ADSWERR-TR-72-6

EFFECT OF STRAIN RATEIqN THE MECHANICAL PROPERTIES OF tR.MO-V STEEL

AT ELEVATED TEMPERATURES

TECHNICAL REPORT

Dr. Kailasam R. Iyer

February 1972 V; -

NATIONAL TECHNICALINr!ORMATION SERVICE

~ '~ Va 2I

.J,,

RESEARCH DIRECTORATE H-

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

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The findings of this. report are not to be construed asan official Department of the Army position unless so designatedby other authorized documents.

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U. S. Army Weapons Command UnclassifiedResearch, Dev. and Eng. Directorate 2bb. GROUP

Rock island, Illinois 612013. -•Ei;ORT TITLE

EFFKC!' OF STRAIN RATE ON THE MECHANICAL PROPERTIES OF CR-MO-V STEELA'f ELEVATED TEMPERATURES (U)

4. OrSCMIPTIVr NOTES (I)Kpe of report and inclusive datea)

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Kailasam R. Iyer

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February 1972 22 9,. CONTRACT OR GRANT NO. 4. ORIGINATOR,., PORT NUII,,,(S)

b. PROJECT NO. SWERR-TR-72-6DA lW562604A607

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IS. TIW9TRACT

The mechanical properties of quenched and tempered Cr-Mo-V steel weredetermined at temperatures between 8000F and 13000F at strain ratesranging from 2X10-3 to 20/min~by the Research Directorate,Weapons Laboratory at Rock Island. Experimental data were analyzed asfunctions of strain rate and temperature on the basis of three semi-empirical mechanical equations of state. It was found that Cr-Mo-Vsteel is strain-rate sensitive, and that this strain-rate sensitivityincreases with temperature. Deviations in the mechanical behavior ofCr-Mo-V steel from idealized equations are explained on the basis ofrecovery and recrystallization which take place at elevated tempera-tures. Despite the deviations, the analysis is useful for the pre-diction of strength levels of the material at high temperatures andhigh strain-rates. (U) (Iyer, Kailasam R.)

DD "'- .1473 " "° ° " "U8" UnclassifiedS s ecurity •Iasificaton

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ROLE WT ROLE WT ROLE Iw

I. Cr-Mo-V Steel

2. Mechanical Properties

3. Strain Rate

4. Temperature

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Uncl as~si fiedSecurity Classirication

AD

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

TECHNICAL REPORT

SWERR-TR-72-6

EFFECT OF STRAIN RATEON THE MECHANICAL PROPERTIES OF CR-MO-V STEEL

AT ELEVATED TEMPERATURES

Dr. Kailasam R. Iyer

February 1972

rif *~ Of

DA IW562604A607 AMS Code 5524.11.80700.03

Approved for public release, distribution unlimited.

i

ABSTRACT

The mechanical properties of quenched and temperedCr-Mo-V steel were determined at temperatures between 800OFand 1300*F at strain rates ranging from 2XlO-13 to 20/minby the Research Directorate, Weapons Laboratory at RockIsland. Experimental data were analyzed as functions ofstrain rate and temperature on the basis of three semi-empirical mechanical equations of state. It was foundthat Cr-Mo-V steel is strain-rate sensitive, and that thisstrain-rate sensitivity increases with temperature. Devi-ations in the mechanical behavior of Cr-Mo-V steel fromidealized equations are explained on the basis of recoveryand recrystallization which take place at elevated tempera-tures. Despite the deviations, the analysis is useful forthe prediction of strength levels of the material at hightemperatures and high strain-rates.

ii

CONTENTS

Title Page

Abstract ii

Table of Contents iii

List of Illustrations

Introduction

Experimental Procedure 2

Results and Discussion 6

Conclusions 15

Future Work 15

Literature Cited 16

Distribution 17

OD Form 1473 (Document Control Data - R&D) 21

iii

ILLUSTRATIONS

Figure Page

1 Diagram of test specimen. 3

2 Photograph of the experimental setup. 4

3 Yield strength of Cr-Mo-V steel versus 7strain rate at elevated temperatures.

4 Tensile properties of Cr-Mo-V steel 8at elevated temperatures.

5 Tensile specimens of Cr-Mo-V steel 9tested at 1300 0 F.

6 Tensile strength of Cr-Mo-V steel 11versus strain rate at elevatedtemperatures.

7 Stress and equivalent strain rate for 12Cr-Mo-V steel.

8 Stress and velocity-modified tempera- 13ture for Cr-Mo-V steel.

9 Tensile load-deformation diagrams for 14Cr-Mo-V steel.

iv

INTRODUCTION

The study of materials to improve the service perfor-mance of military hardware is one of the missions of theResearch Directorate, Weapons Laboratory at Rock Island,U. S. Army Weapons Command. Since Cr-Mo-V steel is exten-sively used as a small arms gun barrel material, its me-chanical properties at elevated temperatures were studied.

The engineering properties of materials are usually de-termined at a strain rate of 5X10 3 /min. In many applica-tions, the materials are stressed at a rate considerablyhigher than that normally recommended for testing. Becausethe mechanical properties of materials are strain-rate de-pendent, especially at elevated temperatures,' the effectof stresses on materials at different rates of loading mustbe understood to properly design structural elements, toresolve failure mechanisms, and to evaluate the stabilityof structures under different loading conditions.

Small arms gun barrels are subjected to cyclic burstsof fire by which the barrel material is stressed at dif-ferent rates (as high as 103 to 105 /min) over a wide rangeof temperatures. During firing, the material is heated toa high temperature. With the firing of one shot, the bar-rel material is subjected to a stress applied at a veryhigh rate. Between two shots, the material is subjectedto stresses at a much slower rate because of thermal gra-dients. During the gradual heating and cooling for eachlong burst, stresses at still lower rates predominate. Ifthe stresses at crack tips, frictional forces and stressesdue to possible phase transformations are considered, thegun barrel material is subjected to different magnitudesof stresses at strain rates ranging over a wide spectrum.Yet, currently in the design of gun barrels, empiricalallowances are assigned to compensate for the dynamicstressing of the material.

The experimental results and the analysis of an in-vestigation to determine the mechanical properties of gunbarrel steel at elevated temperatures as a function ofstrain rate are presented in this report. An analysis isincluded to enable the calculation of strength levels athigher strain rates. The analysis does not purport toaccurately predict the mechanical properties of Cr-Mo-Vsteels under gun barrel environments since the actualstrain-rate values are orders of magnitude higher thanthose utilized for this work, and no allowances were madefor the rate of heating and duration for which the specimenis held at temperatures above 1200'F before testing. These

important criteria are lacking, and every attempt should bemade to determine the properties of Cr-Mo-V steel underconditions which closely simulate those of a gun barrelduring service. This work is the first step in that di-rection.

EXPERIMENTAL PROCEDURE

Specimen Preparation

All test specimens were machined from tý,e same heatof gun barrel blanks which were quartered in tre cross sec-tion for the fabrication of test specimens to avoid defectsalong the axis of the blank. The chemical composition ofthe test specimens is given below:

C Mn P S Si Cr V Mo Cu AI Fe

0.41 0.88 0.013 0.014 0.27 1.19 0.22 0.64 0.06 0.03 Rem.

The blanks were subjected to a conventional thermomechanical

treatment described below:

1. Normalized at 1700'F for 2-1/2 hours and air-cooledto room temperature,

2. Austenitized at 1575°F for 2-1/2 hours, quenchedin circulating oil,

3. Tempered at 1210'F for 2-1/2 hours and air-cooled,

4. Straightened,

5. Stress-relieved at 1100'F for 2-1/2 hours andi r-cooled.

The test specimens were degreased, cleanea in alcohol,and stored in a dessicator intil the time of testing. Thesize and shape of the speci ,en are illustrated in Figure 1.

Test ing

Tensile tests were conducted in an Instron Testing Ma-chine dt crosshead speeds ranging from 0.002 inch per minuteto 20 inches per minute. The experimentai setup is shownin Figure 2. Flow curves v.;ere recorded autographically.

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For crosshead speeds of 5 and 20 inches per minute, the out-put of the load cell was amplified and recorded as a func-tion of time in a precalibrated oscillograph. Necessarycorrections were made to correlate crosshead motion withengineering strain. The specimens were protected fromatmospheric contamination by testing them in a vacuum cap-sule. Temperatures were measured by a platinum - platinum-rhodium thermocouple resting on top of the specimen. Thetemperature of the specimen could be controlled within ± 5'Fof the test temperature. The standard procedure was firstto heat the furnace to the test temperature, introduce thevacuum capsule into the furnace and let the specimen grad-ually become heated to the test temperature. Duplicatetests were conducted at 800, 900, 1000, 1100, 1200, and1300*F. The minimum cross-sectional area in the neck regionwas measured with an optical tool analyzer after the speci-men was cooled to room temperature. Engineering stresses,strains, and elongations were measured from the autographicrecords.

The plastic behavior of materials is generally repre-sented as a relation between the true stress oa at a con-stant true strain e and strain rate or temperature. Thetrue stresses and strains can be calculated from the follow-ing relations:3

I aE : Ao (I+ e) I

in (1 + e) (2)

where

P = Applied load read off from the•./ (

e = Engineering strain ( autcgraphic records

True strain

STrue stress at strain c

A Origini.! area of cross sectionA° of the gage length of the specimen

These relations are true only when the deformation is uniformalong the gage length of the specimen, i.e., prior to necking.

5

.

RESULTS AND DISCUSSION

Engineering properties were calculated from load-ex-tension records on the basis of the original area of crosssection and gage length of the specimen. The values ofyield strength (0.2 per cent offset), elongation, and re-duction in area are presented in Figures 3 and 4. Quali-tatively, the mechanical properties of Cr-Mo-V steel aresimilarly affected by increases in the test temperatureor" decreases in the strain rate. The effect of strain rateis significant at high temperatures and low strain rates.At 13000F, elongation values in excess of 150 per centhave been observed. The tensile specimens after testingat various extension rates at a temperature of 1300*F areshown in Figure 5.

Previous attempts have L.een made by various investi-gators to express plastic deformation characteristics ofmaterials as functions of strain rate (*) and temperature(T). Thr -different methods will be discussed in thefollowing art.graphs with reference to the experimentaldata obtained from this investigation. The equations de-veloped in the past are semiempirical and have limitedapplicability. Yet, these equations are helpful in gen-eralizing the plastic behavior of Cr-Mo-V steel, and use-ful information can be obtained when the relations are useddiscriminately.

The parameter "true ultimate tensile strength" (trueUTS) is chosen for analysis. The reasons for choosing trueUTS are discussed by Hart. 4 Values of log (aUTS) plottedagainst log ý for the various temperatureq are shown inFigure 6. The relation between aUTS and c can be repre-sented by the equation

u = A~m

The exponent 'i', which is the slope of the graphs betweenlog (a UTS) and 'log F, is usually referred to as the strain

rate sensitivity. The strain rate sensitivity increases"with temperature. At high temperatures and low strain rates,the linear relationship between log (aUTS) and log ý is not

valid. This is attributed to the onset of dynamic recoveryat elevated temperatures while the specimen is being strained.Further reference will be made to this phenomenon. Obviously,in this region of temperature and strain rate, the value ofm can be determined only by instantaneous changes in strainrate during testing.

Zeners suggested that the flow stress of the materialwas related to both the strain rate and temperature through

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A graph between lný and lI/T at constant stress shouldbe a straight line with slope Q/R. The value of Q forCr-Mo-V steel was calculated with the data taken from Figure6. From a knowledge of the value of Q, P was calculatedfor each combination of strain rate and temperature. Valuesof log (GUTS) plotted against P are shown in Figure 7. Note

that the same value of Q is not applicable to regions ofhigh and low P values.

McGregor and Fisher 6 introduced the concept of velocitymodified temperature (Tm) in describing the plastic behaviorof materials. This concept is based on an absolute reactionrate theory approach to the creep properties of materials.Tm is defined as

Tm = T (l-kln6.--)S0

where the constant k has to be determined from the steadystate creep properties of the material. No published infor-mation of the creep properties of the gun steel is avail-able at the present time. However, Lubahn 7 has studied thecreep behavior of a similar Cr-Mo-V steel. Extrapolatedvalues of steady state creep rates of Cr-Mo-V steel at 800'Fand 900IF were used to obtain the value of k. This value kwas used to plot aGUTS versus Tm in Figure 8. A functionalrelationship does appear to exist between oUTs and Tm.

A close analysis of Figures 7, 8, and 9 reveals thatgeneralized empirical equations do not fully describe theplastic behavior of Cr-Mo-V steel at higher temperaturesand lower strain rates. This is to be expected because ofthe many changes which can take place in the material if itis kept under stress for extended periods of time. Suchchanges will undoubtedly affect the mechanical propertiesof the material. One example is shown in Figure 9 in whichthe load extension records of tensile tests conducted at11001F, 1200'F, and 1300OF are reproduced. They clearlydemonstrate the onset of dynamic recovery in the materialat elevated temperatures.' 9

Despite these limitations, the graphs in Figures 7,8, and 9 can be used to predict the strength of Cr-Mo-Vsteel at elevated temperatures and at high strain rates.

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CONCLUSIONS

1. The mechanical properties of quenched and temperedCr-Mo-V steel are strain rate sensitive and the strain ratesensitivity increases with temperature.

2. The concepts of "equivalent strain rate" and"velocity modified temperature" have limited applicabilityfor the description of the mechanical properties of Cr-Mo-Vsteel.

3. The deviations from semiempirical relations bywhich the mechanical behavior of Cr-Mo-V steel is describedoccur at high temperatures and low strain rates.

4. Such deviations are attributable to microstruc-tural changes and dynamic recovery which take place atelevated temperatures when sufficient time is allowed.

FUTURE WORK

1. The mechanical behavior of Cr-Mo-V steel at highstrain rates will be investigated by the Research Directorate,Weapons Laboratory at Rock Island.

2.• Cr-Mo-V steel shows increased dependence of strainrate sensitivity on strain rate at low strain rates, andenhanced elongation at temperatures above 1200,F. Theseproperty characteristics, as well as strain-induced micro-structural changes, will be further investigated.

15

LITERATURE CITED

1. Nadai, A., and M. J. Manjoine, J. Ap pl. Mech., Vol. 8,pp. A77-A91, (1941).

2. AMCP 706-252, "Gun Tubes (Gun Series)," EngineeringDesign Handbook Series, p. 462, U. S. ArmyMateriel Command, (1T964).

3. Dieter, G., Mechanical Metallurgy, pp. 255-259,

McGraw Hill Book Company, (1961).

4. Hart, G., Acta Metallurgica, Vol. 18, p. 603, (1970).

5. Zener, C., and J. H. Holloman, J. Appl. Phys., Vol.15, pp. 22-32, (1944).

6. MacGregor, C. W., and J. C. Fisher, J. App1. Mech.,Vol. 13, All-A16, (1946).

7. Lubahn, J. D., and R. P. Felgar, Plasticity and Creepof Metals, John Wiley and Sons, p. T58 and p"4-46, (1961).

8. Jonas, J. J., C. M. Sellars, and W. J. McG. Tegart,Met. Rev., Vol. 14, p. 11, (1969).

9. Nicholls. J. H.. and P. G. McCormick, Met. Trans.,Vol. 1, p. 3469, (1970).

16