11^^^ United States Department of Commerce

I Ai1^31 National Institute of Standards and Technology

NISTIR 3964

CONTINUOUS-COOLING

TRANSFORMATION CHARACTERISTICS

AND HIGH-TEMPERATURE FLOW

BEHAVIOR OF A MICROALLOYED

SAE1141 STEEL

Yi-Wen Cheng

Avinoam Tomer

NISTIR 3964

CONTINUOUS-COOLING

TRANSFORMATION CHARACTERISTICS

AND HIGH-TEMPERATURE FLOW

BEHAVIOR OF A MICROALLOYED

SAE 1141 STEEL

Yi-Wen Cheng

Avinoam Tomer*

Materials Reliability Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

February 1991

*Guest researcher from Nuclear Research Center - Negev, Beer Sheva, Israel

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

CONTENTS

Page

ABSTRACT vii

I. INTRODUCTION 1

II. EXPERIMENTAL PROCEDURES 2

III. EXPERIMENTAL RESULTS AND DISCUSSION 3

Continuous-Cooling Transformation (CCT) Characteristics 3

Metallography 5

Hardness Measurements 8

True Stress-vs.-True Strain Curves 8

Using Recrystallization to Control Austenite Grain Size 10

IV. SUMMARIES AND CONCLUDING REMARKS 11

V. ACKNOWLEDGMENTS 13

VI. REFERENCES 14

111
LIST OF TABLES

Page

1. Chemical composition of the microalloyed SAE 1141 steel (mass percent). 2

2. Experimental conditions 3

3. Results of microhardness measurements 8

4. Values used to calculate Q and C in equation (4) 9

5. Computed austenite grain sizes after different deformation schedules. ... 11

IV
LIST OF FIGURES

Page

1. The CCT diagram of the microalloyed SAE 1141 steel after reheated

to 1218 °C. F: ferrite; P: pearlite; B: bainite; M: martensite 16

2. The CCT diagram of the microalloyed SAE 1141 steel after reheated

to 1218 °C and compressed 50% at 900 °C. F: ferrite; P: pearlite;

B: bainite; M: martensite 17

3. The CCT diagram of the microalloyed SAE 1141 steel after reheated

to 1218 °C and compressed 50% at 1000 °C. F: ferrite; P: pearlite;

B: bainite; M: martensite 18

4. The CCT diagram of the microalloyed SAE 1141 steel after reheated

to 1218 °C and compressed 50% at 1100 °C. F: ferrite; P: pearlite;

B: bainite; M: martensite 19

5. Strain distribution along AA' of a deformed specimen with a 50%

height reduction. The original height of the specimen was 18 mm.

There is small or no strains near the top (A) and bottom (A')

surfaces. The strains are higher at locations close to the

middle (O). The strain is small near the point D. The strain

is calculated using the grain height measurements explained in

figure 6 20

6. The microstructural technique used to estimate the strain distribution

after a cylindrical specimen was compressed. The technique is

applicable only if the deformation is carried out at temperatures

where no grain recrystallization takes place 20

7. Picture of a production yoke 21

8. Microstructure of a yoke (MA SAE 1141 steel) produced with a

production schedule 21

9. Microstructure of a laboratory specimen (dO condition) cooled

at a comparable rate of a production yoke. The ASTM austenite

grain size number is 3.5 22

V
10. Microstructure of a laboratory specimen (dO condition) with a

cooling time of 45.9 s to cool from 800 to 500 °C 22

1 1 . Microstructure of a laboratory specimen (dO condition) with a

cooling time of 24.8 s to cool from 800 to 500 °C 23

12. Microstructure of a laboratory specimen (dO condition) with a

cooling time of 15.9 s to cool from 800 to 500 °C 23

13. Microstructure of a laboratory specimen (dO condition) with a

cooling time of 12.3 s to cool from 800 to 500 °C 24

14. Microstructure of a laboratory specimen (d2 condition) with a

cooling time of 170.8 s to cool from 800 to 500 °C. The ASTM

austenite grain size number is 6 25

15. Microstructure of a laboratory specimen (d3 condition) with a

cooling time of 129.2 s to cool from 800 to 500 °C. The ASTM

austenite grain size number is 5.5 25

16. Microstructure of a laboratory specimen after compressed 50%

at 900 °C. The cooling time from 800 to 500 °C is 14.6 s. The banded structure is a result of transformation from flattened

nonrecrystallized austenite grains 26

17. Microstructure of a laboratory specimen after compressed 50%

at 900 °C. The cooling time from 800 to 500 °C is 189 s. The prior austenite grain boundaries were not well outlined

because ferrite nucleated not only along the prior austenite

grain boundaries but also within austenite grains 26

18. The true stress-true strain curve of the microalloyed SAE 1141

steel tested at 900 °C with a strain rate of 10 s‘^ 27

19. The true stress-true strain curve of the microalloyed SAE 1141

steel tested at 1000 °C with a strain rate of 10 s'^ 28

20. The true stress-true strain curve of the microalloyed SAE 1141

steel tested at 1100 °C with a strain rate of 10 s'^ 29

21. Log (stress)-vs.-log (strain) curves for tests at different

temperatures. . 30

VI
CONTINUOUS-COOLING TRANSFORMATION CHARACTERISTICS

AND HIGH-TEMPERATURE FLOW BEHAVIOR OF

A MICROALLOYED SAE 1141 STEEL

Yi-Wen Cheng

Avinoam Tomer*

Materials Reliability Division

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

This report presents the results of a thermomechanical processing (TMP) study on

a microalloyed SAE 1141 forging steel. The primary objective of the study is to investigate

the effects of deformation temperature on the phase-transformation kinetics and to

determine the high-temperature flow characteristics of the steel. One-hit compression tests

at a constant true strain rate of 10 s’^ were performed with a TMP simulator. Tests were

performed at 900, 1000, and 1100 °C.

The results show that flow stress increased with decreasing temperature. In the strain

range 0.35 to 0.6, the effect of temperature on the flow stress can be described by the

equation, o (MPa) = 2.93 exp[4944/T (K)]. Continuous-cooling transformation (CCT)

diagrams determined following deformation at 1000 and 1100 °C were similar. However,

deformation at 900 °C shifted the ferrite-plus-pearlite nose to a shorter time and produced

a much finer ferrite-plus-pearlite microstructure. This is because the steel does not

recrystallize at 900 ®C after deformation imposed in this study. The usefulness of the CCT

diagram and the relationship between deformation and austenite recrystallization are

discussed.

Key words: continuous-cooling transformation; flow stress; forging steels; microalloyed

steels; recrystallization; thermomechanical processing; true stress-true strain curves.

* Guest researcher, on leave from Nuclear Research Center-Negev, Beer Sheva, Israel.

I. INTRODUCTION

Microalloyed (MA) medium-carbon forging steels have been introduced to the

automobile industry as economical substitutions for some quenched-and-tempered (Q-T)

grades. Cost reduction is the driving force for developing MA steels, which are to be used

in the as-cooled condition. Cost reduction is realized through the elimination of heat

treatment, straightening, stress relieving, and improved machinability. MA steels can achieve

tensile strengths comparable to those of Q-T steels, but with inferior impact properties.

Research to improve the impact properties of the directly cooled MA steels is increasing.

Several approaches to raising the toughness while lowering the ductile-to-brittle transition

temperature have been cited in the literature [1-3]. These include: (1) lowering the carbon

content from 0.5% to 0.35 or 0.25%; (2) lowering the reheating temperature and the finishforging temperature to control the austenite (y) grain size; (3) adding Ti (to produce TiN

particles) to control y grain size; (4) modifying the steel chemistry, such as increasing Mn

or Si content; (5) controlling MnS inclusions to increase intragranular ferrite nucleation; and

(6) producing low-carbon bainitic steels.

Lowering the deformation (finish-forging) temperature will change the y grain

morphology, which will, in turn, influence the austenite-to-ferrite transformation kinetics and

the final microstructure. Lowering the deformation temperature will also increase the flow

resistance of the steel and reduce the forgeability. The goal of the present study is to

investigate the effects of deformation temperature on the transformation kinetics of a MA

SAE 1141 steel in terms of continuous-cooling transformation (CCT) diagrams, and to

determine the high-temperature flow characteristics of the steel. Attempts were also made

to generalize the effects of deformation temperature on flow stress, and the recrystallization

of y after deformation.

1
II. EXPERIMENTAL PROCEDURES

The material used in this study is a MA SAE 1141 steel, which was continuously cast

and rolled into 25.4 mm round bars. The steel was received in the as-rolled condition. The

chemical composition of the steel in mass percent is given in table 1. Cylindrical specimens,

9 mm in diameter by 18 mm in height, were taken from the bars. Specimens were reheated

to 1218 °C with an induction heater at a rate of 1 °C s'^ and soaked for 5 min. One series

of specimens was cooled directly from 1218 °C with forced helium gas to ambient

temperature at different rates to establish a continuous-cooling transformation (CCT)

diagram.

To investigate the effects of deformation on the CCT characteristics and to determine

the flow behavior at different temperatures, three series of specimens were treated with the

above mentioned reheating and soaking conditions and then cooled at a rate of 1 s’’ to

three different deformation temperatures: 900, 1000, and 1100 °C (with one series of

specimens at each temperature). The specimens were compressed 50% in height at the

deformation temperatures with a constant true strain rate of 10 s\ Following the

compression, the specimens were cooled immediately to ambient temperature at different

rates to establish CCT diagrams. The experiments were performed with a hot-deformation

apparatus, described in a previous report [4]. The detailed procedures to determine the

austenite-to- ferrite (or other transformation products) transformation temperatures and the

true stress-true strain curves were previously described [5].

Table 1. Chemical composition of microalloyed SAE 1141 steel (mass percent).

c Mn P S Si Cu Ni Cr Mo Nb Sn

0.42 1.49 0.015 0.099 0.26 0.16 0.06 0.06 0.009 0.039 0.006

2
Following the thermomechanical (TMP) treatments, all specimens were examined

with a light microscope to study the microstructures. Macro- and microhardness testers were

also used to characterize the specimens.

III. EXPERIMENTAL RESULTS AND DISCUSSION

Continuous-Cooling Transformation (CCT) Characteristics

A CCT diagram of a steel reveals, in a time-temperature space, the phase

transformations that take place during continuous cooling at different cooling rates. The

diagram can be used to estimate the hardenability of a steel and to predict the roomtemperature microstructure and its related properties, such as tensile strength, hardness,

machinability, and fatigue strength. However, the CCT diagram of a steel is not unique.

Its relative position and shape in the time-temperature space are influenced by the y

condition prior to transformation, such as grain size, the state of alloying elements (dissolved

in Y or precipitated as compounds), and the state of y (fully recrystallized or deformed).

Figures 1 through 4 present the CCT diagrams of the MA SAE 1141 steel with the

experimental conditions described in table 2. The AC3 temperature of 789 ®C shown on the

figures was calculated using equation (

1 ),

AC3 CQ = 910 - 203C - 15.2Ni + 44.7Si + 104V -f 31.5Mo + 13. IW. (1)

Table 2. Experimental conditions.

Condition Deformation Temperature, °C

dO No Deformation

dl 900

d2 1000

d3 1100

3
Also shown on the CCT diagrams are the progression of transformation from y to the

product phases and the Rockwell C (RJ hardness numbers of specimens cooled with

different rates. The percentages shown on the figures indicate the volume fraction of y that

has transformed to the product phases. For instance, 10% means that 10% of y has

transformed to the product phases such as ferrite plus pearlite (F + P), and the remainder,

90%, is still y. Based on the information on progression of transformation, we can estimate

the volume fraction of each individual microconstituent at room temperature. Thus, roomtemperature mechanical properties can be estimated.

With reheating and no deformation (condition dO), the tip of (F + P) nose is at 20

s. With deformation, the (F + P) nose shifts to a shorter time. This is attributed to smaller

effective y grains prior to transformation. The small effective y grains can be either

physically small grains after recrystallization or heavily deformed grains without

recrystallization. In the present study, conditions d2 and d3 are physically small and

condition dl is heavily deformed. As shown in figure 2, the heavily deformed grains are very

effective in enhancing the nucleation of ferrite. This results in not only shifting the (F + P)

nose to a much shorter time (8 s in condition dl), but producing a finer ferrite-plus-pearlite

structure (figure 17). The heavily deformed y grains provide ferrite-nucleation sites not only

along the grain boundaries but also at areas of high stored energy such as substructures and

dislocation bands.

In addition to enhancing the (F + P) formation, deformation at 1000 and 1100 °C

(conditions d2 and d3) seems to promote the formation of upper bainite at slow cooling

rates (figures 3 and 4). The reason for this effect is not clear at the present. The formation

of upper bainite will result in lower toughness and higher strength [7,8].

When interpreting the phase-transformation temperatures shown by the CCT

diagrams, the effects of nonuniform cooling within the specimen have to be considered. The

specimens used for the present experiments were cylinders 9 mm in diameter by 18 mm in

height. Nonuniform temperature distribution within a specimen is expected during cooling.

4
especially in cases of rapid cooling with forced gas. The exact temperature distribution

within a specimen under different cooling conditions has not been characterized and the

temperature was measured at the peripheral mid-height of the cylinder. The temperature

at the central portion inside the cylinder is expected to be higher than that measured at the

surface. This has to be taken into consideration when one interprets the phasetransformation temperatures. For a slow cooling, nevertheless, the measured ferrite-start

temperature at 630 °C of this study is in good agreement with the calculated temperature

of 638 °C* [9]. In addition to temperature inhomogeneity, there are segregation and

nonuniform strain distribution during and after deformation in a specimen, as shown in

figure 5. The inhomogeneities in temperature, chemistry, and strain distribution will

influence the transformation kinetics.

The intended application of the MA 1141 steel is in the directly cooled condition.

Thus, the CCT diagrams presented in figures 1 through 4 are directly applicable to

production conditions, providing the thermal and the deformation histories are similar, that

is, similar y conditions prior to phase transformation. For example, the thermal profile of

a fan-cooled forged yoke (figure 7) shown as the dashed line in figure 1 [10] is comparable

to that shown in the far-right curve. The microstructures, as shown in figures 8 and 9, of the

yoke (finish-forging temperature was about 1218 °C) and a laboratory specimen with a

comparable cooling profile are similar. The average R<. hardness is approximately 20 for

both the yoke section and the laboratory specimen.

Metallography

Unless specified, microstructures were taken at locations away from free surfaces, so

that they represent the bulk property. In the case of laboratory specimens, the locations are

close to point O in figure 5. The specimens were etched with either 2 % nital or Vilellas'

reagent.

*The temperature was calculated using the formula: 878 - 295C - 78Mn (2)

5
Figure 8 represents the microstructure of a forged yoke produced in a production

plant: it was fan cooled after forging. The predominant microconstituent is pearlite plus

ferrite. Ferrite appears in the allotriomorphic form along the prior y grain boundaries and

in the form of Widmanstatten plates across the prior y grains. As shown in figure 9, a

similar microstructure has been reproduced in a laboratory specimen that received a

comparable thermal treatment. With increases in cooling rate, the microstructure changes,

which are shown in figures 10 through 13 and are described as follows. The rate of cooling

is reported in terms of the time, denoted as At, to cool from 800 to 500 °C.

Figure 9: At is 159.0 s and the microstructure is predominantly massive pearlite, which

appears as dark areas. The allotriomorphic a forms along the prior y grain boundaries and

the Widmanstatten a forms across the grains.

Figure 10: At is 45.9 s. The volume fraction of the allotriomorphic a diminishes and the

microstructure is predominantly fine Widmanstatten oc (needle-like white areas) plus pearlite

(dark areas).

Figure 11 : At is 24.8 s. The microstructure consists of a mixture of fine Widmanstatten a,

pearlite, bainite, and martensite. With a light microscope, it is difficult to differentiate

between Widmanstatten a plus pearlite and bainite. Bulky white areas that appear away

from prior y grain boundaries, are usually martensite.

Figure 12 : At is 15.9 s. The Widmanstatten a plus pearlite disappears, and the

microstructure consists of bainite and martensite. The bainite mainly forms along the prior

Y grain boundaries and appears in dark in these micrographs.

Figure 13 : At is 12.3 s. The predominant microconstituent is martensite with a small

amount of bainite present.

6
After deformation at 1000 (d2 condition) and 1100 °C (d3 condition), the y grains

recrystallized. No pancaked grains were observed. The effect of deformation at these two

temperatures is the decrease in recrystallized y grain size. This effect is evident by

comparing figures 9, 14, and 15. The y grain-size numbers for specimens with treatments

of dO, d2, and d3 were determined to be 3.5,6, and 5.5 using the comparison procedure [11].

The corresponding equivalent grain diameters are 121, 60, and 51 pm, respectively. The

more Widmanstatten a shown in figure 15 is attributed to a higher cooling rate in this

specimen. At is 129.2 s for this specimen, 159.0 s in specimen shown in figure 9 and 170.8

s for that shown in figure 14.

The observed grain diameters are consistent with the theoretical predictions.

Assuming only static recrystallization takes place (no dynamic recrystallization), which is

reasonable for the present testing conditions, the recrystallized grain diameter can be

calculated using equation [12],

= D d„

0.67 ^-0.67

( 3)

where d^* is the recrystallized grain diameter, d^, is the grain diameter before deformation,

G is the total true strain, and D is a constant. Deformation temperature has little or no

effect on d^x [12,13]. Grain growth after completion of static recrystallization is negligible

for Nb-treated steels with deformation temperatures below 1200 °C [12]. Using values of

do = 121 pm, G = 0.69 (50% thickness reduction), and d^x =51 and 60 pm, we obtained

values of D = 1.60 and 1.88 pm°^^. These calculated values of D are in good agreement

with those reported in the literature [12].

Deformation at 900 °C (condition dl) flattened the y grains, which are shown in

figure 16. The banded darker areas around the prior y grain boundaries are a mixture of

polygonal ferrite and bainite. The large lighter areas are martensite. No recrystallized y

grains were observed. The microstructure shown in figure 16 was obtained with a specimen

that was rapidly cooled after deformation. The cooling time from 800 to 500 °C was 14.6

7
s. With a cooling time from 800 to 500 °C equal 189 s, ferrite nucleated along prior y grain

boundaries as well as within the grains. As shown in figure 17, this results in less well

defined prior y grain boundaries than those shown in figure 9. The continuous ferrite grains

and the size of pearlite colonies in figure 17 are much smaller than those in figures 8 and

9. This type of microstructure is desirable because it improves toughness and strength

simultaneously. We conclude from the metallographic examination that the

nonrecrystallization temperature for the MA SAE 1141 steel is between 900 and 1000 °C

under the present testing condition.

Hardness Measurements

Both the Rc hardness and the Vickers microhardness were measured on selected

specimens after different TMP treatments. The results of R^-hardness measurements are

reported in figures 1 through 4 in the CCT diagrams. After a cooling equivalent to that in

still air, the specimens have R^-hardness numbers ranging from 20 to 22 for all the

specimens tested. The hardness number increases with increasing cooling rate. The

microhardness measurements were conducted with a load of 490 N. The microhardness

numbers (DPH) correspond to individual microconstituents are presented in table 3.

True Stress-vs.-True Strain Curves

The true stress-vs.-true strain curves are shown in figures 18, 19, and 20 for tests at

900, 1000, and 1100 °C, respectively. The curves show no occurrence of dynamic

recrystallization during the tests at any of the testing temperatures. Occurrences of dynamic

Table 3. Results of microhardness measurements.

Ferrite Pearlite Bainite Martensite

198 203 258-297 396-444

8
recrystallization will show a decrease of stress with increasing strain. The drop in stress close

to the end of each test, as shown in figures 18 through 20, is an inherent deficiency of the

mechanical testing machine. It should not be considered as indications of dynamic

recrystallization. As shown in the figures, the stress at a given strain decreases with

increasing temperature.

Without structural changes such as precipitation, strain aging, or recrystallization, the

temperature dependence of flow stress at constant strain and strain rate can be represented

by [14]

a = C exp[Q/(RT)], (4)

where a = flow stress, MPa,

Q = an activation energy for plastic flow, kJ/mol,

R = universal gas constant, 8.314 J/(mol K),

T = testing temperature, K, and

C = a constant.

Using the values given in table 4, we obtain Q and C equal to 41.1 kJ/mol and 2.93 MPa,

respectively. At a strain rate of 10 s'\ equation (5) is adequate to predict the flow stress in

the strain range 0.35 to 0.6,

a = 2.93 exp(4944/T). (5)

Table 4. Values used to calculate Q and C in equation (4).

T (K) a (MPa)* (1/T)xl0^ (n a

1173 196.5 0.853 5.28

1273 146.5 0.786 4.99

1373 106.0 0.728 4.66

* values obtained from figures 18 through 20

at a strain level equal to 0.4.

9
In figure 21, (log a) is plotted vs. (log e) for tests at different temperatures, e is the

true strain. Evidently, d(log a)/d(log e) is not constant. It decreases with increasing strain.

This indicates that the a-e behavior does not conform to the simple power-curve relation,

a = Ke". The slope of (log a) vs. (log e), as shown in figure 21, ranges from 0.15 to 0.25.

Using Recrystalli2^tion to Control Austenite Grain Size

Equation (3) can be used to estimate the recrystallized y grain size after deformation.

It can also be used to design a workable deformation schedule for achieving the optimal y

grain refinement. Of course this has to take into consideration factors such as machine

capacity and product shape control. Austenite grain refinement through recrystallization can

be illustrated with the following example: Calculations were made to predict the final y

grain sizes after four different deformation schedules. The calculations were performed with

the following conditions:

a. The workpiece is a flat product with an initial thickness of 2.5 cm.

b. The workpiece is compressed to a final thickness of 1 cm with four different

deformation schedules.

c. Deformations are performed so that the static recrystallization has completed

between deformations. No grain growth occurs after recrystallization. No

deformation is performed at temperatures below T„rd. The values of D and d^ in equation (3) are 1.60 and 121, respectively.

The results are presented in table 5. A close examination of table 5 gives the

following conclusions:

a. With the same initial and final dimensions, the least number of hits produces the

smallest austenite grain size (series 1).

b. With the same number of hits, the austenite grain size can be refined with an

optimum deformation schedule. For example, series 4 produced a smaller

10
Table 5. Computed austenite grain sizes after different deformation schedules.

After Hit(s) Thickness, cm ^rex5

Series 1 0.25000E+01 0.12100E+03

1 0.15000E-f01 0.62380E-H02

2 O.lOOOOE+01 0.46717E+02

Series 2 0.25000E+01 0.12100E-h03

1 0.20000E+01 0.10865E-H03

2 0.15000E+01 0.85269E+02

3 O.lOOOOE+01 0.57600E+02

Series 3 0.25000E+01 0.12100E-h03

1 0.20000E+01 0.10865E+03

2 0.15000E-h01 0.85269E-F02

3 0.12500E+01 0.98399E+02

4 O.lOOOOE+01 0.94596E-h02

Series 4 0.25000E+01 0.12100E-1-03

1 0.22000E-H01 0.15781E+03

2 0.17500E-H01 0.12764E+03

3 0.15000E+01 0.14428E+03

4 O.lOOOOE-hOl 0.81933E+02

recrystallized y grain than series 3. Note that series 4 is not an optimum schedule,

which can be obtained through careful trial-and-error simulations.

IV. SUMMARIES AND CONCLUDING REMARKS

The main objectives of the study include:

1. investigating the effects of finish-forging temperature on the continuous-cooling

transformation (CCT) characteristics and the final microstructure of the microalloyed

SAE 1141 steel, and

2. determining the flow behavior of the steel at high temperature under high strain rate.

11
True stress-vs.-true strain curves were obtained at 900, 1000, and 1100 °C with a

constant true strain rate of 10 s ^ Flow stress increases with decreasing temperature. In the

strain range 0.35 to 0.6, the effect of temperature on the flow stress can be described by the

equation,

a = 2.93 exp(4944/T) (5)

where o is flow stress in MPa and T is temperature in K.

The main factors that influence the CCT characteristics are the type and amount of

alloying elements dissolved in austenite and the austenite grain size before transformation.

For the microalloyed SAE 1141 steel, deformation (forging) temperature has only a slight

effect on the recrystallized austenite grain size after deformation if the temperature is above

the nonrecrystallization temperature, T„r, and below 1200 °C. Based on the results obtained

in the present study, T^^ of the steel is between 900 and 1000 °C. The most prevailing factor

in determining the recrystallized grain size is the strain of each deformation. The present

study shows that there is no observed difference in CCT characteristics between 50%

thickness reduction at 1000 ®C and at 1100 °C. Therefore, the microstructures at room

temperature are similar in both cases.

Deformation below T„„ such as at 900 °C, flattens the austenite grains and produces

high dislocation density and substructures inside the austenite grains. These promote ferrite

formation. Ferrite then forms at a shorter time during continuous cooling below Ar3

temperature, and ferrite grains are small. This is a desirable microstructure because it offers

an excellent combination of strength and toughness. However, flow resistance of the steel

is relatively high at temperatures below T^^. Therefore, this practice might not be practical

for most of the forging applications.

A small ferrite grain size at room temperature can also be achieved by austenite grain

refinement through recrystallization. By proper design of the deformation schedule using

12
the equation, d^x = D the recrystallized austenite grains can be minimized. In

the above equation, d^x is the recrystallized austenite grain diameter after each deformation,

do is the austenite grain diameter before each deformation, e is the true strain of each

deformation, and D is a constant. The grain refinement through recrystallization seems to

be suitable for forging applications.

CCT diagrams can be used to predict the room-temperature microstructure and its

related properties. CCT diagrams can also be used as a guide to assess the cooling-rate

sensitivity of properties. For instance, in the area close to the tip of the nose, a small

change in cooling rate will have a large influence on the final microstructure and properties.

To ensure uniform and consistent properties, we should avoid cooling of the steel at rates

that will pass through the area close to the tip of the nose.

V. ACKNOWLEDGMENTS

This study was performed in collaboration with J. H. Hoffmann of Chrysler Motors

Corporation and A. D. McCrindle of Stelco Fastener & Forging Company, who provided

materials and helpful discussions during the course of the study. The assistance of C. L.

Sargent in performing data reduction is acknowledged.

13
VI. REFERENCES

[1] M. Korchynsky and J. R. Panics, "Microalloyed Forging Steels - A State of the Art

Review," Int’l Congress and Exposition, Feb. 27-Mar. 3, 1989, Detroit, MI, Paper No.

89081.

[2] T. Ouchi, T. Takahashi, and H. Takada, "Improvement of the Toughness of Hot

Forged Products through Intragranular Ferrite Formation," Proc. of the 20th

Mechanical Working and Steel Processing Conf., Oct. 23-26, 1988, Dearborn, MI, pp.

65-72.

[3] K. Matysumoto, et. al., "Development of Low-Carbon Bainitic Bar Steel for Hot

Forged Use," ibid., pp. 73-81.

[4] Y. W. Cheng, Y. Rosenthal, and H. I. McHenry, "Development of a ComputerControlled Hot-Deformation Apparatus at NIST," NISTIR 89-3925, National Institute

of Standards and Technology, Boulder, CO (October, 1989).

[5] Y. W. Cheng and C. L. Sargent, "Data-Reduction and Analysis Procedures Used in

NIST’s Thermomechanical Processing Research," NISTIR 90-3950, National Institute

of Standards and Technology, Boulder, CO (August, 1990).

[6] K. W. Andrews, "Empirical Formulae for the Calculation of Some Transformation

Temperatures," JISI, Vol. 203 (July, 1965), pp. 721-727.

[7] K. J. Irvine, "A Comparison of the Bainite Transformation with Other Strengthening

Mechanisms in High-Strength Structural Steel," Symposium: Strengthening

Mechanisms in Steel, Climax Molybdenum Company, AMAX, Ann Arbor, MI (1969),

pp. 55-65.

[8] The Making, Shaping and Treating of Steel, 9th edition, edited by H. E. McGannon,

United States Steel, Pittsburgh, PA (1971), p. 1091.

•[9] M. Atkins, Atlas of Continuous Cooling Transformation Diagrams for Engineering

Steels. American Society for Metals, Metals Park, OH, 1980.

[10] J. H. Hoffmann, Chrysler Motors Corporation, Highland Park, MI, private

communication.

[11] ASTM designation: E 1 12-88, Standard Test Methods for Determining Average Grain

Size, in: Annual book of ASTM Standards, 1990.

14
[12] C. M. Sellars, "The Physical Metallurgy of Hot Working," in: Hot Working and

Forming Processes, edited by C. M. Sellars and G. J. Davies, The Metals Society,

London (1980), pp. 3-15.

[13] W. Roberts et al., "Prediction of Microstructure Development during Recrystallization

Hot Rolling of Ti-V Steels," in: HSLA Steels - Technology & Applications, edited by

M. Korchynsky, ASM, Metals Park, OH (1984), pp. 67-84.

[14] G. E. Dieter, Mechanical Metallurgy, second edition, McGraw-Hill Book Company,

New York, 1976.

15
1000c

1141d0

3 ‘9jn:).BJ9dm9x

16

Figure

1.

The

CCT

diagram

of

the

microalloyetj

SAE

1141

steel

after

reheated

to

1218°C.

F:

ferrite;

P:

pearlite;

B:

bainite;

M:

martensite.
lOOOir

1141dl

3 ‘0j;n:).«j9dui9x

17

Time,

s

Figure

2.

The

CCT

diagram

of

the

microalloyed

SAE

1141

steel

after

reheated

to

1218®C

and

compressed

50%

at

900*^1^.

F:

ferrite;

P:

pearlite;

B:

bainite;

M:

martensite.
lOOOtr

1141d2

3 ‘ajn^BjaduiQx

18

Time,

s

Figure

3.

The

CCT

diagram

of

the

microalloyed

SAE

1141

steel

after

reheated

to

1218°C

and

compressed

50%

at

1000°C.

F:

ferrite;

P:

pearlite;

B:

bainite;

M:

martensite.
GPTtTT

^OOOT

3 ‘ajTi:).Bjadraax

19

Figure

4.

The

CCT

diagram

of

the

microalloyed

SAE

1141

steel

after

reheated

to

1218°C

and

compressed

50%

at

llOO^C.

F:

ferrite;

P:

pearlite;

B:

bainite;

M:

martensite.
A

G

COu

cn

bD

G

a;

<uG

’Sd GW

100

90 h

80

70-

60-

50-

40-

30-

20

10

0

0

» *

1 I I I I I 1 ^ L_J L_J ^ ^ I ^ ^ ^ \ I 123456709 10

Distance from Top Surface, mm

Figure 5. Strain distribution along AA' of a deformed specimen with a 50% height

reduction. The original height of the specimen was 18 mm. There is small or

no strain near the top (A) and bottom (A') surfaces. The strains are higher

at locations close to the middle (O). The strain is small near the point D.

The strain is calculated using the grain height measurements explained in

figure 6.

Engineering Strain (%)

ao - a

X 100

a 0

Figure 6. The microstructural technique used to estimate the strain distribution after a

cylindrical specimen was compressed. The technique is applicable only if the

deformation is carried out at temperatures where no grain recrystallization

takes place.

20
Figure 7. Picture of a production yoke.

Figure 8. Microstructure of a yoke (MA SAE 1141 steel) produced

with a production schedule.

O 1 ^ 1
Figure 9. Microstructure of a laboratory specimen (dO condition)

cooled at a comparable rate of a production yoke. The

ASTM austenite grain size number is 3.5.

Figure 10. Microstructure of a laboratory specimen (dO condition)

with a cooling time of 45.9 s to cool from 800 to 500°C.

TOZ. _
Figure 11. Microstructure of a laboratory specimen (dO condition)

with a cooling time of 24.8 s to cool from 800 to 500°C.

Figure 12. Microstructure of a laboratory specimen (dO condition)

with a cooling time of 15.9 s to cool from 800 to 500°C.

23
Figure 13. Microstructure of a laboratory specimen (dO condition)

with a cooling time of 12.3 s to cool from 800 to 500°C.

24
Figure 14. Microstructure of a laboratory specimen (d2 condition)

with a cooling time of 170.8 s to cool from 800 to 500°C.

The ASTM austenite grain size number is 6.

Figure 15. Microstructure of a laboratory specimen (d3 condition)

with a cooling time of 129.2 s to cool from 800 to 500°C.

The ASTM austenite grain size number is 5.5.

25
Figure 16. Microstructure of a laboratory specimen after compressed

50% at 900°C. The cooling time from 800 to 500°C is 14.6 s. The banded structure is a result of transformation from

flattened nonrecrystallized austenite grains.

Figure 17. Microstructure of a laboratory specimen after compressed

50% at 900°C. The cooling time from 800 to 500X is 189 s. The prior austenite grain boundaries were not well outlined

because ferrite nucleated not only along the prior austenite

grain boundaries but also within austenite grains.

26
o

TfCV?Oa0CD^C\2O00CDTt^C\2

Cv2 C\2 C\2

isjp^ ‘ss9j:^S

21

True

Strain

Figure

18.

The

true

stress-true

strain

curve

of

the

microalloyed

SAE

1141

steel

tested

900°C

with

a

strain

rate

of

10

s\
Microalloyed

SAE

1141

Steel

o

CO

o

iCi

o

o

CO

o

C\2

o

oooooooooooo ^C\2000COTt^C\2000CO^C\2

CM CM Cv2

a

;h

0)

P

H

Oo

^dW ‘ss0j:^s

28

Figure

19.

The

true

stress-true

strain

curve

of

the

microalloyed

SAE

1141

steel

tested

at

1000°C

with

a

strain

rate

of

10

s

^
o:

t o

CO

CD

CO

o

C\2

o

OOOOOOOOOOOO Oo ^(MO00CDt^hC\2OC0CD^C\2

CM CM CM

i3dH ‘ss9j:^s 91^-^X

29

True

Strain

Figure

20.

The

true

stress-true

strain

curve

of

the

microalloyed

SAE

1141

steel

tested

at

llOO^C

with

a

strain

rate

of

10

s

‘.
Log

(Stress)

Log

(Stress)

o

I g

I 1 1 1 1 1 1 1 X 1 1 1 1 t 1 1 1 I I 1 1 1 1 1 1 1 1 I 1 I 1 1 I I I 1 1 1 1 1 I I 1 I I I I 1 I I I -2.5 -2.0 - 1.5 - 1.0 - 0.5 -0.0

Log (Strain)

Figure 21. Log (stress)-vs.-log (strain) curves for tests at

different temperatures.

30
114A

90)

PUBLICATION OR REPORT NUMBER

NISTIR 3964

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE

j TITLE AND SUBTITLE February 1991

°McroanoSfSaI

Characteristics and High-Temperature Flow Behavior of

i-Wen Cheng and Avinoam Tomer’

I PERFORMING ORGANIZATION OF JOINT OR OTHER THAN NIST. SEE INSTRUCTIONS)

I.S. DEPARTMENT OF COMMERCE

lATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY lOULDER, COLORADO 80303-3328

PONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

CONTRACT/GRANT NUMBER

^PE OF REPORT AND PERIOD COVERED

juPPLEMENTARY NOTES

:uest researcher from Nuclear Research Center-Negev, Beer Sheva, Israel.

llcroalloyed'^slE U ? rg ng sL" Processing (IMP) study on a

i a constant t-mo _ir -.r^ _i i a constant true strain rate nf in o-l ^ Steel. One-hit compression tests

me steel. Une-hit co

jrformed at 900, 1000, and 1100 °C. ^ ®““lator. Tests were

|ige 0.35 to^o!6!^tL''efLct°of*temperaturron‘^the*'n temperature. In the strain

Jjation, a (MPa) = 2.93 exp[4944/T (K) 1 Ormti described by the

.agrams determined following deformitio; at 100ranrilOo“c <CCT)

trormation at 900 °C shift#:>a ^ ^ 1100 C were similar. However,

loduced a much finer feJriL-plus-La'r'li;'

I: recrystallize at 900 “C afterdefomati

”^^'^°®^’^“^ture. This is because the steel does

I WWfcWIVWj

S:ta"S:tio“f t“:TMLic";i^^?:ce"s^^^^ trul':L:s:!«^Lltr^°?n^°u:ve:"^'^^

ILABIUTY

UNUMITED

FOR OFFICIAL DISTRIBOTIOH. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

S5“?nTM o"

"

documents. U.S. GOVERNMENT PRINTING OFFICE.

14. NUMBER OF PRINTED PAGES

40

15. PRICE
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