NATIONAL BUREAU OF STANDARDS REPORT

9997

MICROPLASTICITY II, MICROSTRAIN BEHAVIOR OF

NORMALIZED 4340 STEEL AND ANNEALED INVAR

To

Materials and Processing Branch

Naval Air Systems Command

Department of the Navy

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

31204 10 NBS REPORT

9997

MICROPLASTICITY II, MICROSTRAIN BEHAVIOR OF

NORMALIZED 4340 STEEL AND ANNEALED INVAR

by G. W. Geil

and

I. J. Feinberg

Engineering Metallurgy Section

Metallurgy Division

To

Materials and Processing Branch

Naval Air Systems Command

Department of the N avy

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, thi whole or in part, is not author Bureau of Standards, Washingtc

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Approved for public release by the

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Standards and Technology (NIST) on October 9, 2015

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Microplasticity II, Microstrain Behavior of

Normalized 4340 Steel and Annealed Invar

G. W. Geil and I. J. Feinberg

The testing techniques and some precautions necessary for obtaining

accurate measurement of small microstrains were discussed in a previous

report (l). The results obtained in a study of the microplastic behavior

of normalized 4340 steel and annealed Invar at 2b .20 C ± 0.01 C are

presented in this report.

The vacuum melted 4340 steel had the following chemical composition,

in percent by weight: carbon 0.40; manganese 0 . 68 ; phosphorus <0 . 01 ;

sulphur <0 . 01 ; silicon 0.33; nickel 1 . 80 ; chromium 0.79; molybdenum 0.24;

balance iron. The normalizing treatment consisted of holding the sample

at 900 C (I 65O F) for one hour followed by air cooling.

The Invar sample had the following chemical composition in percent by

weight: carbon 0.14, manganese 0 . 50 , nickel 34.31; iron 64.64, sulphur

0.004, phosphorus 0 . 002 , and a 0.4 balance of silicon, chromium, copper

and cobalt. It was given the annealing treatment, usually prescribed, of

holding at 83O C (1525 8 ) for one hour followed by quenching in water.

Tensile and microplasticity specimens were prepared from the 4340

steel and Invar samples after heat treatments. They were finished by

careful grinding to their final dimensions.

The designs of the microplasticity specimens and the capacitance gage

assembly of three gages spaced 120 ° apart around the specimen and the

testing procedures used in these microplasticity studies are described

in the previous paper (l). The temperature of the insulated thermal

chamber surrounding the specimen and capacitance gage assembly was

maintained at .'4.20 C ± 0.01 C. The microplasticity tests were conducted

using a step 1 oad i ng-un l oad i ng procedure, progressively loading to higher

stresses. Microstrains of the 2-inch gage length of the specimens were

determined to within ± 1 x 10”^ from readings of the three gages, except

readings of one gage only for the intervals of less than one minute after

unloading. The residual microstrains were determined from the initial

gage readings taken prior to stressing the specimen and readings taken

at zero stress after complete unloading of the specimen in each of the

1 oad i ng-un 1 oad i ng steps.

RESULTS AND DISCUSSION

Norma 1 i zed 4 ^40 Steel . The temperature of a specimen of normalized 4j40

steel changes during a microplasticity test; cooling of the specimen

occurs during loading within its elastic range, and heating occurs during

any plastic deformation and during unloading (l,2). An interval of 10 to

15 minutes was usually required for the specimen to regain its initially

controlled temperature (l ). Thus a waiting period of 10 or more minutes

prior to taking microstrain readings after unloading of the specimen was

required for the exclusion of any thermal strains resulting from the

temperature changes of the specimen.

Some of the data obtained in microplasticity tests on a single specimen

of normalized 4340 steel are presented in Fig. 1. The step loading -

unloading stages were conducted at a rate of approxi mate I y 50 ksi/min

( 5.8 MN/m /s). For clarity of presentation of the deformation behavior

at small microstrains, only data for deformation up to microstrains of

2

D x 10"° are presented; peak stresses (maximum stress of each loading -

unloading stage) are plotted versus the residual microstrains determined

at zero loads. The microplastic yield stress , (herei nafter) designated

as "MYS" in this report, is the stress value taken from the peak stress -

residual strain curve at a residual strain of 1 x 10”^.

The peak s t ress- res i dua I strain relationship obtained in the initial

microplasticity test of the specimen (Fig. 1, curve A) indicates a MYS

of 37-5 ksi (2p8 MN/m‘ ) . This stress is approximately one-fourth of the

151 ksi (

1 040 MN/m ) yield strength (0.2% offset) of the normalized 4340

steel specimen. No appreciable residual strains at zero stress were

observed in this test after unloading from peak stresses below jO ksi

(207 MN/rrt"). This test was discontinued after loading to a peak stress

of 50 ksi (3^5 MN/mc ) with a residual strain of 4.8 x 10~^ measured 20

minutes after complete unloading of the specimen. The portion of this

curve designated by "R" represents the measured recovery of residual

strain (contraction of the specimen) of about 0.8 x 10”^ that occurred

under zero stress during an interval of 40 hours. This recovery feature

will be discussed in some detail later.

The influence of pre-microstraining of the specimen on its subsequent

peak stress-residual strain relationships is depicted by the data presented

in curves B to G of Fig. 1. The maximum peak stress of the preceding

microplasticity test and the pre-microstrains of the specimen are reported

in the legend for the figure..- The MYS values of 5$ ksi (400 MN/rrO) and

59 ksi (407 MN/m") for the specimen after small total pre-mi cros tra i ns of

- 3

-

4 x 10"1 and .''8 x 10“° (curves B and C, respectively) are significantly

higher than the 37*5 ks i (2^8 MN/m ) obtained in the initial microplasticity test. This increase in MYS values may be attributed mainly to

some work hardening of the specimen during the previous microstraining

and perhaps to some redistribution of initial residual stresses in the

spec i men

Three pronounced instability characteristics that were introduced

into the specimen by microstraining were revealed by these microplasticity

tests. These are (l) considerable recovery of plastic microstrain at zero

stress during an interval varying from less than one hour to one or more

days after complete unloading of the specimen, (2) appreciable microstraining at very low stresses during reloading of the specimen when there

had been recovery of plastic strain after the previous test; and (3)

reversals in the peak stress-residual strain relationships.

(l) Recovery of plastic microstrain at zero stress.

Some recovery of plastic microstrain at zero stress in an

interval varying from less than one hour to several hours on days after

unloading was observed in all tests, even in the initial test which

involved a total microstrain of only 4.8 x 10”^. The total strain

recovery generally increased with increase in the microstrain during the

test and especially with increase in the total pre-microstrain of the

specimen. This latter feature is illustrated by the recovery strains,

"R", observed in the test on the specimen after a total pre-microstrain

of 1.57 x 10” 3 (Fig. 1, curve G). The intervals between the recovery

measurements are indicated. The first measurement of residual microstrain

- 4 -

(2.4 x I0“p

) was made 20 minutes after complete unloading of the specimen

from the maximum peak stress of the test, so as to eliminate any inclusion

of thermal strains; an appreciably recovery of plastic microstrain may

have occurred in this interval. An increase in the strain recovery was

observed with increase in time up to 2 days. No additional recovery was

observed with further increase in time. Practically all of the microstrain of the specimen in this microplasticity test was recovered.

(2) Microstrains at low stress levels after pre-strain and recovery.

In all of the microplasticity tests on the pre-mi crost ra i ned

specimen small microstrains were observed at peak stresses very much

lower than the MYS of the specimen, or the maximum peak stress of the

previous microplasticity test. These microstrains at low peak stresses

did not depend greatly upon the magnitude of the prestraining in the

previous tests up to total pre-microstrains of 460 x 10'^ (curves B, C,

D and E, Fig. l). However, with much greater total prestrains of the

specimen, such as I .56 x lO

”

3 or 1.57 * 10“^, the microstrains at low peak

stresses during the subsequent microplasticity test were considerably

-6

larger (curves F and G, Fig. l), reaching a value of 1 x 10 at peak

stresses slightly over 40 ksi (280 MN/m2

)

These microstrains at low peak stresses apparently are directly

related to the strain recovery after the preceding microplasticity test.

It is tentatively suggested that during the period of observed strain

recovery edge dislocations move backward, as opposed to their forward

motion under applied stress, perhaps by a double kink mechanism under

the internal back stresses in the specimen after unloading. It is assumed

that on restressing the specimen in a subsequent microplasticity test

the forward movement of the edge dislocations by a double kink mechanism

towards their positions prior to the recovery is facilitated somehow as

a result of their previous movements.

(3) Reversals in the peak stress-residual strain relationships.

Reversals in the peak stress-residual strain curves at peak

stresses ranging from 50 to 70 ks i (350 to 480 MN/m' ) are present

(Fig. 1, curves D,E,F and G) for microplasticity tests conducted after

previous tests to residual strains of 208 x 10 ^ and higher. These

reversals with accompanying small decreases in microstrain (contraction

of the specimen) occurred following peak stresses much smaller than the

highest applied peak stress of the previous microplasticity test. A

trend of a decrease in the peak stress at reversal with increase in the

pre-microstrain of the specimen is indicated in Fig. I by the approximately 10 ks i (70 MN/m' ) lower peak stresses for the reversals in curves

F and G than those in curves D and E.

Normalized 4340 Steel Prestressed to 1.5 Percent Extension . The usual

practice in many microplasticity investigations is to prestrain specimens,

extending them within a range of 0.1 to 5 percent before conducting

microplasticity tests (3). In order to study the effect of prestraining

within the above range on the behavior of the normalized 4340 steel specimen,

it was stressed to 225 ks i (1550 MN/nrf) with a resulting total plastic

extension of 1.5 percent. Microplasticity testing of the specimen was

delayed for 20 days, a period longer than that believed necessary for

completion of all recovery features. The data obtained in the mi crop I as t i c i ty

- .6 -

test conducted at a loading-unloading rate of approximately 100 ksi/min

(11.5 MN/m'Vs) are presented in Figs. 2, 3 and 4 .

The insert in Fig. 2 presents the observed peak stress-residual

strain relationship to a microstrain of 3 x

10”^; using an expanded

abscissa scale similar to the one in Fig. 1 . A very small, probably

insignificant, positive residual microstrain at zero stress was measured

after step loading to a peak stress of 6 ksi ( 4 l HN/m'' ) . However, after

further step loading-unloading stages to peak stresses of 16 ksi

(110 MN/m2

) and 28.5 ksi (196 MN/m'2), negative residual microstrains

(contractions) of 3-5 x 10“^ and 7-5 x 10"^, respectively, were observed.

Positive residual microstrains were again observed after step loading to

successively higher peak stresses. The reversal in the peak stressresidual strain curve of this test occurred at a peak stress between 6 ks i

( 4 l MN/nr) and 16 ksi (110 MN/m2

) . This peak stress value is much lower

than the peak stress values at the reversals shown in Fig. 1 , and confirms

the previously mentioned, general trend of a decrease in peak stress at

the reversal in the peak stress-residual strain relationships with

increase in the prestrain of the specimen. An adequate explanation is

not readily available as to the involved dislocation mechanisms or possible

phase and mi crostructura 1 changes in the specimen that produce the

observed reversals in the peak stress-residual strain relations and the

relatively small residual contractions of the specimen on step loadingunloading to peak stresses just above the peak stress value at the

reversal

-.7 -

The observed contraction of the specimen and reversal in the peak

stress-residual strain curve (insert of Fig. 2) for peak stresses below

hk ks i (303 MN/itt) reveal that the specimen was still dimensionally

unstable even 20 days after the prestrain treatment. Moreover, some

microstrain recovery was observed after unloading of the specimen from

peak stresses of 28.5 ks i (196 MN/nT ) and higher. This is shown in Fig. 2

by the differences between residual microstrain measurements, curve A,

taken 10 to 20 minutes after unloading (interval necessary to eliminate

any inclusions of thermal strains) and microstrain measurements, curve B,

taken approximately 1 hour after unloading. Furthermore, the two residual

strain measurements, curve C, taken 17 hours after unloading from the peak

stresses of 113 ksi (770 MN/nf) and 157 ks i (1080 MN/m?

) indicated

additional microstrain recovery in the interval between approximately 1

hour and 17 hours. Some microstrain recovery of about 7 x 10 ^ was

observed even during the interval between measurements taken 17 and 41

hours after unloading from the peak stress of 157 ksi (1080 MN/nf").

To obtain further information on recovery behavior and thermal strains,

the specimen was stressed to a peak stress of 1 64 ksi ( 1 1 30 MN/nrr). The

temperature changes of the specimen during loading, unloading, and at zero

stress after unloading were determined from mi 1 1

i -mi crovo 1 tmeter readings

of a Chrome 1 -A 1 ume 1 thermocouple t^ped tightly to the surface of the

specimen at mid-point of the gage length section. The temperature data,

(Fig. 3) reveal a relatively large decrease of over 0.6 C during loading

of the specimen and a rise to about 0.2 C above its initial temperature

after complete unloading.

- 8 -

The first microstrain reading was obtained 15 seconds after complete

unloading of the specimen. The data points in Fig. 2 for the instant of

complete unloading (zero time) were determined by extrapol at i on of an

expanded residual strain versus time curve (not shown) extending over an

interval of five minutes. The thermal strain portion of the total microstrain after unloading is based upon the temperature changes of the

specimen after unloading (Fig. 3) and a value of 11. 3 x 10”^ in./in./C

(m/m/C) for the coefficient of linear thermal expansion of 4/40 steel at

ambient temperature.

The time relationships for total contraction, plastic contraction and

thermal contraction of the 4340 specimen at zero load after step loading

to the peak stress of 164 ksi ( 1 1 30 MN/rrf ) and unloading (Fig. 2) are

presented in Fig. 4. Most of the thermal contraction occurred during

the first 10 minutes. Plastic contraction was very rapid at first and

decreased at a continuously decreasing rate over an interval of 648 hours

(27 days). There was a total recovery of plastic contraction of

20.8 X I in ./in. which is approximately two-thirds of the residual

plastic strain ? 34.5 * 10"^ (Fig. 2), generated during the step loadingunloading stage of the 1 64 ksi (1139 MVm2

) peak stress.

The microplasticity data presented in Figs. 1, 2 and 4 clearly reveal

the high degree of instability of the prestrained specimen of normalized

4340 steel and the very long intervals required for completion of the

recovery processes. A general trend is also shown of an increase in

instability and in recovery interval with increase in the prestrain of

the specimen.

- 9 -

Additional studies are planned to determine if the instability

behavior of the normalized 4340 steel is character i st i c of that in other

steels and in 4j40 steel with other microstructures resulting from

different heat treatments.

Annealed Invar . A microplasticity test specimen was prepared from the

sample of annealed Invar. It was finished by careful grinding to its

final dimensions. Peak stress-residual microstrain data obtained in

microplasticity test with loading-unloading steps conducted at a rate of

10 ksi/min (l.l MN/m2 /s) are presented in Fig. 5. The residual microstrain measurements at zero stress were made at various intervals,

ranging from 1 to 135 minutes after unloading the specimen from the

indicated peak stresses. The step loading-unloading and residual strain

measurements were conducted to a maximum peak stress of 15 ksi (103 MN/rrf")

with an accompanying residual microstrain of 15 x 10“ . No microstrain

recovery features, such as those observed with the normalized 4340 steel,

or other instability effects were observed in this test; the residual

microstrain measurements after unloading from each peak stress were

independent of time.

Due to the very low coefficient of linear thermal expansion at ambient

temperature of the specimen of annealed Invar, its temperature changes

during loading and unloading within its elastic range were very small,

less than 0.01 C (l). The specimen regained its initial temperature

within one minute after complete unloading. Thus, the observed residual

microstrain measurements made at intervals ranging from 1 to 135 minutes

after unloading did not include any appreciable thermal strains.

- 10

The data presented in Fig. 5 indicate a MYS of 6.3 ks i (43 MN/m2

)

for the annealed invar. This stress value is about 15 percent of the

determined 43.1 ks i (297 MN/m2

) yield strength (0.2$> offset) of the

annealed specimen.

Annealed Invar Prestrained to 1.94 Percent Extension . The specimen of

annealed Invar was stressed to 50*4 ks i (347 MN/m2

) , with an accompanying

extension of 1.94 percent, to evaluate the effects of prestraining on its

subsequent microplastic behavior. Microplasticity testing was delayed

several days to provide sufficient time for any recovery of strain that

may have occurred after prestraining.

The data obtained in the tests conducted 8 and 9 days later on this

specimen are presented In Fig. 6. Prestraining to 1.94 percent extension

introduced a high degree of Instability in the Invar specimen. This is

revealed in the strain recovery (contraction of the specimen) at zero

stress after each of the step loading-unloading stages up to a peak stress

of 24 ks i (lb5 MN/m2

). The residual microstrain values were independent

of the interval between unloading and strain measurements in the tests up

to 15 ks i (103 MN/m2

) peak stresses and only slightly dependent on the

interval after unloading from peak stresses between 15 ks i (103 MN/m"")

and 24 ks i (165 MN/m ) . However, after the load! ng-un loadi ng steps to

peak stresses greater than 24 ksi (165 MN/m2

), some initial extension of

the specimen followed by appreciably recovery of resrdua-1 strain with

increase in time after unloading was observed. Moreover, this recovery

generally increased with increase in the microstrain that occurred during

the step loading-unloading stages. For example, most of the residual

microstrain that occurred during the loading step to a peak stress of

39 ks i (270 MN/m ) and unloading was recovered within 20 hours after

unloading the specimen. No additional recovery of microstrain was

observed during the next 100 hours. Approximately two thirds of the

total recovery of microstrain occurred during the interval of one hour

after specimen unloading.

Any designation of a specific MYS value for this prestrained specimen

of Invar is certainly questionable as it depends greatly upon the interval

between specimen unloading and measurement of residual microstrain. For

instance, a MYS of approximately 35 ksi (240 MN/nT~) is indicated (Fig. 6)

by measurements taken 2 minutes after unloading, whereas, values of

approx i mate 1 y 37 ks i (255 MN/m''") and 39 ksi (270 MN/m''), respectively,

could be selected from the observed measurements at one hour and at 4 or

more hours after unloading. These stresses are very much higher than the

MYS of 6.3 ksi (43 MN/m2

) observed (Fig. 5) for this same specimen in its

annealed condition.

The microplastic behavior of the specimen of normalized 1+34-0 steel

prestrained to an extension of 1.5 percent and that of the specimen of

annealed invar prestrained to an extension of 1.9 percent clearly

demonstrate that prestraining introduces a high degree of instabili ty;

their microplastic behaviors were greatly different than those observed

with the specimens in their initially heat-treated conditions. Thus, for

these two materials and possibly for others, the somewhat general practice

of prestraining the specimens to an extension of 0.1 to 5 percent prior

to observing their microplastic behaviors does not provide desirable

.12

information on the mi crop 1 as t i c behavior of the materials in their

initial condi t i ons

SUMMARY

The microplastic behavior of normalized 4340 steel subjected to

tensile stresses at ambient temperature, 24.2 C (75*7 F)> was investigated.

A step loading-unloading procedure, progressively loading to higher peak

stresses was employed. Residual microstrains of the 2-inch gage length

of the specimen were determined at zero stress after each unloading stage,

following the procedures described in 1 he previous report, "Microplasticity

1 - Measurement of small microstrains at ambient temperature."

An initial microplastic yield stress ( MY > ) of 37-5 ks i at a strain of

1 x 10”° was observed for the normalized 4340 steel specimen. Microstraining or prestraining the specimen introduced instabi 1 i t / behavior

during subsequent microplasticity tests. The following instability

characteristics were observed: (l) significant recovery (contraction of

specimen) occurred within intervals up to 24 hours or longer after complete

unloading from peak stresses that produced appreciable plastic microstrains

( 2 ) microstraining occurred at very low stresses on subsequent restressing

if the specimen had undergone strain recovery following the previous

plastic microstrain (these stresses being very much lower than the microplastic yield stress at 1 x 10 -tJ strain, or the maximum stress of the

preceding test); ( 3 ) reversals were observed in the p.e.ak stress-residual

strain curves at stresses below the maximum peak stress of the preceding

test; and (4) prestraining to 1.5 percent extension, a procedure often

employed in microplasticity investigations, enhanced rather than

- 13 - reduced instability.

Microplasticity tests were conducted on annealed Invar maintained at

24 . 2 ° C to a maximum peak stress of 15 ksi with an accompanying microstrain of 15 x 10"^. No instability behavior such as that reported for

the normalized 4340 steel, was observed. Moreover, no appreciable thermal

microstrains were observed as the cooling of the specimen during loading

within its elastic range and heating during unloading were very small due

to the very low coefficient of linear thermal expansion of annealed Invar

at ambient temperatures.

Prestraining of the annealed Invar specimen to an extension of 1.94

percent introduced instability characteristics similar to those observed

with the 4340 steel.

The microplastic behavior of the specimen of normalized 4340 steel

prestrained to 1.5 percent and that of the specimen of annealed Invar

prestrained to 1.94 percent was greatly different than that observed with

specimens of these alloys in their initial heat-treated conditions. Thus,

for these two materials, and possibly for others, the often used practice

of prestraining specimens up to 5 percent extension prior to observing

their microplastic behaviors does not provide the desired information on

the microplastic characteristics of the materials in their initial

condi t i ons

ACKNOWLEDGMENTS

The authors wish to express their appreciation for the partial

financial support for this investigation by the Naval Air Systems Command

of the Navy Department. The authors with to thank W. D. Jenkins for his

assistance in the measurement of temperature changes of the specimen and

to Dr. M. R. Meyerson for advice and helpful discussions.

14 -

REFERENCES

1. G. W. Gei I and 1. J. Feinberg, Hi crop 1 ast i c i ty 1 .

Small Hicrostrains at Ambient Temperature.

2 . C. Zener, "Elasticity and Anelasticity of Metals",

of Chicago Press (I9U8).

Me

3 . Microplasticity - book edited by C. J. Mahon, Jr.,

Publishers, Division of John Wiley & Sons, (1968).

- 15 -

Measurement of

The Uni vers i ty

I nterscience

USCOMM-NBS-DC

FIG. 1 . Peak stress-residual strain relationships observed in microplasticity tests on a single specimen of normalized 4340 steel.

Maxi mum stress Strain Total

Curve Previ ous test Previous test Prior strain

ks i MN/m2

A 0 0 0 0

B 50 345 4 x 10"° 4 x I0"8

C 69 480 24 x 10'° 28 x 10"°

D 9b 650 181 x 10"6 209 x 10"°

E 1 10 760 19 x 10"§ 46

0

X 10-8

F 148 1020 730 x 10"° 1560 x 10"°

G 138 950 8 x 10"6 1568 x 10"8

FIG. 2. Peak stress-residual strain relationships observed in microplasticity test on prestrained

specimen of normalized 4-340 steel. Specimen had been prestressed to 225 ksi (1550 MN/m ) and extended 1.5 percent.

TEMPERATURE

CHANGE

FIG. 3. Effect of loading at a rate of 100 ksi (11.5 MN/m2 /s) to a stress of 1 64 ksi (1130 MN/m ) with immediate unloading, on the temperature change of the prestrained specimen of

normalized 4340 steel. Specimen had been prestressed at 225 ksi (1550 MN/m ) and

extended 1.5 percent.

FIG. 4. Strain recovery-time relationships observed after unloading the prestrained

specimen of normalized 4340 steel from the peak stress of 1 64 ksi (1130 MN/m2

) Specimen had been prestressed to 225 ksi (1550 MN/m2

) and extended 1.5 percent

FIG. 5. Peak stress-residual strain relationship observed in microplasticity test on

of annealed Invar.

— 100

- 80 —- MN/m

60

- 40

- 20

0

16

spec i men

RESIDUAL STRAIN , IQ

6

FIG. 6. Peak stress-residual strain relationships observed in microplasticity test on prestrained specimen of annealed Invar. Specimen had been prestressed to 50.4 ks i (350 MN/m2

) and extended to 1.9 percent.