MTL TR 90-19 0ADAD-A221 086

MICROVOID FORMATION DURINGSHEAR DEFORMATION OF ULTRAHIGHSTRENGTH STEELS

JOHN G. COWIE and MORRIS AZRINU.S. ARMY MATERIALS TECHNOLOGY LABORATORYMETALS RESEARCH BRANCH

GREGORY B. OLSONNORTHWESTERN UNIVERSITYEVANSTON, IL

April 1990

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LABORATORY COMMAND U.S. ARMY MATERIALS TECHNOLOGY LABORATORYmiinWS TnMt uWIMTON Watertuwn, Massachusetts 02172-0001

90 04 , 30 070'

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MICROVOID FORMATION DURING SHEAR DEFORMATION OFULTRAHIGH STRENGTH STEELS 6 PERFORMING ORG. REPORT NUMBER

7. AUTHOR(s) a. CONTRACT OR GRANT NUMBER(s)

John G. Cowie, Morris Azrin, and Gregory B. Olson"

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U.S. Army Materials Technology Laboratory,Watertown, Massachusetts 02172-0001SLCMT-EMM

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U.S. Army Laboratory Command April 19902800 Powder Mill Road 13. NUMBER OF PAGESAdelphi, Maryland 20783-1145 13

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Pubiished in Metallurgicai Transactions A, Volume 20A, January 1989,p. 143 - 153.

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High strength steels VoidsStress strain

Stress

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ABSTRACT

Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamicconditions. aimed at elucidating the fundamental mechanisms of shear localization underlyingboth adiabatic shear localization and fracture processes. Experiments were also devised to studythe effect of hydrostatic pressure and austenitizing temperature on the critical strain to localiza-tion. Experimental evidence strongly suggests that strain localization in the steels investigatedis driven by inicrovoid softening controlled by nucleation at 100 nm scale particles. This issupported by the observed pressure dependence of the instability strain, enhanced resistance toshear instability with particle dissolution, and direct observation of microvoids at these particlesin deformed material. For the steels investigated with approximately equivalent strength levels,a direct correla.ion bctwen dhC ,ra,k extension force and shear instability is demonstrated.Consequently. both fracture toughness and shear localization are dependent on the size. type,and distribution of second phase particles. f•

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Microvoid Formation during ShearDeformation of Ultrahigh Strength Steels

J.G. COWIE. M. AZRIN. and G.B. OLSON

Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamicconditions, aimed at elucidating the fundamental mechanisms of shear localization underlyingboth adiabatic shear localization and fracture processes. Experiments were also devised to studythe effect of hydrostatic pressure and austenitizing temperature on the critical strain to localiza-tion. Experimental evidence strongly suggests that strain localization in the steels investigatedis driven by microvoid softening controlled by nucleation at 100 nm scale particles. This issupported by the observed pressure dependence of the instability strain, enhanced resistance toshear instability with particle dissolution, and direct observation of microvoids at these particlesin deformed material. For the steels investigated with approximately equivalent strength levels.a direct correlation between the crack extension force and shear instability is demonstrated.Consequently. both fracture toughness and shear localization are dependent on the size, type.and distribution of second phase particles.

1. INTRODUCTION and Tuler I- describe the various modeling approaches:

THE phenomenon of plastic shear instability and flow flow softening, deformation heating. textural softening.and void nucleation softening. The review also describeslocalization is of considerable interest due to its impor- the relationship between shear instability and fracturetant role in fracture processes in many high strength steels, toughness. The reviews of Rogers' and Bedford et o/!"especially in the ultrahigh-strength (UHS) range. It is tovghnexcelle rview of o ge nel rdhenomenonalsoof pecal oncrn n Ary apliatins f tese give an excellent overview of the general phenomenonalso of special concern in Army applications of these including the microstructures resulting from localizedsteels because of its role as a principal failure mode in flow. The continuum plasticity theory of adiabatic flowballistic penetration. The latter case has been tradition- localization is treated by CliftonP/I and a general surveyally modeled as a continuum plastic instability arising of strain localization is given by Argoni t i A concisefrom the thermal softening associated with deformation treatment of the specific role of adiabatic shear in ar-under adiabatic conditions. Models based on isother- tament of thespcic role o ibtherin armally derived empirical constitutive relations have been mamens and blistics n e found inteare ouse toacoun fo srai loalzaton onitins b- Samuels and Lam ornI 1 lson et al.""'l endeavored toused to account for strain localization conditions ob- computer model the ballistic penetration of high strengthserved in high ;train rate shear tests, but recent experi- steelC steelusing experimentally derived constitutive flow re-ments have cast doubt on the validity of this approach."' lations. but met with limited success. The authors pro-Here we report further results of shear tests conducted posed that the material exhibited a pressure dependenceunder both quasistatic and dynamic conditions, aimed at that the conventional thermal softening models could notelucidating the fundamental mechanisms of shear local- describe. Experiments were designed to determine howization underlying both fracture and ballistic penetration the pressure dependence affects the deformation of UHSprocesses. steels. More recent research by Azrin et al."I showed

that the critical strain for shear localization in high strength4340 steel is nearly identical for both isothermal (quas-

11. BACKGROUND istatic) and adiabatic (dynamic) loading conditions. WhileThe phenomenon of deformation localization as it oc- thermal softening undoubtedly provides a contribution to

curs in ballistic penetration is illustrated in Figure I.' the measured stress-strain relations, this result of nearlyshowing the localized deformation mode of failure, com- identical instability strains and shear localization behav-monly referred to as adiabatic shear. The plastic flow ior in UHS steels at both high and low strain rates in-after the onset of shear instability is concentrated in thin dicated that another flow softening phenomenon wasshear bands which appear white after metallographic equally important. As was reviewed in Reference 10,etching. The through-thickness localized flow produces observations that the instability strain is strongly influ-a shear plugging- failure mode in which the material enced by the hydrostatic component of stress, togetherahead of the projectile is ejected as a solid piece ab- with metallographic evidence of microvoid nucleation,sorbing relatively little associated energy. indicate that fracture related processes can also contrib-

Much has been written over the past 40 years on the ute to the strain softening effects underlying plastic shearsubject of shear localization. In a recent review, Cowie localization. 1'1.'- 1 Such phenomena must also be taken

into account for a complete understanding of shear local-ization and shear banding.

J G COWIE. Materials Research Engineer. and M. AZRIN. Su- The present study was initiated to obtain experimentalpervisory Metallurgist. are with the U.S. Army Materials Technology evidence of microvoid nucleation preceding localizedLaboratory. Watertown. MA 02172-0001. G.B OLSON is Professorat Northwestem Universitv. Evanston. IL 60208 deformation during simple shear of UHS steels. Exper-

Manuscript submitted March 28. 1988 iments were also devised to study both the effect of

METALLURGICAL TRANSACTIONS A L S GOVERNMENT WORK VOLUME 20.A. JANUARY IK9 143NOT PROTECT, BY U S COPYRIGHT

DOUBLE LINEAR SHEAR SPECIMEN

1427 9 795. 1-.27

I ]I 5.08 10.0Fwi. I -Photomicrograph of a 4340 steel Rc 52 plate (5.6 mm thick)Ithat underwent a localized deformation mode of failure during ballistic R=5.56 Timpact. Note the white etcned shear bands benc.ith the area otf impact.

53 34hydrostatic pressure and austenitizing temperature on thecritical strain to localization. Two different simple shearspecimen geometries were compared. Because of its in-fluence on fracture related processes of potential impor-tance to shear instability, the role of melt practice was 5.08 '0.,,also examined. Stress-strain data and corresponding strainprofile~s were obtained from each test in order to quantify /1the flow behavior in addition to accurately determining -the instability strain.

0.65 0.65

Ill. MATERIALS AND METHODS ALL DIMENSIONS IN MILLIMETERS

Two specimen geometries, thin wall torsion and dou-ble linear shear configurations. were tested under bothadiabatic and isothermal conditions. Dynamic torsion testswere performed on a torsional impact apparatus incor-porating a flywheel. Torque to the specimen is appliedby a steel bar pneumatically engaged to lugs on the ro-tating tlywheel. Flywheel rotational frequency is moni-tored by a velocimeter. while applied torque is measuredby a strain gage load cell adjacent to the specimen. PERSPECTIVE DRAWING

Wooden dowels were inserted into the specimen to min-imize buckling. These dynamic tests were performed atan imposed strain rate of 10: per second to ensure es-sentially adiabatic deformation conditions in the speci-mens. Isothermal torsion tests were run on an MTSservo-hydraulic test machine at a strain rate of 10 ' persecond. Results from the thin wall torsion experiments Pwere reported previously.'''

Double linear shear specimens were also tested underboth quasistatic and dynamic conditions. These speci-mens are machined from standard sized Charpy blanks.They have two narrow gage sections which are displacedsimultaneously in simple shear (Figure 2). Dynamic testswere performed in a modified instrumented Charpy im- LOADING SCHEMATIC DIAGRAMpact machine. The Charpy specimen fixture was re-placed with one that rigidly holds the ends of the double Fig. 2-Mechanical dra ving, perspective drav.nv., and loading she-linear shear specimen. In addition, the pendulum weight nliatic diagram ot a double linear shear specimen. h ,peclmen is

w sp h rsheared " ithin the tvwo reduced sections at a preselected speed (V)was increased by 60 pct. The shear fixture may also ap- wAhile an axial load 11,,) can be superimposed on one endply up to 2250 N normal compressive load to the spec-imen's end while the specimen is being sheared. Theload-time curve generated by the strain gage nstru- Typical shear stress-shear strain curves generated frommented tup is recorded and stored in the memory ot a these tests are presented in Figures 3 and 4. These rep-Nicolet high speed digital oscilloscope. These dynamic resent a rare earth modified (REM) 4130 steel (compo-tests were performed at an imposed strain rate in excess sition presented in Table 1). austenitized at 840 _C andof 10 per second. Quasistatic double linear shear tests tempered I hour at 535 )C and 200 C to give hardnesseswere run on a Tinius Olsen hydraulic tension/compres- of Rc 33 and 48, respectively.sion test machine at a strain rate of 10 ' per second. To determine precise strain profiles, four longitudinal

144 V ItFit; 20A. JAN( ARY 199 MirALI .t'R . [iRANt C.,\TR)NS \

1100

700

1000

600~

500 Shear Stress-Strain Curver7REM 4130 SteelV c Shea rM4 StesSranCuv

Rc 33REM 4130 SteelRc 48

300 L 7001 1

J 0.25 0.50 0.75 1.00 1.25 1.50 0 0.25 0.50 0 75Shear Strain, y Shear Strain. y

Fia. 3-Plastic shear ,tress-train curve obtained from a quasistatic Fig. 4-Plastic shear stress-strain curve obtained from a quasistaticlinear shear test ot REM 4130 steel at Rc 33 (535 'C temper). Shear linear shear test of REM 4130 steel at Rc 48 (2(W C temperi Shearstrain at maximum stress is 110 pet. strain at maximum stress is 45 pet

scribe marks were drawn onto the gage section of each melt practice was also examined. Three commercial heatsspecimen before testing. After testing. shear strain as a of 4340 steel processed by air melt (AM). electroslagfunction of the position along the specimen axis was remelt (ESR). and vacuum arc remelt (VAR) practicesdetermined from the local angle of the scribe marks rel- were evaluated (compositions presented in Table 11. Theative to the specimen centerline as measured in a Leitz shear specimens were machined from heat-treated blanksuniversal measuring microscope. Dynamic tests were run austenitized at 840 C and tempered I hour at 650 'C.to failure, while quasistatic tests were generally run until 535 C. 335 'C, and 160 C to give hardness levels ofsufficient flow localization to produce a load drop of ap- Rc 30, 40, 48, and 56. respectively. Dynamic and quas-proximately 10 pct. istatic tests revealed no significant effect of melting

Typical strain profiles obtained from isothermal tests practice. with one exception. Under isothermal condi-are presented in Figures 5 and 6 which correspond to the tions. the air-melted material tempered at 535 'C (Stagestress-strain curves in Figures 3 and 4, respectively. In- Ill temper) proved consistently to have a higher insta-tense strain localization is found adjacent to the gage bility strain than its VAR and ESR counterparts. Struc-section walls. This is consistent with both a computer turally. the only observable difference was the slightlysimulation of simple shear of a rectangular body"t " and smaller prior austenite grain size for the AM material.a finite element stress analysis of the double linear shear The structure of the 4340 steel given this temperingspecimen geometry and loading by Tracey and Per- treatment contains intra-lath cementite. It is possible thatrone."l Note that the shear strain plateaus (henceforth the shear localization resistance is influenced by the lengthdefined as the macroscopic instability strain) on the strain of void-nucleating cementite particles which, in turn. scaleprofiles correspond with the shear strains at peak stress with the martensite packet and prior austenite grain sizes.on their corresponding stress-strain curves. Comparisonof the instability strain data for both torsion and linear-shear test specimen geometries (Figures 7 and 8) con- B. Effect of Strain Ratefirms that the two configurations give nearly identical Plotting all the tnstability strain data for both adiabaticresults. (Figure 7) and isothermal (Figure 8) test conditions to-

gether (Figure 9., we observe little effect of strain rateIV. RESULTS on the instability strain. particularly at the higher strength

A. Effect of Melting Practice levels. The linear shear tests spanned seven orders ofmagnitude in strain rate from isothermal to adiabatic test

Because of its influence on fracture related processes conditions. The resulting instability strains for the high-of potential importance to shear localization, the role of est strength material (Rc 56) were nearly identical

Table I. Rare Earth Modified 4130 Steel Composition (Wt Pct)

C Mn P S Si Cr Mo Ti B Ce0.30 0.62 0.004 0.(X)8 0.29 1.05 0.20 0.051 0.002

2.0.

V AA

S1.5~

-'-71.0. 4

0. 0.

05 ~~ Strain Profile02.A REM 4130 Steel .

0 0.2 0.4 0.6 0.8 1.0 Fig. 7-C oparison of instability s.trains measured InI d~nainiic for-Position, mm 'I on and linear shear tests (it AM. ESR. and VAR 4340) stcvl

Fie. 5-Strain profiles obtained roin a quasistatic linear shear test ofREM - 4130) stcel at Rc 33 i535 'C temrpet . The mecan plateau strain, Finite element .alculations indicate that the stress corn-detined as the miacroscopic instability, strain. I, I 110 pct. ponent arising from this axial load reasonabl,. approxi-

mates uniaxial stress once plastic shearing is underway.(Figures 9 and 10). The flow stress increased nearly 50 Axial compressive stresses of 1/6 and '/1 the material'spct from quasistatic to dynamic test conditions. tensile yield stress were used. The stress-strain results

The results of these tests imply that there is little effect quite clearly show pressure dependent behavior (Figuresof thermal softening (absent in the isothermal tests) on 10 and lbI. As the axial stress is increased from 0 to 1/3the instability strain at high strength levels. This implies of the yield stress. the instability strain is increased quitea dominant role of a microstructural instability in the shear substantially for all four hardness levels investigated.localization behavior under pure shear conditions. The microstructure of the highest strength (Stage I

tempered) material consists of a martensitic lath matrixC Efct of Pressure with a fine epsilon carbide dispersion. In addition, there

are submicron sized second phase particles which act asIn order to test whether UHS steels exhibit the pres- gri0 eies(iue1) hs eodpae uni-sure dependent behavior previously observed in lower solved during austenitizing) have been identified bystrength steels.''". double linear shear tests were run qua- Goe 1 tbepialyloyFeCM)caidsfthsistatically with a superimposed axial compressive load, type V117 with a mean di'ameter of' 171 nm and a vol

fraction of 0.44 pct when austenitized at 870 TC. Thecarbides, with an interparticle mean free path of 670 nim.

1.0

It.

1.00

. AMT. 7 AM

S0.75. 0 .

0.50

0.25 1 Strain ProfileREM 4130 Steel 8AR

0. 0.2 1. V

____ - Position, min I'8 .0 10 Q : W5 88 81 W8 4,' 41 .h A

Fig 6-Strain profiles obtained fromt a quasistatic linear shear iest ofREM 4130) steel at Re 49 12(M C tempert The mecan plateau strain. Fig 9-onaio .ntblt strains measured in quasisftiic tor-defined a% the macroscopic insiability strain, is 45 pet. sion and linear shear tesis tit AM4. FSR. and VA-R 4140 steel

40--- vOlUIME 20A. JANUARY 19K9M \tf-lI. R(;t( \t. VR \',\TIONS *

Table I1. 4340 Steel Compositions (Wt Pct)\1elting Practice C Mn P S Si Cr Ni Mo Cu

AM 0.41 0.75 0.005 ).01 I 0.25 0.79 1.71 0.24 (). 13ESR 0.41 0,70 0,008 0.001 0.26 0.90 1.73 0.22 (. 21VAR 0.42 (.46 0.009 001 0.28 0.89 1.74 021 0. 19

are expected to interact during plastic deformation.' to debond the second phase particles from the matrix andleading to cooperative microvoid nucleation. Interac- to form microvoids. This interpretation is supported bytions between dispersoids occur when the plastic zones the recent analysis by Hutchinson and Tvergaardl'" onof the neighboring particles touch.""' The critical stress microvoid nucleation softening as a basis for shearfor nucleation has been found to be dependent on the instability.interparticle spacing to particle radius ratio."' Large orclosely spaced dispersoids nucleate voids at relatively D. Effect of Austenitizing Temperaturesmall strains, and ultimately degrade the material's duc-tility.11 l Figure 13 presents a TEM photomicrograph of A further test of the role of second phase particles insheared material showing a pair of carbides linked by shear localization can be made by raising the austenitiz-microvoids. The photograph has been deliberately over- ing temperature to dissolve alloy carbides. Gore

'"' hasdeveloped to clearly show the voids, and a schematic demonstrated that raisin, the austenitizing temperaturerepresentation is also shown for clarity. This thin foil from 870 0C to 1200 °C in the same VAR 4340 steelspecimen was removed from the gage section of a linear examined here full\ dissolves the Cr-Mo allo\ carbide,.shear specimen which was strained to instability. The leaving primarily tine 80 nm Ti (C.N) paticles at a muchmicrograph was taken in the region which received only reduced vol fraction of 0.05 pct. Such a changeuniform deformation and had stopped straining once in- in the amount and character of undissolved second-stability occurred. It was observed that many carbides phase particles can be expected to alter significantly theexhibited this type of behavior. The directions of void critical strain for microvoid nucleation softening. Thisgrowth appear aligned in the direction of the principal should be most pronounced in Stage I tempered materialstress. Shear cracks linked pairs of carbides predomi- where the ultrafine epsilon carbides precipitated duringnantly along the direction of the imposed shear direction, tempering should not contribute to microvoid nucleation.45 deg to the principal stress direction. These experi- and the role of the undissolved particles would thus bemental observations are consistent with the finite ele- greatest. Although the size, type. and distribution ofment investigation of interacting void pairs by Tracey the fine dispersoids found in VAR 4340 and AM 4340and Perrone." 6 ' Although specimen preparation by elec- are not identical, the tendency for their dispersoidstropolishing may have enlarged the voids, no such voids to dissolve into the matrix is the same. Therefore, awere observed in identically prepared foils taken fromde undcfo-med grip cr,- 2f the pecirnens We thu,

conclude that the voids are genuinely produced by the 17(0 yplastic deformation.

Summarizing. the pressure dependent behavior of the 1600 EFFECT OF PRESSURE ANDinstability strain can be attributed to the stress required STRAIN RATE

1500 AM 4340 Steel

. ......... 1400 Rc 56

t

, :L " ' - -- 1300

- 1200 -ic

Y AM!

0" 0 1U.... 1100 -/oy 1 rX 1\\900

700

2i

" "_,. .. 0- ' 0.4 0.8 1.2 1.6 2.0 2.4Q a,',,,..Shear Strain,

V, 1 6 i 4 2 0 W S ¢ 0 6 Fig. IO-Plastic shear stress-strain curves obtained from both dynam-,c and quasistatic linear shear tests of an AM 4340 steel at Rc 50,

Fig 9-Cn arison of instabihty %trains measu ,d both d.nam, hith three different imposed axial ....,e 0. 1 . and I

and quasIstatc shear tests (iO AM. ESR. and VAR 4340 steeL the yield strength

METALLURGICAL TRANSACTIONS A 'OUE20lA. JA\NUARY I, t-14'

10. using the same four tempering conditions as in Figure S.IMaterial well tempered in Stage Ill to Rc 30 shoxs nosignificant increase in instability strain, consistent with

Q a dominant role of the 10) nm cementite particles pre-4 cipitated during tempering. For lower tempering tern-

I.o0 UO . { peratures (high hardness. , where the carbides precipitated4 on tempering are finer, there is a pronounced increase

in instability strain with II(X)M C austenitizing. sugges-tive of an important role of the undissolved alloy car-bides in shear localization.

EFFECT OF POrSSwpe

A 1/6a 8400C A~sr in the 11(X)0 C austenitized material is represented ina Oc J Figure 15 comparing the behavior without and with a

normal compressive stress of ', the yield stress. Thoughthe controlling particles may be changed by austenitizingconditions, the persistent pressure sensitivity implies that

28 10 54 56 58 40 4' 44 46 48 50 S2 54 S6 s8 microvoid softening continues to be the dominant strainHiC softening mechanism. As depicted in Figure 14. the AM

Fig I I -Comparison ot intahii strains ot .\M 4401 seloci %kith 4340 tempered in Stage I at I t,( 'C to Rc 56 hardnessthrc dilffcrcnt iiiDp.IctJ A lMA :Mnprcsisc strcsscs n. . i the show s an increase in the instability ,train troll [ict. I 12cnlc old .rrcn.th to 0). IS range t , 0.23 %%hen the autcniti/n tcmperature

is raised from 840 ,C to 1 100 C. This is not as largecomparison of the shear instability of AM 4340 to the an increase as observed for the Stage If and earl\ Stagedissolution of the fine particles in VAR follows. I! temper conditions (Re 50 and 40. respectivelyi. A

A comparison of the shear instability strains measured further investigation by Gore"'I of the effect of austen-in linear shear tests on the AM 4340 austenitized at 840 'C itizing temperature in the range of 870 to 12X) 2 'C on the(as in Figure 8) and I100 'C is presented in Figure 14 instability strain in the VAR 4340 tempered in Stage I

at 2(X) C to Rc 52 has shown a slight monotonic de-crease with increasing austenitizing temperature. The

7"AN% effect has been attributed to a contribution of crystal-"'t " plasticity-based strain softening mechanisms which may

, ' .. . be promoted by grain coarsening. As mentioned previ-ouslv in discussion of melt practice comparison in Figure

"* .. " 8. the AM 4340 shows greater grain coarseninL resis-tance than the cleaner VAR 4340 and such grain coars-ening effects may not ha\,e had as strong an influencein the 840 to 1100 'C comparison for the AM material.

b... Pressure sensitivity of the very coarse-grained materialhas not yet been evaluated to test for a change in strain

- softening mechanism.The observed increase in the instability strain k'.ith

particle dissolution in the AM 4340 lends further support, for the role of microvoid nucleation softcmng min shear

localization in conventionall, t cat.:d material. Ho\%-*,, ever. the indication that severe grain coarsening can in-

. troduce other strain softening contributions limits thepractical utility of high temperature austenitizing treat-ments for enhancement of mechanical properties.

E. Role of Tempering Stage

Al While our studies have focused primarily on the be-havior of 4340 steel, the rare-earth modified (REM) 4130steel represented in Figures 3 to 6 was included to ex-tend the hardness range that could be examined in eachstage of tempeiing. Figure 16 summarizes the combinedlinear-shear test results of the AM 4340 and REM 4130steels in a linear plot of instability strain vs Rc hardness.Grouping the data for the three tempering stages. StageI tempering appears to show a superior comnination of

Fig. 12-TEM micrograph of the microstructure of 4340 steel at shear instabilit resistance and hardness strength). ThisRe 55. which consisted of undissolved alloy carbides in a mainx ot can be attributed to the fine scale of the epsilon carbidesheavily dislocated lath marlensite. precipitated on tempering, which should not act as

14- VO.t.:Mt- 20A. JANtARY 199 10tT, i.tR( TI( \l_ YR \N \('T()Ns

Ir,, /

I I 13 11 \1 m or¢graph ,,I llll rowiHd 1LtCleation armiund a pair ,of undi~s, led alho sarhILIC, File 1llpdlI. hCT1ldflK 111u~traic, (he,Jirc.'t.i l of[ ,hearll .in,, p,i iop.l~l tr,':,

mic.ro)void nucleation sites. For Stage III temlpering. The much low'er she.ar instability resistan.ce ol the Stage,,trenoth rs Provided by, prec.ipitated ce'mentite which is 11 tempered mic.rostructure c.orrelates with a minimum in,,utficientiv coarse to participate directly in earlie:r mi- the Charpy impact energy coninonk, known as the tern-crovoid nucleation at a gviven hardness level. The general i-ered martensite embrittlerncnt trough. This implies adow'nw'ard trend ot instability qtrain with hardness for connec.tion between low Charpy energ-, and early ,sheareach class of inicrostructure is presum~ablyv a.,ssociated with loc.alization..As the instability strain is still pressurereaching! c.ritic.al interface stresses at lower strains When ,sensitive in this sta,-,e (Fig'ure I I ).the embrittlement(he matrix flow ,,tress is increased.

iO..U

II

/,/

//

I . i N Ilk.La il'Il1~r~ >i nclai~i ioud piriiunm'OI>d aI i eahd [h .Icc>'pan ir hei Il ~Ii ih

0. 1 1 1 [ 1 1 I Hy"

1'l 15 .4l~~lll I n,,.11l11l\ of[.ii, . 11i 1 0 4 41) stel . ' aul'-Fieg 14 Comparlon i hll irani, ,ht an 00 441 steel preci- pittedtd caeni ht ch i th dtmperre' Imposed ic\otr trcpro'rret th 'rmi 'mmcno.ondnuclean a a01) Ci h s l) T rd , then temrd ,rethnl

%I[-,[M..II R(,I( MI. [R \NS M ( ~)Ns \1 'o I \I \ IV\'I \k) 1,s, I', I 's

ISTABILITY STRAINS OF OUENCHED iNSTABILITY STRAIN CofIPARISON OF AF 1410 At.D

AND UEPERED STEELS JUECHED AND TW.PERED STEELS

0 i -TempI 1 0

0 5 A

2 tW - 0I13r Tepe

N AF1410Conentinaly Trate

). A AF 1410 Rare Earth Moiied.5

S.. Underaged at 420VC3 1 ~Stagel 11'1]

I0 I I 0, I I 10520.15 203032 34 3638 4042 444648050 52 54 'o Si

,,RC .R

Itl-vrtlflpIrl~on ot lflStdhilit\ .train :urT' m~~ quenched and FL 17-Crnparisori oI inqrhihit% t1rain, .11 .)ue~hed and tecmrcrc:JreCyIpecTd [eel, ha~jfl! Siajic 1. Sice 11, ,nd StMac III truciurcN feel,' and three dittcrenII\ prccescd .\F I-Ili (eel,

phenomena associated with retained austenite decom- tive insensitivity to melt practice. The 420 -C temperedposition in Stage 11 presumably cause earlier microvoid material shows a much lower instability strain consistentnucleation, with the expected role of undissolved cementite in mi-

crovoid nucleation.F Comparison with Secondary Fracture toughness of the same three materials inea-Hardenini Steel: Toug~hness Correlations sured at Carpenter Steel11tt shows a similar trend vs hard-

Secodar hadenig A141 stel ahiees igh ness. Expressed as critical crack extension force G, . theSecodar hadenig AI4I stel ahiees igh toughness is compared with available data for 4340 steel

strength trom the precipitation of tine MC carbides dur- in Figure 18. A test of the apparent G,-instability cor-ingbit eins- ate 510eve 'C pi mum5 u s tr eh/ oh es- relation is plotted in Figure 19. including data for thecombicnition ar achive inmpaeti-horM, tepr eer- VAR 4340 austenitized at 840 TC and tempered 2 hours

iced coditin fr whch ompetio ofM.C recpi- at 200 TC to a comparable hardness level of Rc 50. Thetation causes dissolution of coarser cementite particles good correlation observed is in line with the proposedwhich precipitate earlier during tempering. Optimum melt roles of second-phase particles in shear localization andpractice generally involves rare-earth treatments for get- dciefatrs 9 0

teringz of sulfur. .The shear instability resistance in the utefracsuprt. for th2 motatrl f irvistandard 5 10 'C/5 hours tempered c'ondition was inea- nuleingprte inr the dilefrtatre behair ofsured for two heats with and without the rare earth treat- AFcleatisg paroidled bn the sution ratmren behavio ofment (compositions presented in Table 111). Also ex- Schm1idan prviephiby'the suintrcaet gnssd en-

amind ws rre-erthtreted ateialtempred5 hurs hancement is obtained on raisina the solution treatmentat 420 'C to provide an *underaged- material of com- temperature from 830 'C to 885' 'C. After conventionalparable hardness but with the cementite particles present. treatment at 830 3C, the microstructure contains

The results are compared with those ot the Stage I and chromium-rich ' 1,C, carbides of 900 to 1800 nm di-Stage 11 tempered 4340 and 4130 steels in a sem-log eralnwihsler4to8nm oldnm-cplot v.s hardness in Figure 17. For the standard 510 'C/ amC abdAter 885- wiC soluer40to0n treatm en henl.Ch

5 hors empr. he FI4O shws ubsantall hiher carbides are fully dissolved, leavine only the finer Nicshear instability resistance compared to the Stage I tem--pered steels. The material without rare-earth treatment prilsis shown to contain relatively fine chromium sulfides.- G. Mirov-1 Oh-'~tinReplacement by rare-earth sulfides and oxysulfides in the - " " ((t'irare-earth treated material evidently enhances shear in- The fine 10W nm scale of microvoid formation make,,stability resistance, in contrast to the 4340 steels' rela- microscopy observations difficult compared to the more

Table (if. AF1410 Steel Compositions* (WVt Pctl

Alto%, C M n P S Si Cr N i M1o Co T i AlI N** (P CcConventional I ) 176 - 0.0(1 0.X)2 0) ((03 - 1).01 2 O8 10.3 1.0b 14 3 < 0. 01 0.003 I) 's to I 1 0 11REM 0. 167 ().02 0.003 0 W(2 0.0 1 2.08 10. 2 1 05 14.3 0.013 0)004 -10 is 0 005

*Courtesv ot Carpenier Technolo~g'. Corporation,-Parts per million

I150.- V011 Wi. 20IA. JANI \R' I1XY \ii;T\i_[.L RGi( \I- FR \'US \C(I )Ns

1 7 TOUGHNN! COMPARISON OF AF 1410 AND 1400OL .HED AND TEMAPEREO SrCELS 1

* AF 1410 Rare Earth Modified.-0 Crvera1ed at 5100C

8 U AF 1410 Conventionally Treate . 1200Overagea at 510

0C 14

£ AF 1410 Rare Earth Mof4ied.'5 unOerageo at 420'C

120 1000

50t S 1 Interrupted Shear Tests100 n P' Plastic t r ss-Strn Curvs, ,, .100 _Plastic Stress-Strain Cu rves

0- goo - VAR 4340 Steel25 80 Rc 55

40 4 44 46 48 50 52 54 5 5 0 600

HRC.r-

Ft, I-TouLhncs% Cormpart.on ofl AFl4i) and quenched and tern- u 0. Yieldpered ftee, 400 A, Before Maximum Stress

B. At Maximum St-?ssC. After Maximum Stress

150 D. Near FractureSTRAIN - TOUGHNESS RELATIONSHIP 200 -

_125E .700_ 100 0.00 0.04 0.08 0.12 0 16 0.20 0.24

Shear Strain, '

Fig. 20-Shear stress-strain curve of a VAR 4340 steel at Rc 5575 Interrupted tests were performed to the points indicated (A to DiHRC-50

50 s AF 1410 Steel 1.0Interrupted Shear Tests

A VAR 4340 Steel Strain Profiles' 25 VAR 4340 Steel2R

c 55

0 y I I -- I - I -I - I 0o1e g Before Maximum Stress

0 0.5 1.0 1.5 2.0 2 5 3.0 3.5 4.0 a er Maium StressShear Instability Strain * At Miaaximum Stress

* After flaxim Stress

Fit 19-1nstabilty strain-fracture toughness relationship measured * Near Fractureon three AFI410 steels and one VAR 4340 steel,

familiar problem of >I00 nm scale "primarv voidformation, which has been well studied by light metal-lography. An effort is underway to find further directevidence for microvoid formation beyond the thin-foilTEM observations of Figure 13. using SEM metallog- 04raphy of lightly etched polished surfaces taken from thegage sections of interrupted shear tests. Referring to theshear stress-strain curve for Rc 55 VAR 4340 in Figure20. specimens have been examined for the strained con-ditions denoted by: 02

(A) before maximum stress(B) at maximum (instability) stress(C) after maximum stress(D) near fracture.

0,0The shear strain profiles taken from these four specimens 0.00 o.1s 0.o 045 0.60 0 75are presented in Figure 21, showing the progressive de- Positon, onvelopment of a strain plateau at the instability strain and Fig 21-Strain proties obtained frm the interpted shear tests ofthe growth of strain peaks representing shear bands. Fig- a VAR 4340 steel at Re 55 Interrupted tests were perlormed to theure 22 shows SEM micrographs taken from the uniformly pcots indicated (A to Di in Fig 20

METALLURGICAL TRANSACTIONS A VOLLIME 20A, )ANL'ARY 148Q- I51

? ~ deformied portionls of the four specimen gage sections.\licrovoids commensurate %kith the carbide particle sizeappear abruptly at the instability strain (B) with little

change in microvoid density within the uniformly strainedTechniques for enhancing the contrast between micro-voids and particles are under inetgto.The abruptappearance of microvoids supports a nucleation-control led rather than growth-control led softe ~1in Cmechanism.cInitially, the authors believed that the voids observed

in both the thin foil specimens and polished and etched2 Urn specimens were the result of preferential etching. I-ow-

ever. these voids were not observed in the unstrainedspecimens. Additionally. microvoid sheets with void di-

-Irv ameters roughly equivalent to the interparticle spacing~ ~-'~*were observed on the fracture surfaces of the shear spec-

~ * ' ~imens tested to fracture (Ficzure 23).

*~.t ~-V. SUMMARYt The combined experimental ev-idence strongl% sug-

gests that strain localization in these LI-S steels under-A pure shear loading conditions is driven by microvoid

softenine controlled by nucleation at 100) rnm scale second-phase particles. These tests emphasize a void nucleation

~ rather than void growth as the destabilizing event. ThisA21 M_ is supported by an observed pressure dependence of the

instability strain, enhanced resistance to shear instabilitywith particle dissolution, and direct observation of mi-crovoids at these particles in deformed material. Theseexperimental results coupled with analytical analy-ses. [ 16.17.IX( indicate that microvoid nucleation and. hence.shear insiabblt,, are dependent on the interpairticledistance-to-particle radius ratio. In 4340 and 4 130 steels.superior shear instability resistance for a given har( lessis obtained in Stage I tempered microstructure wher. 'heonly microvoid-nucleating particles appear to be th se

-undissolved dui2austenitizinL,. Fine dispersoids r .

b

Md AWI.>

~~fhA6lA4 .#

0 1P & 4.f~~2-Puncmrp..o hLnuruid~~a ci pcnco I 3t-~oraho rn I'cr pcnc 5NI4~t('c

iiVR4-)'.cid e55 a er m~ru Ic'':b.1nu- .iR 61IAc nt oft Iu~iac~mcimad~cii~p raurmum ~ ~ ~ ~ ~ ~~~A Ir%. jaIitrm~mmdc% d erfdlr chn~

152JVI orit'I 2a,~ mAI'R 9~ d-V\A( I I RSS\Ii\

a g-reater role than the lareer inclusions in void fOrmiation 6 A. Bedfoird. A Wireriise. and K. Thomrpso n. J .tust 1 1m . M-etals.in ,hear. The secondarv hardeninty AF 1410) shows a much 197-4, %ol 9, pp. 01-74,

hi~~~her~Z re7tnet ha oaiainwe eetc R Clifton- Report No. NMAB-556. National Materials AdvisorxNhi,_herresstace o searlocliztio whn cmeniteBoard Committee. Washineon. DC. 19801. ch. 8.

particle., are full%- dissolved by alloy carbide precipita- 8 As. Argon: The lnhrommnei u-it Phixn, iDelion,. I st Edtion. For these steels at hardness near Rc 50, a direct ASM, Metals Park. OH. 1973. pp 161-S9cor.-elation bet\een G, fracture toughness and shear in- 1) L Sanmuels and 1. Lamborn. in M,';a/lov'ria.hs tin foilire Aonil-stahilit\ strain is demonstrated. Continued studv of mji %.%i. kt ed.. Plenum Press. New York. NY. 1979. pp 167-901.

coodnucleation in Iueserxeienswl)alw 1 G. Olson. J Mescall. and VI. ,\tr: in Shod. Wao.,-- andu Hiqh-crovid n pue sear xpeimens wll alowStrain-Rote Phenotnenat in Mi-tas. Plenum Press. Nest York. NY.quantification of the role of a critical component of mii- 1981. ch. 14.crostructure. Deliberate control of rniicrovoid nucleatinu ItI T Walker and M. Shaw in AIvancei, in Maichine Tool Desii'nparticle dispersion offers the potential for desit'n of Ina- and Research. Pergamon Press. Ness York. NY. 1969. p. 241terials with _,reath. enhanced resistance to shear localiza- 11. H. Luonit: Monash University. Victoria. Australia. unpublished

Ph.D). thesis. 1977.tion and ductile fracture. 1 3 H Luong Proi eed intio of tie Aiostalian Conzference5' on Mama -

jaclurini' Eni~ineertngi. Monash Universitv. Victoria. A-ustralia.1977. p. 122.

14. D Tracev and P Perrone ' U.S. Army, Matenals Fcchnolov,%REFERENCES [.ahoratorv, Watertown, %IA. unpublished research. 19M,

lI M . Gore. G Olson,- and %I Cohen: Proi' -dimp' o, th, .?4th SNo-I\1 %/rat -i.' e Coc .ind 6 O lson. loal, o eil PhQ, teinor- m Maite-riali Reorh It Confereiui-, in pres

it. I '156. %sol S5. pp 4i111 il.s), Rcl-ort No M Il FR S7- 10 1) T'races and P Perroe. 'oiil~,' of ti( ,'4i111~eon2.1's \rim, \t itc.il lcmo I aN.ihor'r- \\ .ik-n''n. M, Iim %la,'ril R, seoir~h ( >owercc. in press

tar, Pis8 I- S Argon, J lii1. .iid R. 5itoelu: '.Ital! li-ai, 1. 11J 1 Sescaif aind R t'apirmio 1. if, 1t1,h . 1971)s.9. \o, ol 6A.- pp S2 '5-37.pp _280 31l IS A.S. Aruon and J liti Mitll, irons A, 1975. %ol OA.-

3I Cossie laid 1: Tler: Mat ';, i andlhi' 1987. sol 9S. pp. 839-S I,pp.'39 19 J Hutchinson and V Tverizaard:. Proceedtnt!% of the 340zil Sil

4 H. Rioters: .- RO Technical Report. U.S Arrn, Research Ottice. iwmore Arrms Materials Research Confe-rentce. in press.turham. NC. MIas 1974, 20 M. Schmidt and R Hemphill. Priceedin of thei 34th Saia,,ire

S5 H. Rocers: -Inn. Rev. Mater, Sci. -1979. vol 9). pp. 281-311I Arim . 4ouriils Research Canfi-rence. in press

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