used as an upper bound for the estimated fracture toughness,

i.e., when the J–R curve estimated from the chemical composition is higher than the minimum

fracture toughness of unaged cast stainless steels, the latter is used as the saturation J–R

curve of the material.

Estimation of mechanical properties at any given time and temperature of service, i.e.,

service time properties, is described in Section C. The initial impact energy of the unaged material is required for these estimations. If not known, the initial impact energy can be assumed

to be 200 J/cm2 (118 ft·lb). However, similar to Section B, initial fracture toughness of the

unaged material or the minimum fracture toughness of unaged cast stainless steels is used as

a upper bound for the estimations. The initial tensile properties of the unaged material are

needed for estimations of the tensile strength and Ramberg/Osgood strain hardening parameters. The JIC value and tearing modulus can be determined from the estimated J–R curve and

flow stress.
6

Estimate for known composition and service history

Yes Yes

No Sat. J–R

> Min. J–R

Unaged

No

Minimum J–R

Curve Unaged

Estimate for known composition unknown service history

Sat. J–R

> Initial J–R

Unaged

No

No Saturation

J–R Curve &

Impact Energy

Yes Input Initial

Impact Energy

Initial

Impact

Energy

Estimate Initial

J–R Curve

Minimum J–R

Curve Unaged

Initial

J–R Curve

Unaged

Minimum

J–R Curve

Unaged

Yes Yes

Estd. J–R

> Min. J–R

Unaged

No

Initial Impact

Energy 160 J 

No

Estimate

 ServiceTime

Impact Energy &

J–R Curve

Yes

Section B: 

SATURATION

Yes

CMTR

ESTIMATION SCHEME

Input 

Composition &

Casting Process

Estimate Saturation

Impact Energy & 

J–R Curve

Service

Time &

Temp.

Input Service

Temp. & Time

Section C: 

SERVICE TIME

Service Time

J–R Curve &

Impact Energy

Estimate Initial

J–R Curve

Estimate

 ServiceTime

Impact Energy &

J–R Curve

Estd. J–R

> Initial J–R

Unaged

Initial

Impact

Energy

Yes Input Initial

Impact Energy

No

Estimate for unknown composition

Lower–Bound

J–R Curve for

CF–3, 8, & 8M

Lower–Bound

J–R Curve for

Ferrite Range

No

Yes

Section A: 

LOWER 

BOUND

Ferrite No

content 

Minimum

J–R Curve

Unaged

Initial

J–R Curve

Unaged

Initial

Flow

Stress

Yes

Estimate Service

 Time Flow Stress,

Yield Stress, & 

Ramberg/Osgood

Parameters

No

Figure 2. Flow diagram for estimating mechanical properties of aged cast stainless steels in

LWR systems

3.1 Estimate for Steels of Unknown Composition: Lower–Bound Values

For cast stainless steels of unknown chemical composition within ASTM Specification

A 351, the lower–bound fracture toughness J–R curve is defined for a given method of casting,

material grade, and temperature. The lower–bound curve is based on the “worst case” material

condition, e.g., >20% ferrite content. The cast SSs used in the U.S. nuclear industry generally

contain <15% ferrite. The lower–bound fracture properties for a specific casting method and
7

grade of steel may therefore be very conservative for most steels. More realistic estimates of

lower–bound properties are obtained if the ferrite content of the steel is known. The ferrite

content of a cast SS component can be measured in the field with a ferrite scope. The

predicted lower–bound J–R curves at RT and 290°C for static– and centrifugally cast CF–3,

CF–8, and CF–8M steels with >15%, 10–15%, and <10% ferrite are shown in Figs. 3 and 4.

3.1.1 Ferrite Content >15%

A lower–bound fracture toughness J–R curve at RT, for static–cast CF–3 steel with >15%

ferrite, is given by

Jd = 287[∆a]0.39; (3.1.1)

for static–cast CF–8 steel with >15% ferrite, by

Jd = 261[∆a]0.37; (3.1.2)

and for static–cast CF–8M steel with 15–25% ferrite,* by

Jd = 119[∆a]0.33. (3.1.3)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for static–cast CF–3 steel with

>15% ferrite, is given by

Jd = 264[∆a]0.35; (3.1.4)

for static–cast CF–8 steel with >15% ferrite, by

Jd = 251[∆a]0.34; (3.1.5)

and for static–cast CF–8M steel with 15–25% ferrite,* by

Jd = 167[∆a]0.31. (3.1.6)

A lower–bound fracture toughness J–R curve at RT, for centrifugally cast CF–3 steel with >15%

ferrite, is given by

Jd = 334[∆a]0.39; (3.1.7)

for centrifugally cast CF–8 steel with >15% ferrite, by

Jd = 304[∆a]0.37; (3.1.8)

and for centrifugally cast CF–8M steel with >15% ferrite, by

Jd = 149[∆a]0.33. (3.1.9)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for centrifugally cast CF–3 steel

with >15% ferrite, is given by

Jd = 347[∆a]0.35; (3.1.10)

for centrifugally cast CF–8 steel with >15% ferrite, by

Jd = 330[∆a]0.34; (3.1.11)

 

* The lower–bound J–R curve is not applicable for those compositions of static–cast CF–8M steel for which the

estimated CVsat is <25 J/cm2 (<15 ft·lb); these compositions contain >25% ferrite.
8

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Static–Cast Steels

Room Temp.

J = 261(∆a)0.37

CF–8

Crack Extension ∆a (in.)

5

J = 119(∆a)0.33

CF–8M

J = 287(∆a)0.39

CF–3

0.00 0.05 0.10 0.15

>15% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Static–Cast Steels

290°C

J = 251(∆a)0.34

CF–8

Crack Extension ∆a (in.)

5

J = 167(∆a)0.31

CF–8M

J = 264(∆a)0.35

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

>15% Ferrite

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Static–Cast Steels

Room Temp.

J = 307(∆a)0.38

CF–8

5

J = 149(∆a)0.35

CF–8M

J = 342(∆a)0.40

CF–3

0.00 0.05 0.10 0.15

10–15% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Static–Cast Steels

290°C

J = 274(∆a)0.35

CF–8

5

J = 192(∆a)0.32

CF–8M

J = 290(∆a)0.36

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

10–15% Ferrite

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Static–Cast Steels

Room Temp.

J = 394(∆a)0.40

CF–8

Crack Extension, ∆a (mm)

5

J = 211(∆a)0.36

CF–8M

J = 400(∆a)0.40

CF–3

0.00 0.05 0.10 0.15

<10% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Static–Cast Steels

290°C J = 313(∆a)0.37

CF–8

Crack Extension, ∆a (mm)

5

J = 238(∆a)0.33

CF–8M

J = 331(∆a)0.39

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

<10% Ferrite

Figure 3. Predicted lower–bound J–R curves at RT and 290°C for static–cast SSs with ferrite

contents >15, 10–15, or <10%
9

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Centrifugally Cast Steels

Room Temp. J = 304(∆a)0.37

CF–8

Crack Extension ∆a (in.)

5

J = 149(∆a)0.33

CF–8M

J = 334(∆a)0.39

CF–3

0.00 0.05 0.10 0.15

>15% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Centrifugally Cast Steels

290°C J = 330(∆a)0.34

CF–8

Crack Extension ∆a (in.)

5

J = 195(∆a)0.31

CF–8M

J = 347(∆a)0.35

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

>15% Ferrite

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Centrifugally Cast Steels

Room Temp.

J = 357(∆a)0.38

CF–8

5

J = 186(∆a)0.35

CF–8M

J = 398(∆a)0.41

CF–3

0.00 0.05 0.10 0.15

10–15% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Centrifugally Cast Steels

290°C J = 360(∆a)0.35

CF–8

5

J = 223(∆a)0.32

CF–8M

J = 382(∆a)0.36

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

10–15% Ferrite

0

200

400

600

800

1000

01234

Deformation J (kJ/m 2

)

Lower–Bound J–R Curve

Centrifugally Cast Steels

Room Temp.

J = 458(∆a)0.41

CF–8

Crack Extension, ∆a (mm)

5

J = 264(∆a)0.36

CF–8M

J = 507(∆a)0.43

CF–3

0.00 0.05 0.10 0.15

<10% Ferrite

01234

Deformation J (in.–lb/in.2

Lower–Bound J–R Curve )

Centrifugally Cast Steels

290°C

J = 412(∆a)0.37

CF–8

Crack Extension, ∆a (mm)

5

J = 276(∆a)0.33

CF–8M

J = 435(∆a)0.39

CF–3

0.00 0.05 0.10 0.15

5000

4000

3000

2000

1000

0

<10% Ferrite

Figure 4. Predicted lower–bound J–R curves at RT and 290°C for centrifugally cast SSs with

ferrite contents >15, 10–15, or <10%

and for centrifugally cast CF–8M steel with >15% ferrite, by

Jd = 195[∆a]0.31. (3.1.12)
10

The J values at any intermediate temperature can be linearly interpolated from the values at

RT and at 290°C.

 

For static–cast steels with >15% ferrite, the lower–bound fracture toughness J–R curves

corresponding to Eqs. 3.1.1–3.1.6 in British units, i.e., Jd and ∆a expressed in in.–lb/in.2 and

in., respectively, are given by

Jd = 5786[∆a]0.39, (3.1.1a)

Jd = 4933[∆a]0.37, (3.1.2a)

Jd = 1976[∆a]0.33, (3.1.3a)

Jd = 4677[∆a]0.35, (3.1.4a)

Jd = 4305[∆a]0.34, (3.1.5a)

and

Jd = 2599[∆a]0.31. (3.1.6a)

For centrifugally cast steels, the lower–bound fracture toughness J–R curves corresponding to Eqs. 3.1.7–3.1.12 in British units are given by

Jd = 6734[∆a]0.39, (3.1.7a)

Jd = 5745[∆a]0.37, (3.1.8a)

Jd = 2474[∆a]0.33, (3.1.9a)

Jd = 6147[∆a]0.35, (3.1.10a)

Jd = 5660[∆a]0.34, (3.1.11a)

and

Jd = 3035[∆a]0.31. (3.1.12a)

 

3.1.2 Ferrite Content 10–15%

A lower–bound fracture toughness J–R curve at RT, for static–cast CF–3 steel with 10–15%

ferrite, is given by

Jd = 342[∆a]0.40; (3.1.13)

for static–cast CF–8 steel with 10–15% ferrite, by

Jd = 307[∆a]0.38; (3.1.14)

and for static–cast CF–8M steel with 10–15% ferrite, by

Jd = 149[∆a]0.35. (3.1.15)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for static–cast CF–3 steel with

10–15% ferrite is given by
11

Jd = 290[∆a]0.36, (3.1.16)

for static–cast CF–8 steel with 10–15% ferrite by

Jd = 274[∆a]0.35, (3.1.17)

and for static–cast CF–8M steel with 10–15% ferrite by

Jd = 192[∆a]0.32. (3.1.18)

A lower–bound fracture toughness J–R curve at RT for centrifugally cast CF–3 steel with

10–15% ferrite, is given by

Jd = 398[∆a]0.40; (3.1.19)

for centrifugally cast CF–8 steel with 10–15% ferrite, by

Jd = 357[∆a]0.38; (3.1.20)

and for centrifugally cast CF–8M steel with 10–15% ferrite, by

Jd = 186[∆a]0.35. (3.1.21)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for centrifugally cast CF–3 steel

with 10–15% ferrite, is given by

Jd = 382[∆a]0.36; (3.1.22)

for centrifugally cast CF–8 steel with 10–15% ferrite, by

Jd = 360[∆a]0.35; (3.1.23)

and for centrifugally cast CF–8M steel with 10–15% ferrite, by

Jd = 223[∆a]0.32. (3.1.24)

The J values at any intermediate temperature can be linearly interpolated from the values at

RT and at 290°C.

 

For static–cast steels with 10–15% ferrite, the lower–bound fracture toughness J–R curves

corresponding to Eqs. 3.1.13–3.1.18 in British units, i.e., Jd and ∆a expressed in in.–lb/in.2

and in., respectively, are given by

Jd = 7122[∆a]0.40, (3.1.13a)

Jd = 5993[∆a]0.38, (3.1.14a)

Jd = 2640[∆a]0.35, (3.1.15a)

Jd = 5306[∆a]0.36, (3.1.16a)

Jd = 4854[∆a]0.35, (3.1.17a)

and

Jd = 3087[∆a]0.32. (3.1.18a)

For centrifugally cast steels, the lower–bound fracture toughness J–R curves corresponding to Eqs. 3.1.19–3.1.24 in British units are given by
12

Jd = 8561[∆a]0.40, (3.1.19a)

Jd = 6969[∆a]0.38, (3.1.20a)

Jd = 3295[∆a]0.35, (3.1.21a)

Jd = 6990[∆a]0.36, (3.1.22a)

Jd = 6377[∆a]0.35, (3.1.23a)

and

Jd = 3585[∆a]0.32. (3.1.24a)

 

3.1.3 Ferrite Content <10%

A lower–bound fracture toughness J–R curve at RT, for static–cast CF–3 steel with <10%

ferrite, is given by

Jd = 400[∆a]0.40; (3.1.25)

for static–cast CF–8 steel with <10% ferrite, by

Jd = 394[∆a]0.40; (3.1.26)

and for static–cast CF–8M steel with <10% ferrite, by

Jd = 211[∆a]0.36. (3.1.27)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for static–cast CF–3 steel with

<10% ferrite, is given by

Jd = 331[∆a]0.39; (3.1.28)

for static–cast CF–8 steel with <10% ferrite, by

Jd = 313[∆a]0.37; (3.1.29)

and for static–cast CF–8M steel with <10% ferrite, by

Jd = 238[∆a]0.33. (3.1.30)

A lower–bound fracture toughness J–R curve at RT, for centrifugally cast CF–3 steel with <10%

ferrite, is given by

Jd = 507[∆a]0.43; (3.1.31)

for centrifugally cast CF–8 steel with <10% ferrite, by

Jd = 458[∆a]0.41; (3.1.32)

and for centrifugally cast CF–8M steel with <10% ferrite, by

Jd = 264[∆a]0.36. (3.1.33)

At 290°C (554°F), a lower–bound fracture toughness J–R curve, for centrifugally cast CF–3 steel

with <10% ferrite, is given by

Jd = 435[∆a]0.39; (3.1.34)
13

for centrifugally cast CF–8 steel with <10% ferrite, by

Jd = 412[∆a]0.37; (3.1.35)

and for centrifugally cast CF–8M steel with <10% ferrite, by

Jd = 276[∆a]0.33. (3.1.36)

The J values at any intermediate temperature can be linearly interpolated from the values at

RT and at 290°C.

 

For static–cast steels with <10% ferrite, the lower–bound fracture toughness J–R curves

corresponding to Eqs. 3.1.25–3.1.30 in British units, i.e., Jd and ∆a expressed in in.–lb/in.2

and in., respectively, are given by

Jd = 8330[∆a]0.40, (3.1.25a)

Jd = 8205[∆a]0.40, (3.1.26a)

Jd = 3861[∆a]0.36, (3.1.27a)

Jd = 6674[∆a]0.39, (3.1.28a)

Jd = 5915[∆a]0.37, (3.1.29a)

and

Jd = 3952[∆a]0.33. (3.1.30a)

For centrifugally cast steels, the lower–bound fracture toughness J–R curves corresponding to Eqs. 3.1.31–3.1.36 in British units are given by

Jd = 11634[∆a]0.43, (3.1.31a)

Jd = 9851[∆a]0.41, (3.1.32a)

Jd = 4830[∆a]0.36, (3.1.33a)

Jd = 8770[∆a]0.39, (3.1.34a)

Jd = 7786[∆a]0.37, (3.1.35a)

and

Jd = 4583[∆a]0.33. (3.1.36a)

 

3.2 Estimate for Steels of Known Composition and Unknown Service

History: Saturation Values

3.2.1 Charpy–Impact Energy

When a CMTR is available, the saturation RT impact energy of a specific cast stainless

steel is determined from the chemical composition and ferrite content of the material. The

ferrite content is calculated from chemical composition in terms of Hull's equivalent factors32
14

Creq = Cr + 1.21(Mo) + 0.48(Si) – 4.99 (3.2.1)

and

Nieq = (Ni) + 0.11(Mn) – 0.0086(Mn)2 + 18.4(N) + 24.5(C) + 2.77. (3.2.2)

The concentration of N is often not available in a CMTR; if not known, it is assumed to be

0.04 wt.%. The ferrite content δc is given by

δc = 100.3(Creq/Nieq)2 – 170.72(Creq/Nieq) + 74.22. (3.2.3)

The measured and calculated values of ferrite content for the various heats used in studies at

ANL,4 GF,15 Electricité de France (EdF),17 National Power (NP),18 FRA,19 and EPRI21 are

shown in Fig. 5. For most heats, the difference between the estimated and measured values is

±6% ferrite. The few heats for which the calculated ferrite content is significantly lower than

the measured values generally contain ≥10% nickel.

Different correlations are used to estimate the saturation RT impact energy of the various

grades of cast stainless steel. To ensure that the estimates are either accurate or conservative

for all heats, the saturation RT impact energy for a specific cast stainless steel is determined by

two different expressions, and the lower value is used for estimating fracture properties. For

CF–3 and CF–8 steels, the saturation value of RT impact energy CVsat is the lower value

determined from

log10CVsat = 1.15 + 1.36exp(–0.035Φ), (3.2.4)

0

10

20

30

40

50

0 10 20 30

Calculated Ferrite (%)

40 50

ANL

Measured Ferrite (%)

FRA

GF

EdF

CEGB

Open Symbols: CF–3, CF–8

Closed Symbols: CF–8M

EPRI Figure 5.

Measured and calculated ferrite content of

various heats of cast stainless steel. The

band represents ±6% deviation.

where the material parameter Φ is expressed as

Φ = δc(Cr + Si)(C + 0.4N), (3.2.5)

and from

log10CVsat = 5.64 – 0.006δc – 0.185Cr + 0.273Mo – 0.204Si

+ 0.044Ni – 2.12(C + 0.4N). (3.2.6)

For CF–8M steel with <10% Ni, the saturation value of RT impact energy CVsat is the lower

value determined from
15

log10CVsat = 1.10 + 2.12exp(–0.041Φ), (3.2.7)

where the material parameter Φ is expressed as

Φ = δc(Ni + Si + Mn)2(C + 0.4N)/5; (3.2.8)

and from

log10CVsat = 7.28 – 0.011δc – 0.185Cr – 0.369Mo – 0.451Si

– 0.007Ni – 4.71(C + 0.4N). (3.2.9)

For CF–8M steel with >10% Ni, the saturation value of RT impact energy CVsat is the lower

value determined from

log10CVsat = 1.10 + 2.64exp(–0.064Φ), (3.2.10)

where the material parameter Φ is expressed as

Φ = δc(Ni + Si + Mn)2(C + 0.4N)/5; (3.2.11)

and from

log10CVsat = 7.28 – 0.011δc – 0.185Cr – 0.369Mo – 0.451Si

– 0.007Ni – 4.71(C + 0.4N). (3.2.12)

If not known, the N content can be assumed to be 0.04 wt.%. The saturation values of RT impact energy for CF–3, CF–8, and CF–8M steels observed experimentally at ANL,4–6 GF,15

Westinghouse (WH),16 EdF,17 NP,18 FRA,19 and EPRI21 are plotted as a function of the material parameter Φ in Fig. 6. The solid lines represent best–fit curves for the data, i.e., Eq. 3.2.4

for CF–3 or CF–8 steels and Eqs. 3.2.7 and 3.2.10 for CF–8M steel. The chemical composition,

ferrite content, and saturation RT Charpy–impact energy of the materials from ANL, GF, FRA,

WH, and EPRI studies are given in Table 1. The difference between the predicted and observed

values is <±15% for most of the CF–3 and CF–8 steels and <±25% for the CF–8M steels.

The correlations described above for estimating CVsat are somewhat different from those

presented earlier in Ref. 9. The differences between the new and old set of correlations are as

follows:

• The correlations have been optimized by using a larger data base (e.g., ≈80 different compositions of cast stainless steel) and mechanical–property results on materials that were

aged up to ≈58,000 h at 290–350°C (554–662°F).10,11 Data on ≈45 compositions of cast

stainless steel and for aging times up to 30,000 h were used in developing the earlier correlations.

Table 1. Chemical composition, ferrite content, and kinetics of thermal embrittlement for

various heats of cast stainless steels

Chemical Composition (wt.%) Ferrite (%) CVsat Constants Q

Heat Cr Mo Si Ni Mn C N Calc. Meas. (J/cm2) β θα (kJ/mole)

Argonne 

52 19.49 0.35 0.92 9.40 0.57 0.009 0.052 10.3 13.5 161.8 – – – –

51 20.13 0.32 0.86 9.06 0.63 0.010 0.058 14.3 18.0 115.9 0.139 3.53 1.15 204.7

47 19.81 0.59 1.06 10.63 0.60 0.018 0.028 8.4 16.3 163.7 0.069 2.29 1.20 195.7

P2 20.20 0.16 0.94 9.38 0.74 0.019 0.040 12.5 15.6 141.3 0.258 2.83 1.09 218.6

I 20.20 0.45 0.83 8.70 0.47 0.019 0.032 20.4 17.1 134.3 0.094 2.10 1.00 250.0

69 20.18 0.34 1.13 8.59 0.63 0.023 0.028 21.0 23.6 76.7 0.214 3.21 1.07 175.9
16

P1 20.49 0.04 1.12 8.10 0.59 0.036 0.057 17.6 24.1 53.7 0.305 2.57 0.75 252.7

61 20.65 0.32 1.01 8.86 0.65 0.054 0.080 10.0 13.1 93.3 0.214 3.48 1.20 197.8

59 20.33 0.32 1.08 9.34 0.60 0.062 0.045 8.8 13.5 89.1 0.197 3.14 1.20 249.4

68 20.64 0.31 1.07 8.08 0.64 0.063 0.062 14.9 23.4 47.1 0.301 2.88 0.68 161.1

60 21.05 0.31 0.95 8.34 0.67 0.064 0.058 15.4 21.1 44.8 0.291 2.89 0.88 210.9

56 19.65 0.34 1.05 9.28 0.57 0.066 0.030 7.3 10.1 117.6 – – – –

74 19.11 2.51 0.73 9.03 0.54 0.064 0.048 15.5 18.4 63.1 0.269 3.44 0.70 95.0

75 20.86 2.58 0.67 9.12 0.53 0.065 0.052 24.8 27.8 32.1 0.436 2.82 0.51 139.0

66 19.45 2.39 0.49 9.28 0.60 0.047 0.029 19.6 19.8 87.9 0.208 3.16 1.57 163.9

64 20.76 2.46 0.63 9.40 0.60 0.038 0.038 29.0 28.4 41.1 0.338 2.81 0.60 147.3

65 20.78 2.57 0.48 9.63 0.50 0.049 0.064 20.9 23.4 59.7 0.260 2.99 0.59 153.8

P4 19.64 2.05 1.02 10.00 1.07 0.040 0.151 5.9 10.0 62.7 0.289 2.70 0.62 158.7

63 19.37 2.57 0.58 11.85 0.61 0.055 0.031 6.4 10.4 126.5 0.119 2.83 1.11 155.5

Georg Fischer Co.

284 23.00 0.17 0.52 8.23 0.28 0.025 0.037 43.6 42.0 20.5 0.551 3.66 0.39 85.9

280 21.60 0.25 1.37 8.00 0.50 0.028 0.038 36.3 38.0 19.6 0.609 3.20 0.73 88.9

282 22.50 0.15 0.35 8.53 0.43 0.035 0.040 29.7 38.0 28.5 0.500 3.65 0.39 91.6

281 23.10 0.17 0.45 8.60 0.41 0.036 0.053 31.4 30.0 17.2 0.618 3.76 0.47 89.8

283 22.60 0.23 0.53 7.88 0.48 0.036 0.032 42.6 42.0 18.6 0.599 3.60 0.44 83.7

278 20.20 0.13 1.00 8.27 0.28 0.038 0.030 18.5 15.0 68.3 0.347 3.90 0.29 63.1

279 22.00 0.22 1.36 7.85 0.37 0.040 0.032 39.5 40.0 23.8 0.546 3.06 0.58 93.5

277 20.50 0.06 1.81 8.13 0.54 0.052 0.019 22.5 28.0 30.7 0.466 3.54 0.49 87.7

291 19.60 0.66 1.59 10.60 0.28 0.065 0.054 4.2 6.0 121.9 0.195 3.65 0.35 71.2

292 21.60 0.13 1.57 7.52 0.34 0.090 0.039 23.9 28.0 17.2 0.373 3.07 0.44 98.8

290 20.00 2.40 1.51 8.30 0.41 0.054 0.050 31.3 32.0 15.8 0.624 3.48 0.12 81.0

288 19.60 2.53 1.70 8.40 0.47 0.052 0.022 35.6 28.0 14.9 0.671 2.96 0.66 105.3

287 20.50 2.58 0.51 8.46 0.50 0.047 0.033 37.2 38.0 20.5 0.555 3.46 0.36 90.3

286 20.20 2.44 1.33 9.13 0.40 0.072 0.062 18.9 22.0 15.5 0.594 3.03 0.72 106.4

289 19.70 2.30 1.44 8.25 0.48 0.091 0.032 22.6 30.0 16.2 0.580 3.29 0.41 90.1

285 18.80 2.35 0.86 9.49 0.48 0.047 0.039 14.0 10.0 61.1 0.313 3.60 0.20 89.3

Framatome

A 18.90 0.10 0.99 8.90 1.14 0.021 0.074 6.0 6.3 166.0 0.090 3.44 0.20 111.7

E 21.04 0.08 0.54 8.47 0.80 0.035 0.051 17.6 16.5 45.7 0.334 2.63 0.65 132.9

F 19.72 0.34 1.16 8.33 0.26 0.038 0.026 17.7 12.0 83.2 0.282 2.45 1.23 176.2

C 20.73 0.13 1.09 8.19 0.91 0.042 0.035 20.9 20.1 51.1 0.393 3.30 0.45 83.1

G 20.65 0.02 1.03 8.08 0.74 0.040 0.073 15.3 17.0 62.5 – – – –

H 20.70 0.05 1.18 8.07 0.71 0.050 0.045 18.3 21.5 50.6 – – – –

D 19.15 2.50 0.94 10.32 1.12 0.026 0.063 12.2 13.9 33.0 0.439 3.30 0.40 89.7

I 19.36 2.40 0.98 10.69 0.70 0.020 0.039 14.1 15.5 150.7 – – – –

K 20.80 2.62 0.75 10.45 1.09 0.060 0.056 15.4 14.0 48.5 – – – –

L 20.76 2.48 0.81 10.56 0.79 0.040 0.042 18.6 19.0 30.4 – 3.00 – –

B 20.12 2.52 0.93 10.56 0.83 0.053 0.042 14.0 17.3 28.2 0.478 2.55 0.47 128.6

Westinghouse

C1488 20.95 2.63 0.53 9.48 1.02 0.061 0.056 22.1 14.0 53.1 – 2.80 – –

Electric Power Research Institute

EPRI 22.04 0.23 0.84 7.93 0.74 0.030 0.045 36.0 32.0 30.0 0.564 2.10 0.60 225.0
17

10

100

0 10 20 30 40

Impact Energy, CVsat (J/cm 2)

50

200

Φ = δc (Cr+Si)(C+0.4N)

ANL

FRA

GF

EdF

CEGB

EPRI

80

60

40

20

log10(CVsat ) = 1.15+1.36exp(–0.035Φ)

CF–3, CF–8

log10(C Vsat ) = 1.05+1.36exp(–0.035Φ)

(a)

10

100

0 10 20 30 40

Impact Energy, CVsat (ft·lb)

50

Φ = δc(Ni+Si+Mn) 2(C+0.4N)/5

ANL

EdF

GF

WH

Ni < 10 wt.%

80

60

40

20

8

6

(b)

log10(CVsat ) = 1.10+2.12exp(–0.041Φ)

CF–8M

log10(CVsat ) = 0.99+2.12exp(–0.041Φ)

10

100

10

100

0 10 20 30 40

Impact Energy, CVsat (J/cm 2)

Impact Energy, CVsat (ft·lb)

50

200

Φ = δc(Ni+Si+Mn) 2 (C+0.4N)/5

ANL

FRA

EdF

Ni ≥ 10 wt.%

80

60

40

20

80

60

40

20

8

6

(c)

log10(CVsat ) = 1.10+2.64exp(–0.064Φ)

CF–8M

log10(CVsat ) = 0.96+2.64exp(–0.064Φ)

Figure 6.

Correlation between RT Charpy–impact

energy at saturation and the material

parameter φ for CF–3, CF–8, and CF–8M

steels. Solid lines represent best fit

curves for the data.

• The saturation RT impact energy is estimated from two differing correlations, i.e., in addition to CVsat being expressed in terms of material parameter Φ, it is also expressed in

terms of the chemical composition alone. The experimental values of saturation RT impact

energy and those estimated in terms of the material parameter (Eqs. 3.2.4, 3.2.7, or

3.2.10) and composition alone (Eqs. 3.2.6, 3.2.9, or 3.2.12) are shown in Fig. 7. For most

heats, the two methods give comparable estimates of CVsat. For a few heats, however, one

or the other set of expressions gives more accurate estimates. It is likely that minor differences in the composition and microstructure of the ferrite caused by differences in production heat treatment and possibly the casting process influence CVsat. These factors cannot

be quantified from the present data base. To ensure that the estimates are either accurate

or conservative for all heats of cast stainless steel within ASTM Specification A 351, the

lower of the two estimated values is used for estimating the fracture properties. A comparison of the experimental and the lower of the two estimated values of CVsat is shown in

Fig. 8. The estimated saturation RT impact energies are within 5–10% of the experimental

values for most of the heats and are conservative for some. Only two of ≈80 heats included

in developing these correlations show nonconservative estimates.

• For CF–8M steel, the expression for the material parameter Φ has been modified* and different correlations between CVsat and Φ are used for steels with <10 wt.% Ni and with

>10 wt.% Ni. The new correlations provide more accurate estimates of CVsat. The old

correlations9 yield conservative estimates of CVsat for most compositions of cast stainless

steel. The values of CVsat estimated from the old and new correlations are shown in Fig. 9.

 

* For CF–8M steel, material parameter Φ was expressed as δcCr(Ni+Si)2(C+0.4N)/100 in Ref. 9.
18

0

50

100

150

200

Estimated CVsat (J/cm 2)

0 50 100 150 200

Measured CVsat (J/cm 2)

Material Parameter

Composition

Estimated from

0

50

100

150

200

Estimated CVsat (J/cm 2)

0 50 100 150 200

Measured CVsat (J/cm 2)

Minimum value estimated

from the two methods

Figure 7. Experimental values of saturation RT

impact energy and those estimated in

terms of the material parameter and

composition alone

Figure 8. Experimental and lower of the two

estimated values of saturation RT

impact energy

0

50

100

150

200

Estimated CVsat (J/cm 2)

0 50 100 150 200

Measured CVsat (J/cm 2)

Open Symbols: New Expressions

Closed Symbols: Old Expressions

CF–3, CF–8

CF–8M

0

10

20

30

40

50

0 10 20 30 40 50

ASTM A 800 (%)

Hull's Equivalent Factors (%)

Open Symbols: CF–3, CF–8

Closed Symbols: CF–8M

Calculated Ferrite 

Figure 9. Experimental values of saturation RT

impact energy and those estimated

from old and new correlations

Figure 10. Estimates of ferrite content

obtained from Hull's equivalent

factors and ASTM A 800

The ferrite content δc used in developing Eqs. 3.2.4–3.2.12 was calculated from Hull's

equivalent factors. Using δc values determined by methods other than Hull's may result in

nonconservative estimates of mechanical properties. For the various heats used in this investigation, the ferrite contents calculated from ASTM Specification A 800/A 800M–8433 and from

the Hull's equivalent factors are shown in Fig. 10. Estimations of ferrite content based on the
19

ASTM procedure are ≈20% lower than those obtained from Hull's method for ferrite levels >12%

and are comparable for lower ferrite levels. Consequently, δc determined by the ASTM method

for cast stainless steels with >12% ferrite may yield nonconservative estimates of fracture

properties.

The correlations expressed in Eqs. 3.2.4–3.2.12 have been validated with Charpy–impact

data from service–aged cast stainless steel components from the decommissioned Shippingport

reactor; KRB reactor in Gundremmingen, Germany; and the Ringhals 2 reactor in Sweden.8

The correlations do not consider the effect of Nb, and may not be applicable for Nb–bearing

steels. Also, they do not differentiate between product form, i.e., static–cast or centrifugally

cast materials.

3.2.2 Fracture Toughness J–R Curve

The saturation fracture toughness J–R curve for a specific cast stainless steel can be estimated from its RT impact energy at saturation. The J–R curve is expressed by the power–law

relation Jd = C∆an, where Jd is deformation J per ASTM Specifications E 813–85 and

E 1152–87, ∆a is crack extension, and C and n are constants. The coefficient C at room or

reactor temperatures and the RT Charpy–impact energy for aged and unaged cast stainless

steels are plotted in Fig. 11. Fracture toughness data from studies at ANL,4–7 FRA,19,20

EPRI,21 The Welding Institute (TWI),22 and Materials Engineering Associates, Inc. (MEA)34 are

included in the figure. At both room and reactor temperatures, the coefficient C decreased

with a decrease in impact energy. Separate correlations were obtained for CF–3 or CF–8 steels

and for CF–8M steels; the latter showed a larger decrease in fracture toughness for a given

impact energy. The correlations used to estimate J–R curves for static–cast materials were

obtained by subtracting the value of standard deviation for the fit to the data from the best–fit

curve; shown as dashed lines in Fig. 11, they help ensure that the estimated J–R curve is

conservative for all material and aging conditions. Best–fit correlations were used for

centrifugally cast materials.

10

100

1000

10 50

RT Impact Energy, CV (J/cm 2)

5 50 100

Coefficient C

ANL

EPRI

KRB

C = 134CV

290 – 320°C 0.28

TWI

C = 57C V

0.41

C = 102CV

0.28

Open Symbols: CF–3, CF–8

Closed Symbols: CF–8M

RT Impact Energy (ft·lb)

5 50 100 10

C = 49C V

0.41

ANL

FRA

500

10

100

1000

10

RT Impact Energy, CV (J/cm 2 )

5 50 100

ANL

EPRI

KRB

C = 57C V

0.52

Room Temp.

MEA

C = 20C V

0.67

C = 49C V

0.52

Open Symbols: CF–3, CF–8

Closed Symbols: CF–8M

RT Impact Energy (ft·lb)

5 50 100 10

C = 16CV

0.67

ANL

FRA

Figure 11. Correlation between RT Charpy–impact energy and coefficient C for cast stainless steel

at 290–320°C and at RT. The solid and dashed lines represent the correlations used

to estimate the J–R curves for centrifugally and static–cast materials, respectively.
20

The exponent n of ∆a is correlated with the Charpy–impact energy CV in Fig. 12. The correlations shown in the figure bound the minimum values of the exponent n observed experimentally, and help ensure that the estimated J–R curves are conservative.

The saturation fracture toughness J–R curve at RT, for static–cast CF–3 and CF–8 steels,

is given by

Jd = 49[CVsat]0.52[∆a]n, (3.2.13)

and for centrifugally cast CF–3 and CF–8 steels, by

Jd = 57[CVsat]0.52[∆a]n, (3.2.14)

where the exponent n for CF–3 steel is expressed as

n = 0.15 + 0.16log10[CVsat], (3.2.15)

and for CF–8 steels, as

n = 0.20 + 0.12log10[CVsat]. (3.2.16)

The saturation fracture toughness J–R curve at RT, for static–cast CF–8M steel is given by

Jd = 16[CVsat]0.67[∆a]n, (3.2.17)

and for centrifugally cast CF–8M steel, by

Jd = 20[CVsat]0.67[∆a]n, (3.2.18)

where the exponent n is expressed as

n = 0.23 + 0.08log10[CVsat]. (3.2.19)

At 290°C (554°F), the saturation J–R curve, for static–cast CF–3 and CF–8 steels is given by

Jd = 102[CVsat]0.28[∆a]n (3.2.20)

and for centrifugally cast CF–3 and CF–8 steels by

Jd = 134[CVsat]0.28[∆a]n, (3.2.21)

where the exponent n for CF–3 steel is expressed as

n = 0.17 + 0.12log10[CVsat], (3.2.22)

and for CF–8 steels as

n = 0.21 + 0.09log10[CVsat]. (3.2.23)

At 290°C (554°F), the saturation J–R curve, for static–cast CF–8M steel is given by

Jd = 49[CVsat]0.41[∆a]n (3.2.24)

and for centrifugally cast CF–8M steel, by

Jd = 57[CVsat]0.41[∆a]n, (3.2.25)

where the exponent n is expressed as

n = 0.23 + 0.06log10[CVsat]. (3.2.26)
21

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Exponent n

1.0

10 100

CF–3 Steel

Room Temp.

n = 0.15 + 0.16log10(CV)

Charpy–Impact Energy CV (J/cm 2)

10 100

CF–3 Steel

290°C

n = 0.17 + 0.12log10(CV) 

Charpy–Impact Energy CV (J/cm 2)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Exponent n

1.0

10 100

CF–8 Steel

Room Temp.

n = 0.20 + 0.12log10(CV)

Charpy–Impact Energy CV (J/cm 2)

10 100

CF–8 Steel

290°C

n = 0.21 + 0.09log10(CV)

Charpy–Impact Energy CV (J/cm 2)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Exponent n

1.0

10 100

CF–8M Steel

Room Temp.

n = 0.23 + 0.08log10(CV)

Charpy–Impact Energy CV (J/cm 2)

10 100

CF–8M Steel

290°C

n = 0.23 + 0.06log10(CV)

Charpy–Impact Energy CV (J/cm 2)

Figure 12. Correlation between RT impact energy and exponent n of the power–law J–R curve at

RT and 290°C for cast stainless steels
22

The J–R curve at any intermediate temperature can be linearly interpolated from the estimated

values of C and n at RT and 290°C.

 

The fracture toughness J–R curves corresponding to Eqs. 3.2.13–3.2.26 in British units,

i.e., saturation RT impact energy CVsat is in ft·lb and Jd and ∆a are expressed in in.–lb/in.2

and in., respectively, are given by

Jd = {368(25.4)n[CVsat]0.52}[∆a]n, (3.2.13a)

Jd = {428(25.4)n[CVsat]0.52}[∆a]n, (3.2.14a)

n = 0.187 + 0.16log10[CVsat], (3.2.15a)

n = 0.227 + 0.12log10[CVsat], (3.2.16a)

Jd = {130(25.4)n[CVsat]0.67}[∆a]n, (3.2.17a)

Jd = {163(25.4)n[CVsat]0.67}[∆a]n, (3.2.18a)

n = 0.248 + 0.08log10[CVsat], (3.2.19a)

Jd = {675(25.4)n[CVsat]0.28}[∆a]n, (3.2.20a)

Jd = {887(25.4)n[CVsat]0.28}[∆a]n, (3.2.21a)

n = 0.197 + 0.12log10[CVsat], (3.2.22a)

n = 0.231 + 0.09log10[CVsat], (3.2.23a)

Jd = {347(25.4)n[CVsat]0.41}[∆a]n, (3.2.24a)

Jd = {404(25.4)n[CVsat]0.41}[∆a]n, (3.2.25a)

and

n = 0.244 + 0.06log10[CVsat]. (3.2.26a)

The expression enclosed in { } represents the coefficient C of the power–law J–R curve.

 

The J–R curves described in Eqs. 3.2.13–3.2.26 are based on fracture toughness data obtained under material and aging conditions that yield Charpy–impact energies ≥25 J/cm2;

there is little or no data for Charpy–impact energies <25 J/cm2 (Fig. 11). Existing data are not

adequate to accurately establish correlations between RT Charpy–impact energy and fracture

toughness parameter C or n for values of CVsat <25 J/cm2. Extrapolation of the results

beyond the data base may yield nonconservative estimates, i.e., Eqs. 3.2.13–3.2.26 may over

estimate the fracture toughness of those compositions of cast stainless steel for which the

estimated RT saturation impact energy CVsat is <25 J/cm2 (<15 ft·lb). The correlations

presented in Eqs. 3.2.1–3.2.12 indicate that the compositions of CF–8M steel for which CVsat is

<25 J/cm2 contain >25% ferrite. However, the correlations presented in this report are

applicable to cast SSs used in the U.S. nuclear industry, the steels contain <25% ferrite in

almost all cases.

The fracture toughness J–R curve at saturation for a specific cast stainless steel can be

obtained from its chemical composition by using the correlations for CVsat given in Eqs.

3.2.1–3.2.12 and then using the estimated CVsat in Eqs. 3.2.13–3.2.26 to obtain the J–R curve.
23

Comparisons of the experimental and estimated J–R curves at saturation, i.e., the minimum

fracture toughness that would be achieved for the material by thermal aging, are shown in

Figs. 13–16. For most heats, the saturation fracture toughness is achieved after aging for

5000 h or more at 400°C (752°F). In most cases, estimated J–R curves show good agreement

with experimental results and are essentially conservative. At RT, the maximum conservatism

is for centrifugally cast Heats P2 and 205 and at 290°C for centrifugally cast Heat P2 and a

static–cast EPRI heat. Estimated curves of these heats are 40–50% lower than those obtained

experimentally.

The correlations described in Eqs. 3.2.4–3.2.26 account for the degradation of mechanical

properties of typical heats of cast stainless steel. They do not consider the initial fracture

properties of the unaged material. Some heats of cast stainless steels may have low initial

fracture toughness and estimates from Eqs. 3.2.4–3.2.26 may be higher than the initial value.

Some knowledge regarding the initial fracture toughness of the material is, therefore, needed to

justify the use of the estimated fracture toughness.

The CMTR for a specific cast stainless steel component provides information on chemical

composition, tensile strength, and possibly Charpy–impact energy of the material; fracture

toughness is not available in CMTRs. The available fracture toughness J–R curve data at

290–320°C (554–608°F) for unaged cast stainless steels are shown in Fig. 17a. Fracture

toughness J–R curves for a few “weak” heats of static–cast stainless steels are lower than those

for wrought stainless steels, Fig. 17b. The fracture toughness of unaged cast stainless steels is

slightly higher at RT than at 290–320°C. At temperatures between RT and 320°C, the minimum fracture toughness, of unaged static–cast stainless steels, can be expressed as

Jd = 400[∆a]0.40, (3.2.27)

and of centrifugally cast stainless steels, as

Jd = 650[∆a]0.43. (3.2.28)

 

The lower–bound fracture toughness J–R curves for unaged static– and centrifugally cast

stainless steels in British units are given by

Jd = 8330[∆a]0.40 (3.2.27a)

and

Jd = 14916[∆a]0.43. (3.2.28a)

 

When no information is available, these minimum fracture toughness J–R curves may be

used as the initial fracture toughness of a cast material. The fracture toughness J–R curve for

unaged material may also be obtained by using the initial RT Charpy–impact energy, CVint, instead of CVsat in Eqs. 3.2.13–3.2.26. However, Eqs. 3.2.27 and 3.2.28 are used as a lower

bound for these estimations. The correlations expressed in Eqs. 3.2.13–3.2.26 are based primarily on data for thermally aged steels and, therefore, are overly conservative for unaged

steels, particularly at 290°C. When the estimation based on CVint and Eqs. 3.2.13–3.2.26 is
24

0

1000

2000

3000

4000

5000

0 2 4 6 8 10

Unaged

10,000 h at 400°C

Saturation

δc = 12%

Deformation J (kJ/m2)

Room Temp.

J = 639(∆a )0.47

Heat P2 CF–3

Crack Extension, ∆ a (in.)

Unaged

J = 1366(∆a )0.57

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 320°C

Centrifugally Cast Pipe

32 efpy

at 290°C

0 2 4 6 8 10

Unaged

10,000 h at 400°C

Saturation

δc = 12%

20000

290°C

J = 492(∆a )0.41

Heat P2 CF–3

Deformation J (in.–lb/in.2)

Crack Extension, ∆ a (in.)

10000

0

Saturation

Unaged

J = 741(∆a )0.49

32 efpy

at 290°C

0.0 0.1 0.2 0.3

32 efpy

at 320°C

Centrifugally Cast Pipe

0

1000

2000

3000

0 2 4 6 8 10

Unaged

10,000 h at 400°C

Saturation

δc = 18%

Deformation J (kJ/m2)

Room Temp.

J =468(∆a )0.41

Heat P1 CF–8

Centrifugally Cast Pipe

Unaged

J = 942(∆a )0.48

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 320°C

32 efpy

at 290°C

0 2 4 6 8 10

Unaged

10,000 h at 400°C

Saturation

δc = 18%

15000 290°C

J =416(∆a )0.37

Heat P1 CF–8

Deformation J (in.–lb/in.2)

10000

5000

Centrifugally Cast Pipe

Unaged

J = 650(∆a )0.43

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 290°C

32 efpy

at 320°C

0

1000

2000

3000

0 2 4 6 8 10

18,000 h at 400°C

Saturation

δc = 21%

Deformation J (kJ/m2)

Room Temp.

J = 350(∆a )0.38

Crack Extension, ∆ a (mm)

Heat 205 CF–8M

Centrifugally Cast Pipe Unaged

J = 855(∆a )0.42

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 320°C

32 efpy

at 290°C

0 2 4 6 8 10

Unaged

10,000 h at 400°C

Saturation

δc = 21%

15000

290°C

J = 328(∆a )0.34

Crack Extension, ∆ a (mm)

Heat 205 CF–8M

Deformation J (in.–lb/in.2)

10000

5000

Unaged

J = 650(∆a )0.43

Saturation

0.0 0.1 0.2 0.3

0

Centrifugally Cast Pipe

32 efpy

at 320°C

32 efpy

at 290°C

Figure 13. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of centrifugally cast CF–3, CF–8, and CF–8M pipes, and

determined experimentally
25

0

1000

2000

3000

0 2 4 6 8 10

δc = 21%

Deformation J (kJ/m 2)

Room Temp.

J = 504(∆a)0.46

Heat 69 CF–3

Crack Extension, ∆a (in.)

32 efpy

at 320°C

Static–Cast Slab

Unaged

J = 845(∆a)0.53

Saturation

0.0 0.1 0.2 0.3

Unaged

10,000 h at 400°C

Saturation

32 efpy

at 290°C

0 2 4 6 8 10

δc = 21%

15000

290°C

J = 358(∆a)0.40

Heat 69 CF–3

Deformation J (in.–lb/in.2 )

Crack Extension, ∆a (in.)

10000

5000

Unaged

Static–Cast Slab

Unaged

J = 473(∆a)0.46

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 320°C

10,000 h at 400°C

Saturation 32 efpy

at 290°C

0

1000

2000

3000

0 2 4 6 8 10

δc = 20%

Deformation J (kJ/m 2)

Room Temp.

J =533(∆a)0.47

Heat I CF–3

Unaged

Static–Cast Pump Impeller

Unaged

J = 748(∆a)0.51

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 290°C

32 efpy

at 320°C

0 2 4 6 8 10

δc = 20%

15000 290°C

J =369(∆a)0.41

Heat I CF–3

Deformation J (in.–lb/in.2 )

10000

5000

Unaged

Static–Cast Pump Impeller

Unaged

J = 443(∆a)0.44

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 290°C

32 efpy

at 320°C

0

1000

2000

3000

0 2 4 6 8 10

δc = 15%

Deformation J (kJ/m 2)

Room Temp.

J = 354(∆a)0.40

Crack Extension, ∆a (mm)

Heat 68 CF–8

32 efpy

at 320°C

Static–Cast Slab

Unaged

J = 856(∆a)0.49

Saturation

0.0 0.1 0.2 0.3

Unaged

10,000 h at 400°C

Saturation

32 efpy

at 290°C

0 2 4 6 8 10

δc = 15%

15000

290°C

J = 296(∆a)0.36

Crack Extension, ∆a (mm)

Heat 68 CF–8

Deformation J (in.–lb/in.2 )

10000

5000

Unaged

Static–Cast Slab

Unaged

J = 476(∆a)0.43

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 320°C

10,000 h at 400°C

Saturation 32 efpy

at 290°C

Figure 14. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–3 and CF–8 steels, and determined

experimentally
26

0

1000

2000

3000

0 2 4 6 8 10

δc = 25%

Deformation J (kJ/m 2)

Room Temp.

J =144(∆a)0.34

Heat 75 CF–8M

Crack Extension, ∆a (in.)

32 efpy

at 290°C

Static–Cast Slab

Unaged

J = 624(∆a)0.42

Saturation

0.0 0.1 0.2 0.3

Unaged

10,000 h at 400°C

Saturation

0 2 4 6 8 10

δc = 25%

15000

290°C

J = 188(∆a)0.32

Heat 75 CF–8M

Deformation J (in.–lb/in.2 )

Crack Extension, ∆a (in.)

10000

5000

Unaged

Static–Cast Slab

Unaged

J = 461(∆a)0.37

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 290°C

10,000 h at 400°C

Saturation

0

1000

2000

3000

0 2 4 6 8 10

δc = 16%

Deformation J (kJ/m 2)

Room Temp.

J =253(∆a)0.37

Heat 74 CF–8M

Unaged

Static–Cast Slab

Unaged

J = 575(∆a)0.42

Saturation

0.0 0.1 0.2 0.3

10,000 h at 400°C

Saturation

32 efpy

at 320°C

32 efpy

at 290°C

0 2 4 6 8 10

δc = 16%

15000 290°C

J =266(∆a)0.34

Heat 74 CF–8M

Deformation J (in.–lb/in.2 )

10000

5000

Unaged

Static–Cast Slab

Unaged

J = 439(∆a)0.37

Saturation

0.0 0.1 0.2 0.3

0

10,000 h at 400°C

Saturation

32 efpy

32 efpy at 290°C

at 320°C

0

1000

2000

3000

0 2 4 6 8 10

δc = 21%

Deformation J (kJ/m 2)

Room Temp.

J = 220(∆a)0.37

Crack Extension, ∆a (mm)

Heat 758 CF–8M

18,000 h at 400°C

Saturation

Unaged

J = 681(∆a)0.42

Saturation

0.0 0.1 0.2 0.3

Static–Cast Elbow

32 efpy

at 320°C 32 efpy

at 290°C

0 2 4 6 8 10

δc = 21%

15000

290°C

J = 244(∆a)0.33

Crack Extension, ∆a (mm)

Heat 758 CF–8M

Deformation J (in.–lb/in.2 )

10000

5000

18,000 h at 400°C

Saturation

Static–Cast Elbow

Unaged

J = 487(∆a)0.38

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 320°C

32 efpy

at 290°C

Figure 15. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–8M steels, and determined experimentally
27

0

1000

2000

3000

0 2 4 6 8 10

δc = 17%

Deformation J (kJ/m 2)

Room Temp.

J =524(∆a)0.47

TWI Heat CF–3

Crack Extension, ∆a (in.)

32 efpy

at 290°C

Static–Cast Plate

Unaged

J = 914(∆a)0.54

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 320°C

0 2 4 6 8 10

δc = 17%

15000 300°C

J =365(∆a)0.41

TWI Heat CF–3

Deformation J (in.–lb/in.2 )

Crack Extension, ∆a (in.)

10000

5000

Unaged

Static–Cast Plate

Unaged

J = 493(∆a)0.46

Saturation

0.0 0.1 0.2 0.3

0

10,000 h at 400°C

Saturation

32 efpy

at 290°C

32 efpy

at 320°C

0

1000

2000

3000

0 2 4 6 8 10

δc = 36%

Deformation J (kJ/m 2)

Room Temp.

J =290(∆a)0.39

EPRI Heat CF–3

Unaged Static–Cast Plate

Unaged

J = 1109(∆a)0.57

Saturation

0.0 0.1 0.2 0.3

7,320 h at 400°C

Saturation

32 efpy

at 290°C

32 efpy

at 320°C

0 2 4 6 8 10

δc = 36%

15000 300°C

J =266(∆a)0.35

EPRI Heat CF–3

Deformation J (in.–lb/in.2 )

10000

5000

Unaged

Static–Cast Plate

Unaged

J = 547(∆a)0.48

Saturation

0.0 0.1 0.2 0.3

0

7,320 h at 400°C

Saturation

32 efpy

at 290°C 32 efpy

at 320°C

0

1000

2000

3000

0 2 4 6 8 10

δc = 19%

Deformation J (kJ/m 2)

Room Temp.

J =151(∆a)0.35

Crack Extension, ∆a (mm)

Heat L CF–8M

7,500 h at 400°C

Saturation

Static–Cast Plate

Unaged

J = 564(∆a)0.41

Saturation

0.0 0.1 0.2 0.3

32 efpy

at 290°C

0 2 4 6 8 10

δc = 19%

15000 320°C

J =194(∆a)0.32

Crack Extension, ∆a (mm)

Heat L CF–8M

Deformation J (in.–lb/in.2 )

10000

5000

7,500 h at 400°C

Saturation

Static–Cast Plate

Unaged

J = 434(∆a)0.37

Saturation

0.0 0.1 0.2 0.3

0

32 efpy

at 290°C

Figure 16. Saturation fracture toughness J–R curves at RT and 290°C, estimated from the

chemical composition of static–cast CF–3 and CF–8M steels, and determined

experimentally (Refs. 19,21,22)
28

0

1000

2000

3000

0 2 4 6 8 10

CF–3

CF–8

CF–8M

Unaged

Cast SS

Deformation J (kJ/m2)

J = 400(∆a )0.40

Crack Extension, ∆ a (mm)

Crack Extension, ∆ a (in.)

0.2 0.3

290°C

Lower–Bound

Static Cast

0.0 0.1

(a)

J = 650(∆a )0.43

Lower–Bound

Centrifugally Cast

Open Symbols: Static–Cast Products

Closed Symbols: Centrifugally Cast Pipe

0 2 4 6 8 10

288°C Pipe Ref. (34)

288°C Pipe Ref. (35)

427°C Plate Ref. (36)

288°C Plate Ref. (37)

427°C Plate Ref. (36)

550°C Plate Ref. (38)

316°C Plate Ref. (39)

304 SS

Wrought SS

0

Lower–Bound

Static–Cast

Crack Extension, ∆ a (mm)

15000

Deformation J (in.–lb/in.2

)

Crack Extension, ∆ a (in.)

0.3

10000

5000

0.2

(b)

Unaged Cast SS

0.0 0.1

(b)

(b)

316 SS

Figure 17. Fracture toughness J–R curve for (a) unaged cast stainless steels at 290°C and

(b) wrought stainless steels at various temperatures

lower than the minimum fracture toughness J–R curve for unaged cast stainless steels (i.e.,

Eqs. 3.2.27 and 3.2.28), the latter is used as the initial J–R curve of the material.

Experimental and estimated J–R curves for the unaged materials are also shown in

Figs. 13–16. It should be noted that the RT J–R curves for unaged static–cast Heats 68, 69,

and 75 are nonconservative. It is believed that the poor fracture toughness of these unaged

static–cast slabs is due to residual stresses introduced into the material during the casting

process or production heat treatment. Annealing these heats for a short time at temperatures

between 290 and 400°C (554 and 752°F) increased the fracture toughness and decreased the

tensile stress without significantly affecting the impact energy.6,7 Consequently, the fracture

toughness of these heats would actually increase during reactor service before it would decrease as a result of thermal aging.

When the initial fracture toughness or initial RT Charpy–impact energy for estimating

fracture toughness of a material is known, and when the J–R curve estimated from CVsat and

Eqs. 3.2.13–3.2.26 is higher than the initial fracture toughness of unaged material, the latter is

used as the saturation J–R curve of the material. Such cases represent low–fracture–toughness materials that are relatively insensitive to thermal aging; thus, fracture toughness of the

material would not change during reactor service.

When no information is available on either the initial fracture toughness or initial RT

Charpy–impact energy for estimating fracture toughness of a material, the minimum fracture

toughness of unaged cast stainless steels is used as the upper bound for the predicted fracture

toughness of the aged material. In other words, if the J–R curve estimated from CVsat and

Eqs. 3.2.13–3.2.26 is higher than the minimum fracture toughness of unaged cast stainless

steels (i.e., Eqs. 3.2.27 and 3.2.28), the latter is used as the saturation J–R curve of a material.

The estimated J–R curves after 32 effective full power years (efpys) of service (i.e., the end

of ≈40–yr design life) at 290 and 320°C (554 and 608°F) are also shown in Figs. 13–16. These

curves provide information about the rate of thermal embrittlement. The results indicate that,
29

for service at 320°C, fracture toughness of these materials will reach saturation or will be close

to saturation within the 40–yr design life. A few compositions of CF–8M steel alone will reach

saturation fracture toughness within the 40–yr design life at 290°C.

3.3 Estimate for Steels of Known Composition and Service History:

Service Time Values

3.3.1 Charpy–Impact Energy

Room–temperature impact energy as a function of time and temperature of aging is

estimated from the RT saturation impact energy CVsat and the kinetics of embrittlement. The

decrease in RT Charpy–impact energy CV with time is expressed as

log10CV = log10CVsat + β{1 – tanh [(P – θ)/α]}, (3.3.1)

where the aging parameter P is defined by

 

P = log10 ( )t – 1000Q

19.143

1

T S + 273 – 1673





 



 (3.3.2)

The constants α and β can be determined from CVint and CVsat as follows:

α = –0.585 + 0.795log10CVsat (3.3.3)

and

β = (log10CVint – log10CVsat)/2. (3.3.4)

If CVint is not known, a typical value of 200 J/cm2 (118 ft·lb) may be used. The value of θ is

not available for cast stainless steel components in the field, and can only be obtained from

aging archival material for 5,000–10,000 h at 400°C (752°F). However, parametric studies

show that the aging response at reactor temperatures is relatively insensitive to the values of

θ.9 A value of 2.9 for θ (i.e., mean value of the experimental data) is used to estimate thermal

embrittlement at 280–330°C (536–626°F). A θ value of 3.3 is used for estimates at temperatures <280°C (<536°F) and 2.5 for estimates at 330–360°C (626–680°F). Activation energy for

thermal embrittlement is expressed in terms of both chemical composition and the constant θ.

The activation energy Q in kJ/mole is given by

Q = 10 [74.52 – 7.20 θ – 3.46 Si – 1.78 Cr – 4.35 I1 Mn

+ (148 – 125 I1) N – 61 I2 C], (3.3.5)

where the indicators I1 = 0 and I2 = 1 for CF–3 or CF–8 steels and assume the values of 1 and

0, respectively, for CF–8M steels.

 

The activation energy Q in kcal/mole is defined by

Q = 178.10 – 17.21 θ – 8.27 Si – 4.25 Cr – 10.40 I1 Mn

+ (353.7 – 298.8 I1) N – 145.8 I2 C, (3.3.5a)

where the indicators I1 = 0 and I2 = 1 for CF–3 or CF–8 steels and assume the values of 1 and

0, respectively, for CF–8M steels.

 
30

100

Estimated Q (kJ/mole)

Estimated Q (kcal/mole)

10

100

ANL

FRA

EdF

GF

NP

Open Symbols: CF–3, CF–8

Closed Symbols: CF–8M

Measured Q (kJ/mole)

40

60

80

200

400

40 60 200 400

30

50

70

Figure 18.

Observed and estimated activation energy

for thermal embrittlement of cast stainless

steels

The estimated and observed values of Q for the ANL,1–6 GF,15 EdF,17 NP,18 and FRA19

heats are plotted in Fig. 18. The predicted values are within the 95% confidence limits for all

the heats. Equation 3.3.5 is applicable to compositions within ASTM Specification A 351, with

an upper limit of 1.2 wt.% for Mn content. Actual Mn content is used when materials contain

up to 1.2 wt.% Mn; for steels containing >1.2 wt.% Mn, 1.2 wt.% is assumed. Furthermore,

the values of Q predicted from Eq. 3.3.5 should be between 65 kJ/mole (15.5 kcal/mole)

minimum and 250 kJ/mole (59.8 kcal/mole) maximum; Q is assumed to be 65 kJ/mole if the

predicted values are lower, and 250 kJ/mole if the predicted values are higher.

Equation 3.3.5 is an updated version* of an expression presented earlier;9 qualitatively it

agrees with the microstructural and mechanical–property data. For example, an increase in

the value of θ decreases the activation energy, as expected. The contributions of Si for all

grades of steel and Mn for CF–8M steels are consistent with their effect on G–phase precipitation. These elements should promote precipitation of G phase: hence, the coefficients for these

elements should have a negative sign, because activation energy for thermal embrittlement is

low for steels that show G–phase precipitation. An increase in C or N in the steel will promote

carbide or nitride precipitation at high temperatures and thus increase the activation energy.

The positive sign of the coefficient for N agrees with this behavior. The coefficient of C in

CF–8M steel, however, has a negative sign. It is likely that C also promotes precipitation of G

phase, a multicomponent phase consisting of Ni, Si, Mo, Cr, Fe, and some Mn and C.25,29

The RT Charpy–impact energy of a specific cast stainless steel as a function of service time

and temperature can be obtained from estimated CVsat (Eqs. 3.2.1–3.2.12) and the kinetics of

embrittlement (Eqs. 3.3.1–3.3.5). The initial Charpy–impact energy of the unaged steel is

needed for estimating the decrease in impact energy as a function of time and temperature of

service. The RT Charpy–impact energy observed experimentally and that estimated from the

chemical composition and initial impact energy of the ANL,4–7 GF,15 EdF,17 FRA,19 and EPRI21

heats aged at temperatures between 290 and 350°C (554 and 662°F) are shown in Figs. 19 and

20. The estimates for 31 of the 47 heats used in developing the correlations for the kinetics of

 

* The updated expression for CF–8M steel is essentially an optimized version of the earlier expression. For CF–3

and CF–8 steels, the effect of Mo and Mn content has been omitted and the effect of C has been added.
31

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 9%

Heat 59 CF–8 Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 10%

Heat 61 CF–8

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 15%

Heat 60 CF–8

Solid lines: Experimental

Dash lines: Estimated

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 15%

Heat 68 CF–8

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 18%

Heat P1 CF–8

Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 6%

Heat P4 CF–8M

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

Figure 19. RT Charpy–impact energy for 16 ANL heats and 1 EPRI heat, observed experimentally

and estimated from the composition and initial impact energy of the materials. Solid

lines represent the best fit of the data at 290–400°C and dash lines are the estimates

at 290–350°C.
32

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 6%

Heat 63 CF–8M Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 20%

Heat 66 CF–8M

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 29%

Heat 64 CF–8M

Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 16%

Heat 74 CF–8M

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 21%

Heat 65 CF–8M

Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 25%

Heat 75 CF–8M

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

Figure 19. (Contd.)
33

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 8%

Heat 47 CF–3 Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

Unaged

290°C

320°C

350°C

400°C

δc = 14%

Heat 51 CF–3

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

0

100

200

300

400

500

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 13%

Heat P2 CF–3 Solid Lines: Experimental

Dash Lines: Estimated

100 102 104 106

200

Unaged

320°C

Aging Time (h)

350°C

400°C

δc = 36%

EPRI Heat CF–3

Impact Energy, C

V (ft–lb)

0

50

100

150

250

Solid Lines: Experimental

Dash Lines: Estimated

0

50

100

150

200

250

300

Impact Energy, C

V (J/cm 2)

100 102 104 106

Unaged

290°C

Aging Time (h)

320°C

350°C

400°C

δc = 21%

Heat 69 CF–3

Impact Energy, C

V (ft–lb)

0

50

100

150

Solid Lines: Experimental

Dash Lines: Estimated

Figure 19. (Contd.)
34

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (J/cm 2

)

Heat 284 CF–3

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 44%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

50

100

150

200 Heat 280 CF–3

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 36%

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (J/cm 2

)

Heat 282 CF–8

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 30%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

50

100

150

200 Heat 279 CF–8

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 39%

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Aging Time (h)

Impact Energy, Cv (J/cm 2

)

Heat 278 CF–8

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 19%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

Aging Time (h)

50

100

150

200 Heat C CF–8

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 21%

Figure 20. RT Charpy–impact energy for the GF, FRA and EdF heats, observed experimentally

and estimated from the composition and initial impact energy of the materials. Solid

lines represent the best fit of the data at 300, 350, and 400°C and dash lines are

the estimates at 300 and 350°C.
35

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (J/cm 2

)

Heat 288 CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 36%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

50

100

150

200 Heat 286 CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 19%

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (J/cm 2

)

Heat 289 CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 23%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

50

100

150

200 Heat 287 CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 37%

0

100

200

300

400

100 102 104 106

Unaged

300°C

350°C

Aging Time (h)

Impact Energy, Cv (J/cm 2

)

Heat B CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 14%

100 102 104 106

Unaged

300°C

350°C

Impact Energy, Cv (ft·lb)

0

Aging Time (h)

50

100

150

200 Heat D CF–8M

400°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 12%

Figure 20. (Contd.)
36

0

100

200

300

400

100 102 104 106

Unaged

300°C

325°C

Aging Time (h)

Impact Energy, Cv (J/cm 2

)

Heat A10 CF–8M

350°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 23%

400°C

100 102 104 106

Unaged

300°C

325°C

Impact Energy, Cv (ft·lb)

0

Aging Time (h)

50

100

150

200 Heat A13 CF–8M

350°C

Solid Lines: Experimental

Dash Lines: Estimated

δc = 16%

400°C

Figure 20. (Contd.)

thermal embrittlement of cast stainless steels are shown in these figures. A θ value of 2.9 was

used for all temperatures of thermal aging. The estimated change in impact energy at temperatures ≤330°C (626°F) is either accurate or slightly conservative for most of the heats. A few

heats show poor agreement because either the estimated CVsat is higher than the experimental

value, e.g., FRA Heat D and ANL Heat 47, or the estimated activation energy is high, e.g., FRA

Heat C and GF Heat 278. Even at 350°C, the estimated impact energies show good agreement

with the experimental results because the θ values for most of the heats shown in the figures

are either greater or only slightly lower than 2.9. The EPRI heat and EdF Heat A10

(experimental θ is 2.1 for both heats) alone show nonconservative estimates at 350°C. A θ

value of 2.5 rather than 2.9 should be used to ensure that the estimates at 330—360°C

(626–680°F) are conservative.

3.3.2 Fracture Toughness J–R Curve

Once the RT Charpy–impact energy CV is known, the service time fracture toughness J–R

curve is determined from correlations described in Section 3.2. For convenience they are repeated here. The service time fracture toughness J–R curve at RT for static–cast CF–3 and

CF–8 steels, is given by

Jd = 49[CV]0.52[∆a]n, (3.3.6)

and for centrifugally cast CF–3 and CF–8 steels, by

Jd = 57[CV]0.52[∆a]n, (3.3.7)

where the exponent n, for CF–3 steel, is expressed as

n = 0.15 + 0.16log10[CV], (3.3.8)

and for CF–8 steels, as

n = 0.20 + 0.12log10[CV]. (3.3.9)

The service time fracture toughness J–R curve at RT for static–cast CF–8M steel, is given by

Jd = 16[CV]0.67[∆a]n, (3.3.10)

and for centrifugally cast CF–8M steel, by
37

Jd = 20[CV]0.67[∆a]n, (3.3.11)

where the exponent n is expressed as

n = 0.23 + 0.08log10[CV]. (3.3.12)

At 290°C (554°F), the service time J–R curve, for static–cast CF–3 and CF–8 steels, is given by

Jd = 102[CV]0.28[∆a]n, (3.3.13)

and for centrifugally cast CF–3 and CF–8 steels, by

Jd = 134[CV]0.28[∆a]n, (3.3.14)

where the exponent n, for CF–3 steel, is expressed as

n = 0.17 + 0.12log10[CV], (3.3.15)

and for CF–8 steels, as

n = 0.21 + 0.09log10[CV]. (3.3.16)

At 290°C (554°F), the service time J–R curve, for static–cast CF–8M steel, is given by

Jd = 49[CV]0.41[∆a]n, (3.3.17)

and for centrifugally cast CF–8M steel, by

Jd = 57[CV]0.41[∆a]n, (3.3.18)

where the exponent n is expressed as

n = 0.23 + 0.06log10[CV]. (3.3.19)

The J–R curve at any intermediate temperature can be linearly interpolated from the estimated

values of C and n at RT and 290°C.

 

The fracture toughness J–R curves corresponding to Eqs. 3.3.13–3.3.26 in British units,

i.e., RT impact energy CV is in ft·lb and Jd and ∆a are expressed in in.–lb/in.2 and in.,

respectively, are given by

Jd = {368(25.4)n[CV]0.52}[∆a]n, (3.3.6a)

Jd = {428(25.4)n[CV]0.52}[∆a]n, (3.3.7a)

n = 0.187 + 0.16log10[CV], (3.3.8a)

n = 0.227 + 0.12log10[CV], (3.3.9a)

Jd = {130(25.4)n[CV]0.67}[∆a]n, (3.3.10a)

Jd = {163(25.4)n[CV]0.67}[∆a]n, (3.3.11a)

n = 0.248 + 0.08log10[CV], (3.3.12a)

Jd = {675(25.4)n[CV]0.28}[∆a]n, (3.3.13a)

Jd = {887(25.4)n[CV]0.28}[∆a]n, (3.3.14a)

n = 0.197 + 0.12log10[CV], (3.3.15a)

n = 0.231 + 0.09log10[CV], (3.3.16a)

Jd = {347(25.4)n[CV]0.41}[∆a]n, (3.3.17a)
38

Jd = {404(25.4)n[CV]0.41}[∆a]n, (3.3.18a)

and

n = 0.244 + 0.06log10[CV]. (3.3.19a)

The expression enclosed in { } represents the coefficient C of the power–law J–R curve.

 

As discussed in Section 3.2, the correlations described in Eqs. 3.3.6–3.3.19 account for

the degradation of fracture toughness of typical heats of cast stainless steel and do not consider the initial fracture properties of the unaged material. Depending on the available information, initial fracture toughness of the unaged material or minimum fracture toughness of

unaged cast stainless steels (Eqs. 3.2.27 or 3.2.28) is used as an upper bound for the estimations (see Section C of flow diagram in Fig. 2). Examples of the experimental and estimated

J–R curves for several partially aged (e.g., 30,000 h at 320°C) cast stainless steels are shown in

Figs. 21 and 22. The estimated J–R curves show good agreement with experimental results.

3.4 Estimation of Tensile Properties

3.4.1 Tensile Strength

Tensile flow stress of aged cast stainless steels can be estimated from correlations between

the ratio of the tensile flow stress of aged and unaged cast stainless steels and a normalized

aging parameter. The ratio of the tensile flow stress of aged and unaged cast stainless steels at

RT and 290°C (554°F) is plotted as a function of a normalized aging parameter in Fig. 23. Flow

stress is characterized as the mean of the 0.2% yield stress and ultimate stress, and the aging

parameter is normalized with respect to a θ value of 2.9. The aging parameter P was determined from Eq. 3.3.2 and experimental values of activation energy. At both temperatures, the

flow–stress ratio increased with thermal aging; the increase in flow stress of CF–3 steels was

the smallest and that of CF–8M steels the largest. The correlations (shown as solid lines in

Fig. 23) were obtained by subtracting the value of standard deviation for the fit to the data

from the best–fit curve. The X axis in Fig. 23 is reduced to P when a θ value of 2.9 is used in

Eqs. 3.3.5 and 3.3.2 to determine P.

Tensile flow stress of aged cast stainless steels can be estimated from the initial tensile

flow stress and the correlations given in Fig. 23. At RT, the tensile–flow–stress ratio

Rf = (σfaged/σfunaged), for CF–3 steel, is given by

Rf = 0.90 + 0.05P (1.00 ≤ Rf ≤1.10); (3.4.1)

for CF–8 steel, by

Rf = 0.84 + 0.08P (1.00 ≤ Rf ≤1.16); (3.4.2)

and for CF–8M steel, by

Rf = 0.77 + 0.10P (1.00 ≤ Rf ≤1.19). (3.4.3)

At 290°C (554°F), the tensile–flow–stress ratio Rf = (σfaged/σfunaged), for CF–3 steel, is given by

Rf = 0.87 + 0.06P (1.00 ≤ Rf ≤1.08); (3.4.4)
39

0

1000

2000

3000

0 2 4 6 8 10

55,000 h at 320°C

δc = 18%

Deformation J (kJ/m2)

Heat P1 CF–8 Room Temp.

Crack Extension, ∆ a (in.)

Centrifugally Cast Pipe

Unaged

J = 724(∆a )0.45

Saturation

0.0 0.1 0.2 0.3

6.3 efpy at 320°C or

51.9 efpy at 290°C

0 2 4 6 8 10

55,000 h at 320°C

δc = 18%

15000 Heat P1 CF–8 290°C

Deformation J (in.–lb/in.2)

Crack Extension, ∆ a (in.)

10000

5000

Centrifugally Cast Pipe

Unaged

J = 526(∆a )0.40

Saturation

0.0 0.1 0.2 0.3

0

6.3 efpy at 320°C or

51.9 efpy at 290°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 350°C

δc = 18%

Deformation J (kJ/m2)

Heat P1 CF–8 Room Temp.

Centrifugally Cast Pipe

Unaged

J = 574(∆a )0.43

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 350°C or

23.1 efpy at 320°C

0 2 4 6 8 10

30,000 h at 350°C

δc = 18%

15000 Heat P1 CF–8 290°C

Deformation J (in.–lb/in.2)

10000

5000

Centrifugally Cast Pipe

Unaged

J = 465(∆a )0.38

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 350°C or

23.1 efpy at 320°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 350°C

δc = 13%

Deformation J (kJ/m2)

Room Temp.

Crack Extension, ∆ a (mm)

Heat P2 CF–3

Centrifugally Cast Pipe

Unaged

J = 816(∆a )0.51

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 350°C or

22.4 efpy at 320°C

0 2 4 6 8 10

30,000 h at 350°C

δc = 13%

15000 290°C

Crack Extension, ∆ a (mm)

Heat P2 CF–3

Deformation J (in.–lb/in.2)

10000

5000

Centrifugally Cast Pipe

Unaged

J = 562(∆a )0.44

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 350°C or

22.4 efpy at 320°C

Figure 21. Fracture toughness J–R curve at RT and 290°C, estimated from the chemical

composition and initial Charpy–impact energy and determined experimentally

for partially aged, centrifugally cast CF–3 and CF–8 pipes
40

0

1000

2000

3000

0 2 4 6 8 10

53,000 h at 320°C

δc = 15%

Deformation J (kJ/m2)

Heat 68 CF–8 Room Temp.

Crack Extension, ∆ a (in.)

Static–Cast Slab Unaged

J = 591(∆a )0.45

Saturation

0.0 0.1 0.2 0.3

6.0 efpy at 320°C or

44.9 efpy at 290°C

0 2 4 6 8 10

53,000 h at 320°C

δc = 15%

15000 Heat 68 CF–8 290°C

Deformation J (in.–lb/in.2)

Crack Extension, ∆ a (in.)

10000

5000

Static–Cast Slab

Unaged

J = 390(∆a )0.40

Saturation

0.0 0.1 0.2 0.3

0

6.0 efpy at 320°C or

44.9 efpy at 290°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 350°C

δc = 15%

Deformation J (kJ/m2)

Heat 68 CF–8 Room Temp.

Static–Cast Slab

Unaged

J = 437(∆a )0.42

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 350°C or

20.9 efpy at 320°C

0 2 4 6 8 10

30,000 h at 350°C

δc = 15%

15000 Heat 68 CF–8 290°C

Deformation J (in.–lb/in.2)

10000

5000

Static–Cast Slab

Unaged

J = 331(∆a )0.37

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 350°C or

20.9 efpy at 320°C

0

1000

2000

3000

0 2 4 6 8 10

312 h at 400°C

δc = 36%

Deformation J (kJ/m2)

Room Temp.

Crack Extension, ∆ a (mm)

EPRI Heat CF–3

Static Cast Plate

Unaged J = 340(∆a )0.41

Saturation

0.0 0.1 0.2 0.3

7.3 efpy at 320°C 

0 2 4 6 8 10

312 h at 400°C

δc = 36%

15000 300°C

Crack Extension, ∆ a (mm)

EPRI Heat CF–3

Deformation J (in.–lb/in.2)

10000

5000

Static Cast Plate

Unaged

J = 290(∆a )0.36

Saturation

0.0 0.1 0.2 0.3

0

7.3 efpy at 320°C 

Figure 22. Fracture toughness J–R curves at RT and 290°C, estimated from the chemical

composition and initial Charpy–impact energy and determined experimentally

for partially aged, static–cast CF–3, CF–8, and CF–8M steels
41

0

1000

2000

3000

0 2 4 6 8 10

50,000 h at 320°C

δc = 21%

Deformation J (kJ/m2)

Heat 69 CF–3 Room Temp.

Crack Extension, ∆ a (in.)

J = 641(∆a )0.49

Static–Cast Slab Unaged

Saturation

0.0 0.1 0.2 0.3

5.7 efpy at 320°C or

34.1 efpy at 290°C

0 2 4 6 8 10

50,000 h at 320°C

δc = 21%

15000 Heat 69 CF–3 290°C

Deformation J (in.–lb/in.2)

Crack Extension, ∆ a (in.)

10000

5000

Static–Cast Slab

Unaged

J = 407(∆a )0.43

Saturation

0.0 0.1 0.2 0.3

0

5.7 efpy at 320°C or

34.1 efpy at 290°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 350°C

δc = 21%

Deformation J (kJ/m2)

Heat 69 CF–3 Room Temp. Static–Cast Slab

Unaged

J = 571(∆a )0.48

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 350°C or

17.2 efpy at 320°C

0 2 4 6 8 10

30,000 at 350°C

δc = 21%

290°C 15000 Heat 69 CF–3

Deformation J (in.–lb/in.2)

10000

5000

Static–Cast Slab

Unaged

J = 383(∆a )0.42

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 350°C or

17.2 efpy at 320°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 320°C

δc = 20%

Deformation J (kJ/m2)

Heat I CF–3 Room Temp.

Static Cast Impeller Unaged

J = 668(∆a )0.50

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 320°C or

25.0 efpy at 290°C

0 2 4 6 8 10

30,000 h at 320°C

δc = 20%

15000 290°C

Crack Extension, ∆ a (mm)

Heat I CF–3

Deformation J (in.–lb/in.2)

10000

5000

Static Cast Impeller

Unaged

J = 417(∆a )0.43

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 320°C or

25.0 efpy at 290°C

Figure 22. (Contd.)
42

0

1000

2000

3000

0 2 4 6 8 10

50,000 h at 320°C

δc = 16%

Deformation J (kJ/m2)

Heat 74 CF–8M Room Temp.

Crack Extension, ∆ a (in.)

Static–Cast Slab

Unaged

5.7 efpy at 320°C or

31.7 efpy at 290°C

Saturation

0.0 0.1 0.2 0.3

J = 357(∆a )0.39

0 2 4 6 8 10

50,000 h at 320°C

δc = 16%

15000 290°C

5.7 efpy at 320°C or

31.7 efpy at 290°C

Heat 74 CF–8M

Deformation J (in.–lb/in.2)

Crack Extension, ∆ a (in.)

10000

5000

Static–Cast Slab

Unaged J = 328(∆a )0.35

Saturation

0.0 0.1 0.2 0.3

0

0

1000

2000

3000

0 2 4 6 8 10

50,000 h at 320°C

δc = 25%

Deformation J (kJ/m2)

Heat 75 CF–8M Room Temp.

Static–Cast Slab

Unaged

J = 186(∆a )0.36

Saturation

0.0 0.1 0.2 0.3

5.7 efpy at 320°C or

23.5 efpy at 290°C

0 2 4 6 8 10

50,000 h at 320°C

δc = 25%

15000 Heat 75 CF–8M 290°C

Deformation J (in.–lb/in.2)

10000

5000

Static–Cast Slab

Unaged

J = 220(∆a )0.33

Saturation

0.0 0.1 0.2 0.3

0

5.7 efpy at 320°C or

23.5 efpy at 290°C

0

1000

2000

3000

0 2 4 6 8 10

30,000 h at 350°C

δc = 25%

Deformation J (kJ/m2)

Room Temp.

Crack Extension, ∆ a (mm)

Heat 75 CF–8M

Static–Cast Slab

Unaged J = 157(∆a )0.35

Saturation

0.0 0.1 0.2 0.3

3.4 efpy at 350°C or

12.3 efpy at 320°C

0 2 4 6 8 10

30,000 h at 350°C

δc = 25%

15000 290°C

Crack Extension, ∆ a (mm)

Heat 75 CF–8M

Deformation J (in.–lb/in.2)

10000

5000

Static–Cast Slab

Unaged

J = 199(∆a )0.32

Saturation

0.0 0.1 0.2 0.3

0

3.4 efpy at 350°C or

12.3 efpy at 320°C

Figure 22. (Contd.)
43

0.8

0.9

1.0

1.1

1.2

1.3

1.4

012345

ANL

FRA

Ratio = 0.90 + 0.05(P–Θ+2.9)

Normalized Aging Parameter (P – Θ + 2.9)

CF–3 Steel

Flow Stress Ratio (aged/unaged)

1.00 ≤ Ratio ≤ 1.10

Room Temp.

012345

ANL

Ratio = 0.87 + 0.06(P–θ+2.9)

Normalized Aging Parameter (P – θ + 2.9)

CF–3 Steel

1.00 ≤ Ratio ≤ 1.08

290°C

0.8

0.9

1.0

1.1

1.2

1.3

1.4

012345

ANL

FRA

Ratio = 0.84 + 0.08(P–θ+2.9)

Normalized Aging Parameter (P – θ + 2.9)

CF–8 Steel

Flow Stress Ratio (aged/unaged)

1.00 ≤ Ratio ≤ 1.16

Room Temp.

012345

ANL

Ratio = 0.83 + 0.09(P–θ+2.9)

Normalized Aging Parameter (P – θ + 2.9)

CF–8 Steel

1.00 ≤ Ratio ≤ 1.14

290°C

0.8

0.9

1.0

1.1

1.2

1.3

1.4

012345

ANL

FRA

Ratio = 0.77 + 0.10(P–θ+2.9)

Normalized Aging Parameter (P – θ + 2.9)

CF–8M Steel

Flow Stress Ratio (aged/unaged)

1.0 ≤ Ratio ≤ 1.19

Room Temp.

012345

ANL

Ratio = 0.69 + 0.14(P–θ+2.9)

Normalized Aging Parameter (P – θ + 2.9)

CF–8M Steel

1.00 ≤ Ratio ≤ 1.24

290°C

Figure 23. Flow stress ratio of aged cast stainless steels at RT and 290°C as a function of the

normalized aging parameter. The solid lines represent correlations obtained by

subtracting σ from the best fit curve.
44

for CF–8 steel, by

Rf = 0.83 + 0.09P (1.00 ≤ Rf ≤1.14); (3.4.5)

and for CF–8M steel, by

Rf = 0.69 + 0.14P (1.00 ≤ Rf ≤1.24). (3.4.6)

The minimum and maximum values of the ratio Rf are given for each grade of steel and temperature, i.e., a minimum or maximum value is assumed, respectively, when the calculated

ratio is smaller than the minimum or greater than the maximum. Equations 3.4.1–3.4.6 are

valid for service temperatures between 280 and 330°C (536 and 626°F) and ferrite contents

>7% for CF–8M steel and >10% for CF–3 and CF–8 steels. Thermal aging has little or no effect

on the tensile strength of cast stainless steels with low ferrite content. The available data base

is inadequate for estimating the tensile properties at service temperatures <280°C (<536°F).

Experimental and estimated tensile flow stress at 290°C (554°F) and at RT for various heats of

aged cast stainless steel are shown in Fig. 24. For each heat, the aging parameter and activation energy were obtained from Eqs. 3.3.2 and 3.3.5 and a θ value of 2.9. Tensile flow stress

was then estimated from Eqs. 3.4.1–3.4.6 and the initial flow stress of the materials. The estimated values are either accurate or conservative for all material and aging conditions.

The fracture toughness JIC values for aged cast stainless steels can be determined from

the estimated J–R curve and flow stress. The experimental and estimated JIC for various heats

aged at temperatures ≤350°C are shown in Fig. 25. The chemical composition and initial

Charpy–impact energy and the flow stress of the unaged material were used for the estimations. The estimated JIC values show good agreement with the experimental results; for most

cases the estimated JIC is lower but within 30% of the observed value.

The data on tensile properties of cast stainless steels indicate that the increase in yield

stress due to thermal aging is much lower than the increase in ultimate stress. At RT, the tensile–yield–stress ratio Ry = (σyaged/σyunaged), for CF–3 steel, is given by

200

300

400

500

200 300 400 500

CF–3

CF–8

CF–8M

Estimated Flow Stress (MPa)

290°C

Measured Flow Stress (MPa)

300

400

500

600

300 400 500 600

CF–3

CF–8

CF–8M

Room Temp.

Open Symbols: ANL

Closed Symbols: FRA

Measured Flow Stress (MPa)

Figure 24. Experimental and estimated flow stress of aged cast stainless steel at 290°C

and RT
45

100

1000

100 1000

CF–3, CF–8

CF–8M

Estimated J

I C (kJ/m2

)

290°C

Measured J

I C (kJ/m2

)

100 1000

CF–3, CF–8

CF–8M

Room Temp.

Measured J

I C (kJ/m2

)

Figure 25. Experimental and estimated values of JIC for aged cast stainless steels

Ry = 0.873 + 0.048P (1.00 ≤ Ry ≤1.07); (3.4.7)

for CF–8 steel, by

Ry = 0.798 + 0.076P (1.00 ≤ Ry ≤1.10); (3.4.8)

and for CF–8M steel, by

Ry = 0.708 + 0.092P (1.00 ≤ Ry ≤1.10). (3.4.9)

At 290°C (554°F), the tensile–yield–stress ratio Ry = (σyaged/σyunaged), for CF–3 steel, is given by

Ry = 0.844 + 0.058P (1.00 ≤ Ry ≤1.05); (3.4.10)

for CF–8 steel, by

Ry = 0.788 + 0.086P (1.00 ≤ Ry ≤1.09); (3.4.11)

and for CF–8M steel, by

Ry = 0.635 + 0.129P (1.00 ≤ Ry ≤1.14). (3.4.12)

The minimum and maximum values of the tensile–yield–stress ratio Ry are given for each grade

of steel and temperature. Equations 3.4.7–3.4.12 are valid for service temperatures between

280 and 330°C (536 and 626°F) and ferrite contents >7% for CF–8M steel and >10% for CF–3

and CF–8 steels.

Experimental and estimated tensile yield stress at 290°C (554°F) and at RT for various

heats of aged cast stainless steel are shown in Fig. 26. As for flow stress estimations, the aging

parameter and activation energy were obtained from Eqs. 3.3.2 and 3.3.5 and a θ value of 2.9.

Tensile yield stress was then estimated from Eqs. 3.4.7–3.4.12 and the initial yield stress of the

materials. The estimated values are conservative for most material and aging conditions.
46

5 0

100

150

200

250

300

350

5 0 100 150 200 250 300 350

CF–3

CF–8

CF–8M

Estimated Yield Stress (MPa)

290°C

Measured Yield Stress (MPa)

150

200

250

300

350

400

450

150 200 250 300 350 400 450

CF–3

CF–8

CF–8M

Room Temp.

Open Symbols: ANL

Closed Symbols: FRA

Measured Yield Stress (MPa)

Figure 26. Experimental and estimated yield stress of aged cast stainless steel at 290°C

and RT

3.4.2 Ramberg–Osgood Parameters

The engineering stress vs. strain behavior of aged cast stainless steel can also be obtained

from the estimated flow stress.23 The engineering stress–vs.–strain curve is expressed by the

Ramberg–Osgood equation

 

ε

εo

= σ

σo

+ α1

σ

σo





 





n1

, (3.4.13a)

where σ and ε are engineering stress and strain, respectively; σo is an arbitrary reference

stress, often assumed to be equal to flow or yield stress; the reference strain εo = σo/E; α1 and

n1 are Ramberg–Osgood parameters; and E is elastic modulus. The Ramberg–Osgood equation

can be rearranged to the form

 

Eε – σ

σf

= α1

σ

σf













n1

, (3.4.13b)

which is more convenient for fitting stress vs. strain data; α1 can be determined at σ/σf = 1 and

n1 can be obtained from the slope of the log–log plot of Eq. 3.4.13b. For all grades of cast

stainless steel, the parameter n1 does not change with thermal aging. The parameter α1

decreases with aging and shows good correlation with the flow stress σf of the material, Fig. 27.

For engineering stress–vs.–strain curves up to 5% strain, the Ramberg–Osgood parameters at

room RT, for CF–3 steels, are given by

α1 = 143.9 – 0.267σf (n1 = 6.1); (3.4.14)

for CF–8 steel, by

α1 = 157.9 – 0.300σf (n1 = 6.4); (3.4.15)
47

0

2 5

5 0

7 5

100

350 400 450 500 550

α1 (<5% Strain)

Flow Stress, σf (MPa)

α1 = 143.9 – 0.267σf

CF–3 Steel

Room Temp.

250 300 350 400 450

Flow Stress, σf (MPa)

α1 = 102.1 – 0.235σf

CF–3 Steel

290°C

0

2 5

5 0

7 5

100

350 400 450 500 550

α1 (<5% Strain)

Flow Stress, σf (MPa)

α1 = 157.9 – 0.300σf

CF–8 Steel

Room Temp.

250 300 350 400 450

Flow Stress, σf (MPa)

α1 = 153.3 – 0.373σf

CF–8 Steel

290°C

0

2 5

5 0

7 5

100

350 400 450 500 550

α1 (<5% Strain)

Flow Stress, σf (MPa)

α1 = 50.9 – 0.072σf

CF–8M Steel

Room Temp.

250 300 350 400 450

Flow Stress, σf (MPa)

α1 = 145.9 – 0.314σf

CF–8M Steel

290°C

Figure 27. Correlation between the Ramberg/Osgood parameter α1 and flow stress at RT and

290°C for cast stainless steels
48

and for CF–8M steel, by

α1 = 50.9 – 0.0724σf (n1 = 5.6). (3.4.16)

At 290°C (554°F), the Ramberg–Osgood parameters for engineering stress–vs.–strain curves up

to 5% strain, for CF–3 steels, are given by

α1 = 102.1 – 0.235σf (n1 = 6.2); (3.4.17)

for CF–8 steel, by

α1 = 153.3 – 0.373σf (n1 = 7.1); (3.4.18)

and for CF–8M steel, by

α1 = 145.9 – 0.314σf (n1 = 6.6). (3.4.19)

Similar correlations have also been developed for stress–vs.–strain curves up to 15% strain or

up to the ultimate stress.23 Examples of engineering stress–vs.–strain curves estimated from

Eqs. 3.4.13–3.4.19 and those observed experimentally for several heats of aged CF–3, CF–8,

and CF–8M steels, are shown in Fig. 28. The estimated curves are essentially conservative.

4 Conclusions

A procedure and correlations are presented for predicting Charpy–impact energy and

fracture toughness J–R curve of aged cast SSs (ASTM A 351) from known material information.

Mechanical properties of a specific cast SS are estimated from the extent and kinetics of thermal embrittlement. Embrittlement of cast SSs is characterized in terms of RT Charpy–impact

energy. The extent or degree of thermal embrittlement at “saturation,” i.e., the minimum impact energy that can be achieved for the material after long–term aging, is determined from

chemical composition of the steel. Charpy–impact energy as a function of time and temperature of reactor service is estimated from the kinetics of thermal embrittlement, which is also

determined from the chemical composition. The initial impact energy of the unaged steel is required for these estimations. The fracture toughness J–R curve for the material is then obtained from correlations between RT Charpy–impact energy and fracture toughness parameters. A common “predicted lower–bound” J–R curve for cast SSs with unknown chemical composition is also defined for a given grade of steel, range of ferrite contents, and temperature.

Typical examples for estimating fracture properties of cast SS components during reactor service are described in the Appendix.
49

0

200

400

600

800

0123456

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 15%

Heat 68 CF–8

Room Temp.

Engineering Strain (%)

0123456

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

0

20

40

60

80

100 δ c = 15%

Heat 68 CF–8

290°C

Engineering Stress (ksi)

Engineering Strain (%)

0

200

400

600

800

01234567

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 21%

Heat 69 CF–3

Room Temp.

Engineering Strain (%)

0123456

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

0

20

40

60

80

100 δ c = 21%

Heat 69 CF–3

290°C

Engineering Stress (ksi)

Engineering Strain (%)

0

200

400

600

800

0123456

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 25%

Heat 75 CF–8M

Room Temp.

Engineering Strain (%)

0123456

Unaged

Aged 53,000 h at 320°C

Predicted Unaged

Predicted Aged

0

20

40

60

80

100 δ c = 25%

Heat 75 CF–8M

290°C

Engineering Stress (ksi)

Engineering Strain (%)

Figure 28. Engineering stress–vs.–strain curve at RT and 290°C, estimated from the chemical

composition and initial tensile strength and determined experimentally for thermally

aged CF–3, CF–8, and CF–8M steels
50

0

200

400

600

800

0123456

Unaged

Aged 55,000 h at 320°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 13%

Heat P2 CF–3

Room Temp.

Engineering Strain (%)

0123456

Unaged

Aged 55,000 h at 320°C

Predicted Unaged

Predicted Aged

0

20

40

60

80

100 δ c = 13%

Heat P2 CF–3

290°C

Engineering Stress (ksi)

Engineering Strain (%)

0

200

400

600

800

0123456

Unaged

Aged 55,000 h at 320°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 18%

Heat P1 CF–8

Room Temp.

Engineering Strain (%)

0123456

Unaged

Aged 55,000 h at 320°C

Predicted Unaged

Predicted Aged

0

20

40

60

80

100 δ c = 18%

Heat P1 CF–8

290°C

Engineering Stress (ksi)

Engineering Strain (%)

0

200

400

600

800

0123456

Annealed

68,000 h service at 284°C

Predicted Unaged

Predicted Aged

Engineering Stress (MPa)

δ c = 28%

KRB Reactor Pump Cover

CF–8

Room Temp.

Engineering Strain (%)

0123456

Annealed

68,000 h service at 284°C

Predicted Unaged

Predicted Aged

δ c = 28%

KRB Reactor Pump Cover

CF–8

290°C

Engineering Strain (%)

0

20

40

60

80

100

Engineering Stress (ksi)

Figure 28. (Contd.)
51

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Properties of Cast Stainless Steels,” in Properties of Stainless Steels in

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at Temperatures Between 300 and 500°C on the Impact Toughness and the Structure

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also in Stainless Steel Castings, V. G. Behal and A. S. Melilli, eds., STP 756, ASTM,

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Centrifugally Cast Type 316 Stainless Steel Pipe after Simulated Thermal Service

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17. S. Bonnet, J. Bourgoin, J. Champredonde, D. Guttmann, and M. Guttmann, “Relationship

between Evolution of Mechanical Properties of Various Cast Duplex Stainless Steels and

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Technol., 6, 221–229 (1990).

18. P. H. Pumphrey and K. N. Akhurst, “Aging Kinetics of CF3 Cast Stainless Steel in

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Oct. 1988, Avignon, France.

21. P. McConnell and J. W. Sheckherd, Fracture Toughness Characterization of Thermally

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Palo Alto, CA (September 1987).
53

22. G. E. Hale and S. J. Garwood, “The Effect of Aging on the Fracture Behaviour of Cast

Stainless Steel and Weldments,” Mater. Sci. Technol., 6, 230–235 (1990).

23. W. F. Michaud, P. T. Toben, W. K. Soppet, and O. K. Chopra, Tensile–Property

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(February 1994).

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and Transmission Electron Microscopy Study of Aging of Cast Duplex Stainless Steels,”

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Formation of Cleavage Cracks in Ferrite in an Aged Duplex Stainless Steel,” Scripta Metall.,

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Reaction in the Ferrite Phase of a Duplex Stainless Steel,” in Properties of Stainless Steels

in Elevated Temperature Service, M. Prager, ed., MPC Vol. 26, PVP Vol. 132, ASME,

New York, pp. 65–78 (1988).

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“Quantitative Atom Probe Analysis of Spinodal Reaction in Ferrite Phase of Duplex

Stainless Steel,” Mater. Sci. Technol., 6, 293–300 (1990).

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Primary Coolant Pipe Steels,” in Environmental Degradation of Materials in Nuclear Power

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Warrendale, PA, pp. 341–349 (1988).

30. O. K. Chopra and L. Y. Bush, Long–Term Embrittlement of Cast Duplex Stainless Steels in

LWR Systems: Semiannual Report October 1989–March 1990, NUREG/CR–4744, Vol. 5,

No. 1, ANL–91/7 (July 1991).

31. A. W. Bowen and G. M. Leak, “Diffusion in BCC Iron Base Alloys,” Metall. Trans., 1, 2767

(1970).

32. L. S. Aubrey, P. F. Wieser, W. J. Pollard, and E. A. Schoefer, “Ferrite Measurement and

Control in Cast Duplex Stainless Steel,” in Stainless Steel Castings, V. G. Behal and A. S.

Melilli, eds., STP 756, ASTM, Philadelphia, PA, pp. 126–164 (1982).

33. Standard Practice for Steel Casting, Austenitic Alloy, Estimating Ferrite Content Thereof,

ASTM Designation A 800/A 800M–84, American Society for Testing and Materials,

Philadelphia, PA (1984).

34. A. L. Hiser, Fracture Toughness Characterization of Nuclear Piping Steels,

NUREG/CR–5118, MEA–2325, Materials Engineering Associates, Inc. (Nov. 1989).
54

35. G. M. Wilkowski, et al., Degraded Piping Program – Phase II, Semiannual Report,

NUREG/CR–4082, Vol. 2 (June 1985).

36. W. J. Mills, “Heat–to–Heat Variations in the Fracture Toughness of Austenitic Stainless

Steels,” Eng. Fracture Mech., 30, 469–492 (1988).

37. M. G. Vassilaros, R. A. Hays, and J. P. Gudas, “Investigation of the Ductile Fracture

Properties of Type 304 Stainless Steel Plate, Welds, and 4–Inch Pipe,” in Proc. 12th Water

Reactor Safety Information Meeting, U.S. Nuclear Regulatory Commission,

NUREG/CP–0058, Vol. 4, p. 176 (Jan. 1985).

38. P. Balladon, J. Heritier, and P. Rabbe, “Influence of Microstructure on the Ductile Rupture

Mechanisms of a 316L Steel at Room and Elevated Temperatures,” in Fracture Mechanics:

Fourteenth Symposium, STP 791, ASTM, Philadelphia, PA., pp. II496–II513 (1983).

39. W. H. Bamford and A. J. Bush, “Fracture Behavior of Stainless Steel,” Elastic–Plastic

Fracture, STP 668, ASTM, Philadelphia, PA, pp. 553–577 (1979).
55
A–1

Appendix

Estimation of Fracture Properties of Cast SSs

The correlations described in this report can be used for assessing thermal embrittlement

of cast SS components. The procedure involves a few simple steps. First, available

information about the material and service condition is obtained from the CMTR; i.e., if known,

determine the chemical composition, grade of steel, casting method, Charpy–impact properties,

and time and temperature of service. Then, on the basis of available information, various

schemes are used to estimate the fracture toughness J–R curve and Charpy–impact energy of

the aged material. Typical examples for estimating fracture properties of cast SS components

during reactor service are presented.

Example 1. A centrifugally cast CF–8 pipe, 0.51 m nominal diameter, in service at 302°C

(575°F) for 18 effective full power years (efpys). The following material information is also

known.

Measured Ferrite Content (%): 14.2

Only the lower–bound fracture toughness J–R curve can be estimated for this steel, because only the grade and ferrite content are known* and the chemical composition of the material is not known. The estimation scheme for this example involves one step and is shown in

Fig. A–1. The lower–bound fracture toughness represents the minimum toughness that can be

achieved after long–term aging by centrifugally cast CF–8 steels containing 10–15% ferrite.

Lower–Bound

J–R Curve for

Ferrite Range

No

Yes

Section A: 

LOWER BOUND

CMTR

ESTIMATION SCHEME

Ferrite

content 

Estimate for unknown composition

Figure A–1. Estimation scheme for Example 1

Step 1. From Eqs. 3.1.20 and 3.1.23 the predicted lower–bound fracture toughness J–R curve

for centrifugally cast CF–8 steel with 10–15% ferrite, at RT, is given by

Jd (kJ/m2) = 357[∆a(mm)]0.38 (A–1.1)

 

*CF–8M steel and >15% ferrite are assumed if the grade and ferrite content of the steel are not known.
A–2

and at 290°C (554°F) by

Jd (kJ/m2) = 360[∆a(mm)]0.35. (A–1.2)

The J–R curve at a service temperature of 302°C can be linearly extrapolated from Eqs. A–1.1

and A–1.2. The curve at 302°C is not significantly different from that at 290°C and can be approximated as

Jd (kJ/m2) = 360[∆a(mm)]0.35. (A–1.3)

Example 2. A static–cast CF–8 check valve in service at 282°C (540°F). The following material

information is also known.

Chem. Comp. (wt.%): Cr, 20.26; Mo, 0.01; Si, 1.45; Ni, 8.84; Mn, 1.10; C, 0.056; N, 0.041

Measured Ferrite Content (%): 10.4

RT Charpy–Impact Energy (J): 150.4

Only the saturation fracture properties can be estimated in this case, because the time of

service is not known. The estimation scheme for this example is shown in Fig. A–2. The saturation fracture toughness represents the minimum toughness that can be achieved by this

specific cast stainless steel after long–term aging.

Step 1. The calculated ferrite content (Eqs. 3.2.1–3.2.3) is

δc (%)= 10.8.

Step 2. The material parameter Φ (Eq. 3.2.5) is 16.94. Saturation RT Charpy–impact energy

in terms of the material parameter (Eq. 3.2.4) is 79.8 J/cm2, and in terms of composition

(Eq. 3.2.6), it is 58.8 J/cm2. The lower of the two values is used for fracture toughness estimations; thus

CVsat (J/cm2) = 58.8.

Step 3. The saturation fracture toughness J–R curve for static–cast CF–8 steel, at RT

(Eqs. 3.2.13 and 3.2.16), is given by

Jd (kJ/m2) = 407.8[∆a(mm)]0.41 (A–2.1)

and at 290°C (Eqs. 3.2.20 and 3.2.23), by

Jd (kJ/m2) = 319.2[∆a(mm)]0.37. (A–2.2)

The saturation fracture toughness J–R curve at a service temperature of 282°C is linearly interpolated from Eqs. A–2.1 and A–2.2. The curve at 282°C is not significantly different from

that at 290°C and can be approximated as

Jd (kJ/m2) = 319.2[∆a(mm)]0.37. (A–2.3)

This corresponds to a Jd value of 579 kJ/m2 (3306 in.lb/in.2) at 5–mm crack extension.
A–3

Estimate for known composition unknown service history

Sat. J–R

> Initial J–R

Unaged

No

No Saturation

J–R Curve &

Impact Energy

Input Initial Yes

Impact Energy

Initial

Impact

Energy

Section B: 

SATURATION

Yes

CMTR

ESTIMATION SCHEME

Input 

Composition &

Casting Process

Estimate Saturation

Impact Energy & 

J–R Curve

Estimate Initial

J–R Curve

Service

Time &

Temp.

Figure A–2. Estimation scheme for Example 2

Step 4. The initial RT Charpy–impact energy* is 150.4 J. The normalized value is obtained by

dividing this value by the cross–sectional area of the Charpy–impact specimen, i.e., 0.8 cm2.

CVint (J/cm2) = 188.0.

Step 5. The initial J–R curve for the unaged material is the higher of either Eq. 3.2.27 or the

J–R curve determined from Eqs. 3.2.13, 3.2.16, 3.2.20, and 3.2.23 using CVint instead of CVsat.

In this example, the initial J–R curve, at 282°C, is obtained from Eqs. 3.2.20 and 3.2.23 as

Jd (kJ/m2) = 441.9[∆a(mm)]0.41. (A–2.4)

This corresponds to a Jd value of 855 kJ/m2 (4881 in.·lb/in.2) at 5–mm crack extension.

Thermal aging decreases the fracture toughness of this steel from 855 to 579 kJ/m2 in the

fully aged condition.

 

* In this report, all values of impact energy are considered to be for a standard Charpy–V–notch specimen (ASTM

Specification E 23), i.e., 10 x10–mm cross section and 2–mm V notch.
A–4

Example 3. A centrifugally cast CF–8M pipe, 0.51 m nominal diameter, in service at 291°C

(555°F). The following material information is also known.

Chem. Comp. (wt.%): Cr, 20.64; Mo, 2.05; Si, 1.02; Ni, 10.00; Mn, 1.07; C, 0.040; N, 0.151

Charpy–Impact Energy (J): 181.6

This example is similar to the previous example and only the saturation fracture

properties can be estimated because the time of service is not known. The estimation scheme

for this example is shown in Fig. A–3. The saturation fracture toughness represents the

minimum toughness that can be achieved by this specific cast SS after long–term aging.

Step 1. The calculated ferrite content (Eqs. 3.2.1–3.2.3) is

δc (%)= 8.8.

Step 2. The material parameter Φ for static–cast CF–8M steel with >10% Ni (Eq. 3.2.11) is

25.80. Saturation RT Charpy–impact energy in terms of Φ (Eq. 3.2.10) is 40.4 J/cm2, and in

terms of composition (Eq. 3.2.12), it is 40.3 J/cm2. The lower of the two values is used for

fracture toughness estimations; thus

CVsat (J/cm2) = 40.3.

Step 3. The saturation fracture toughness J–R curve for static–cast CF–8M steel, at a service

temperature of 291°C (Eqs. 3.2.25 and 3.2.26), can be approximated as

Jd (kJ/m2) = 259.5[∆a(mm)]0.33. (A–3.1)

This corresponds to a Jd value of 441 kJ/m2 (2520 in.·lb/in.2) at 5–mm crack extension.

Step 4. The initial RT Charpy–impact energy is 181.6 J. The normalized value is obtained by

dividing this value by 0.8 cm2, the cross–sectional area of the Charpy–impact specimen.

CVint (J/cm2) = 227.0.

Step 5. The initial J–R curve for the unaged material is the higher of either Eq. 3.2.28 or the

J–R curve determined from Eqs. 3.2.25 and 3.2.26 using CVint instead of CVsat. The initial J–R

curve, at 291°C is essentially the same as that at 290°C

Jd (kJ/m2) = 527.0[∆a(mm)]0.37. (A–3.2)

This corresponds to a Jd value of 956 kJ/m2 (5459 in.·lb/in.2) at 5–mm crack extension.

Thermal aging decreases the fracture toughness of this steel from 956 to 441 kJ/m2 in the

fully aged condition.
A–5

Estimate for known composition unknown service history

Sat. J–R

> Initial J–R

Unaged

No

No Saturation

J–R Curve &

Impact Energy

Input Initial Yes

Impact Energy

Initial

Impact

Energy

Section B: 

SATURATION

Yes

CMTR

ESTIMATION SCHEME

Input 

Composition &

Casting Process

Estimate Saturation

Impact Energy & 

J–R Curve

Estimate Initial

J–R Curve

Service

Time &

Temp.

Figure A–3. Estimation scheme for Example 3

Example 4. A static–cast CF–8M elbow, 0.66 m nominal size, in service at 325°C (617°F) for

9 efpys. The following material information is also known.

Chem. Comp. (wt.%): Cr, 20.21; Mo, 2.09; Si, 1.03; Ni, 10.24; Mn, 0.77; C, 0.037; N, 0.044

Charpy–Impact Energy (J): 209.6

RT 325°C

0.2% Yield Stress (MPa): 272 167

Ultimate Stress (MPa): 526 387

Service time, as well as saturation fracture properties, can be estimated in this case, because both chemical composition and service conditions for the material are known. The

change in tensile properties can also be estimated, because the initial tensile strengths at RT

and at 325°C are also available. The estimation scheme for this example is shown in Fig. A–4.

The saturation fracture toughness represents the minimum toughness that can be achieved by
A–6

this cast SS after long–term aging and the service time fracture toughness is the value after

9 efpys of service at 325°C.

Step 1. The calculated ferrite content (Eqs. 3.2.1–3.2.3) is

δc (%)= 16.1.

Step 2. The material parameter Φ for static–cast CF–8M steel with >10% Ni (Eq. 3.2.11) is

25.56. Saturation RT Charpy–impact energy in terms of Φ (Eq. 3.2.10) is 41.1 J/cm2, and in

terms of composition (Eq. 3.2.12), it is 62.9 J/cm2. The lower of the two values is used for

fracture toughness estimations; thus

CVsat (J/cm2) = 41.1.

Step 3. The saturation fracture toughness J–R curve for this static–cast CF–8M steel, at RT

(Eqs. 3.2.17 and 3.2.19), is given by

Jd (kJ/m2) = 193[∆a(mm)]0.36 (A–4.1)

and at 290°C (Eqs. 3.2.24 and 3.2.26), by

Jd (kJ/m2) = 225[∆a(mm)]0.33. (A–4.2)

The saturation–fracture–toughness J–R curve at a service temperature of 325°C linearly extrapolated from Eqs. A–4.1 and A–4.2 is

Jd (kJ/m2) = 229[∆a(mm)]0.33. (A–4.3)

This corresponds to a Jd value of 389 kJ/m2 (2224 in.·lb/in.2) at 5–mm crack extension.

Step 4. The kinetics for thermal embrittlement of this steel, i.e., the activation energy Q, and

aging parameter P corresponding to 9 efpys (78840 h) of service at 325°C (617°F) are estimated

from the chemical composition and service conditions.

θ = 2.9 for service temperatures of 280–330°C.

Q (kJ/mole) = 117.6 from Eq. 3.3.5 ( with I1 = 1 and I2 = 0).

P = 3.752 from Eq. 3.3.2.

Step 5. The initial RT Charpy–impact energy is 209.6 J. The normalized value is obtained by

dividing by the cross–sectional area of the Charpy–impact specimen, i.e., 0.8 cm2.

CVint (J/cm2) = 262.0.

Step 6. The initial J–R curve for the unaged material is the higher of either Eq. 3.2.27 or the

J–R curve determined from Eqs. 3.2.17, 3.2.19, 3.2.24 and 3.2.26 using CVint instead of CVsat.

In this example, the initial J–R curve at RT is obtained from Eqs. 3.2.17 and 3.2.19 as

Jd (kJ/m2) = 667[∆a(mm)]0.42; (A–4.4)

and at 290°C is obtained from Eqs. 3.2.24 and 3.2.26 as

Jd (kJ/m2) = 481[∆a(mm)]0.38. (A–4.5)
A–7

Yes Input Service

Temp. & Time

Section C: 

SERVICE TIME

Service Time

J–R Curve &

Impact Energy

Estimate

 ServiceTime

Impact Energy &

J–R Curve

Estd. J–R

> Initial J–R

Unaged

Yes

CMTR

ESTIMATION SCHEME

Input 

Composition &

Casting Process

Estimate Saturation

Impact Energy & 

J–R Curve

Service

Time &

Temp.

Estimate Initial

J–R Curve

Initial

Impact

Energy

Yes Input Initial

Impact Energy

No

Initial

Flow

Stress

Yes

Estimate Service

 Time Flow Stress,

Yield Stress, & 

Ramberg/Osgood

Parameters

Figure A–4. Estimation scheme for Example 4

The initial–fracture–toughness J–R curve at a service temperature of 325°C linearly extrapolated from Eqs. A–4.4 and A–4.5 is

Jd (kJ/m2) = 456[∆a(mm)]0.37. (A–4.6)
A–8

This corresponds to a Jd value of 827 kJ/m2 (4723 in.·lb/in.2) at 5–mm crack extension.

Step 7. The constants in Eq. 3.3.1 are obtained from CVsat and CVint.

α = 0.698 from Eq. 3.3.3.

β = 0.402 from Eq. 3.3.4.

Step 8. Information from Steps 2 and 7 are used to determine the Charpy–impact energy CV

after 9 efpys at 325°C.

Cv (J/cm2) = 47.7 from Eq. 3.3.1.

Step 9. The service–time–fracture–toughness J–R curve for static–cast CF–8M steel, at RT, is

given (from Eqs. 3.3.10 and 3.3.12) by

Jd (kJ/m2) = 213[∆a(mm)]0.36 (A–4.7)

and at 290°C (from Eqs. 3.3.17 and 3.3.19), by

Jd (kJ/m2) = 239[∆a(mm)]0.33. (A–4.8)

The service–time–fracture–toughness J–R curve at a service temperature of 325°C linearly extrapolated from Eqs. A–4.7 and A–4.8 is

Jd (kJ/m2) = 242[∆a(mm)]0.33. (A–4.9)

This corresponds to a Jd value of 412 kJ/m2 (2350 in.·lb/in.2) at 5–mm crack extension.

Thermal aging decreases the fracture toughness of this steel from 827 to 412 kJ/m2 after

9 efpy of service at 325°C; the saturation fracture toughness in the fully aged condition

corresponds to a Jd value of 389 kJ/m2.

Step 10. Initial yield stress and flow stress (mean of yield and ultimate stress) are 272 and

399 MPa, respectively, at RT and 167 and 277 MPa, respectively, at 325°C. The change in

tensile strength after 9 efpy at 325°C is estimated from the initial tensile strength and the

kinetics of embrittlement, i.e., a P value of 3.714. Tensile strength of the steel is assumed to

be essentially the same at temperatures of 280–325°C.

σfaged (MPa) = 455 at RT from Eq. 3.4.3.

σfaged (MPa) = 335 at 325°C from Eq. 3.4.6.

σyaged (MPa) = 286 at RT from Eq. 3.4.9.

σyaged (MPa) = 186 at 325°C from Eq. 3.4.12.

Step 11. The engineering stress vs. strain behavior of the steel, up to 5% strain, can be

obtained from Eqs. 3.4.16 and 3.4.19. At RT it is expressed as

 

200,000ε – σ

455 = 17.96 σ455









5.6

(A–4.10)

and at 290°C as
A–9

 

180,000ε – σ

335 = 40.71 σ335









6.6

. (A–4.11)

In this example, the value of fracture toughness JIC and tearing modulus of the aged material

can also be determined from the estimated J–R curve and tensile flow stress. Furthermore,

this information and the estimated Ramberg/Osgood equation may be used for elastic–plastic

fracture mechanics analysis of nuclear power plant piping.

Example 5. A centrifugally cast CF–8M pipe in service at 291°C (555°F) for 17 efpys. The

following material information is also known.

Chem. Comp. (wt.%): Cr, 19.64; Mo, 2.05; Si, 1.02; Ni, 10.00; Mn, 1.07; C, 0.040; N, 0.151

RT

0.2% Yield Stress (MPa): 242

Ultimate Stress (MPa): 495

Similar to Example 4, both service time and saturation fracture properties can be estimated in this case. The change in RT tensile strength can also be estimated from the available

information. The initial RT Charpy–impact energy is assumed to be 160 J, because the value is

not known. The estimation scheme for this example is shown in Fig. A–5. The saturation

fracture toughness represents the minimum toughness that can be achieved by this cast

stainless steel after long–term aging and the service time fracture toughness is the value after

17 efpys of service at 291°C.

Step 1. The calculated ferrite content (Eqs. 3.2.1–3.2.3) is

δc (%)= 5.9.

Step 2. The material parameter Φ for static–cast CF–8M steel with >10% nickel (Eq. 3.2.11) is

17.37. The saturation RT Charpy–impact energy in terms of the material parameter

(Eq. 3.2.10) is 93.0 J/cm2; in terms of chemical composition (Eq. 3.2.12), it is 66.4 J/cm2.

The lower of the two values is used for fracture toughness estimations; thus

CVsat (J/cm2) = 66.4.

Step 3. The saturation fracture toughness J–R curve for this centrifugally cast CF–8M steel,

at 291°C (Eqs. 3.2.25 and 3.2.26), is given by

Jd (kJ/m2) = 318[∆a(mm)]0.34. (A–5.1)

This corresponds to a Jd value of 550 kJ/m2 (3139 in.·lb/in.2) at 5–mm crack extension.

Step 4. The kinetics for thermal embrittlement of this steel, i.e., the activation energy Q, and

aging parameter P corresponding to 17 efpys (148,920 h) of service at 291°C are estimated

from chemical composition and service conditions.

θ = 2.9 for service temperatures of 280–330°C.

Q (kJ/mole) = 139.7 from Eq. 3.3.5 ( with I1 = 1 and I2 = 0).

P = 3.077 from Eq. 3.3.2.
A–10

Estimate for known composition and service history

Minimum J–R

Curve Unaged

Estd. J–R

> Min. J–R

Unaged

No

Initial Impact

Energy 160 J 

No

Estimate

 ServiceTime

Impact Energy &

J–R Curve

Yes

Yes

CMTR

ESTIMATION SCHEME

Input 

Composition &

Casting Process

Estimate Saturation

Impact Energy & 

J–R Curve

Service

Time &

Temp.

Input Service

Temp. & Time

Section C: 

SERVICE TIME

Service Time

J–R Curve &

Impact Energy

Initial

Impact

Energy

Initial

Flow

Stress

Yes

Estimate Service

 Time Flow Stress,

Yield Stress, & 

Ramberg/Osgood

Parameters

Figure A–5. Estimation scheme for Example 5

Step 5. The initial RT Charpy–impact energy is 160.0 J. The normalized value is obtained by

dividing by the cross–sectional area of the Charpy–impact specimen, i.e., 0.8 cm2.

CVint (J/cm2) = 200.0.
A–11

Step 6. In this example, fracture toughness of the unaged material cannot be estimated from

the initial RT Charpy–impact energy, because it is not known. The J–R curve for the unaged

material given by Eq. 3.2.28 is

Jd (kJ/m2) = 650[∆a(mm)]0.43. (A–5.2)

Step 7. The constants in Eq. 3.3.1 are obtained from CVsat and CVint.

α = 0.864 from Eq. 3.3.3.

β = 0.239 from Eq. 3.3.4.

Step 8. Information from Steps 2 and 7 are used to determine the Charpy–impact energy CV

after 17 efpy at 291°C.

Cv (J/cm2) = 103.1 from Eq. 3.3.1.

Step 9. The service–time–fracture–toughness J–R curve for static–cast CF–8M steel, at 291°C

(Eqs. 3.3.18 and 3.3.19), is given by

Jd (kJ/m2) = 381[∆a(mm)]0.35. (A–5.3)

This corresponds to a Jd value of 669 kJ/m2 (3821 in.·lb/in.2) at 5–mm crack extension.

The estimated fracture toughness of this steel after 17 efpy of service at 291°C is 669 kJ/m2;

the saturation fracture toughness in the fully aged condition corresponds to a Jd value of

550 kJ/m2.

Step 10. Only RT tensile properties can be estimated in this example. Initial yield stress and

flow stress (mean of yield and ultimate stress) are 242 and 369 MPa, respectively, at RT. The

change in tensile strength after 17 efpys at 291°C is estimated from the initial tensile strength

and the kinetics of embrittlement, i.e., a P value of 3.077.

σfaged (MPa) = 398 at RT from Eq. 3.4.3.

σyaged (MPa) = 242 at RT from Eq. 3.4.9.

Step 11. The engineering stress vs. strain behavior of the steel, up to 5% strain, can be

obtained from Eq. 3.4.16. At RT it is expressed as

 

200,000ε – σ

398 = 22.08 σ

398









5.6

. (A–5.4)

In this example, the value of fracture toughness JIC and tearing modulus of the aged material

can also be determined from the estimated J–R curve and tensile flow stress. Furthermore,

this information and the estimated Ramberg/Osgood equation may be used for elastic–plastic

fracture mechanics analysis of nuclear power plant piping.
A–12