AD-A230 999

MTL TR 90-54 1`1' L AD

FORMATION OF CONTROLLED

ADIABATIC SHEAR BANDS IN AISI4340 HIGH STRENGTH STEEL

JOHN H. BEATIYU.S. ARMY MATERIALS TELHNOLOGY LABORAIORYMETALS RESEARCH BRANCH

LOTHAR W. MEYERFRAUNHOFER-INSTUT FUR ANGEWANDTE MATERIALFORSCHUNG (IFAM)D-287, BREMEN 17, WEST GERMANY

MARC A. MEYERS and SIA NEMAT-NASSERUNIVERSITY OF CALIFORNIA. SAN DIEGOla JOU.A. CA 92093

November 1990 DTIC

I ELECTEApproved for public release; di~tribution un~imired. S

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RFA INShUICTIONSREPORT DOCUMENTATION PAGE BEFORE COMPLETING FORMI. REPrOr NUMBER 2. GOVr ACCZSSION NO. a RECIPIENTS CATALOG NUMBER

MTL TR 90-541

4, PiRE (ai^d SauMt) 5. TYPE OF REPORT & PERIOO COVERED

FORMATION OF CONTROLLED ADIABATIC SHEAR Final Report

BANDS IN AISI 4340 HIGH STRENGTH STEEL 6.PERFORMINGOR.REPORTNUMBER

7. AUTHORO) & CORACT.Ok 3RANT NUMBER1s)

John H. Beatty, 0othar W. Meyer,"t Marc A. Meyers.t andSia Nemht-Naser'

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U.S. Army Zaboratory Command November 1990280" Powder Mill R uad 13 NUM18MERFPAGES

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IIL 5a.WPUtMEMIARY NOTES

*Fraunhofer-Instut iuf angcvandic Matcrialfor-ichung(IFAM) D)-2,P7. 11reinen 77, \Vest (;crnant Department of Applicd MealJams and Lginceering Sicncc, Univvrmity of C4,d1fornia. Sani Dicgo.

La Jolla, CA 920')3 U.S.A.

•*,tl,, '4sha S cs•-ts,4'-W, steel, Adiibatic flow,

•High stfcagtal, a s, Plugginlg

Fractum•"

(SEE REVELRSE SIDE)

00 1473 -UNI' A.h.11wGD ~473UNCLA.kSSIF.1EL)

UNCLASSIEDSECUW'Y CIASSICATION OF THS PAGE (Itei Data Ejm4

Block No. 20

ABSTRACT

-Adiabatic shear banding is one of the predomiuant failure modes of ultrahighstrength (UHS) steels under high rates of deformation. Though these bands have beenpreviously studied, the specific factors governing when, how, or if a certain material fails

-Z) by shear band localiztion are still relatively unknown. This article examines these ques-tions in terms of the microstructure using a controlled shear strain. Four microstruc-tures of equal hardness (Rc52) with different carbide distributions were produced in a

-VAR 4340-steel by varying the normalizing temperature (one hour at 8450 C, 925°C,1Of-60C, or 1090(C). A short duration reaustenitizing treatment was subsequently usedto produce similar prior austenite grain sizes; this was followed by a 200 0C temper.The controlled shear strain was introduced by the use of a hat-shaped specimen loadedin a split Hopkinson bar rig, producing a shear zone at local strain rates up to5 x 10s'_,,. The use of mechanical stops to arrest the deformation process allowed thedevelopment of the shear bands to be studied under controlled stress conditions. Trans-mission electron microscopy (TEM) studies revealed a microcrystalline structure withinthe shear band with a crystallite size of 8 nm to 20 nm. A gradual change from themicrocrystalline structure within the band to lath martensite was observed. Tensileunloading cracks were observed in the sheared regions. A normalizing temperature or9250C produced the microstructure with the greatest resistance to unstable shearing.

)

'©, ',5c.4OG -. , c"!'

.L.NCLASSIFILD

CONTENTS

Page

INTRODU CTIO ,,4 ...................................................... 1

EXPERIMENTAL PROCEDURE ........................................... 1

R ESU LTS ..... .............. ..... .............. ... ... .. ... ........ .. 3

DISCUSSIONS/CONCLUSIONSS ............................................. 12

ACKNOWLEDGMENTS ...................................... .......... 13

LA-oeeson For[ TIS qPA&I

DTIC TABU!•tffn l•1auced 0

; Dity bu i u/ '

FAvallebi-lity CodesAAVi11 and/or

jt~ Sp specia,

FV1

INTRODUCTION

Adiabatic shear bands have been previously studied because of their importance both inmetal w3rking operations (such as turning, reaming, punching, and forging) and in other pro-cesses at high strain rates (such as armor and projectile failure, and explosive forming). Anadiabatic shear band consists of a narrow region of concentrated strain where the strain ra'.eis sufficient to preclude significant heat transfer away from the sheared region. The materialwithin the band experiences both significant increases in temperature and a large accumulatedstrain. In steels, two types of adiabatic bands have been distinguished using optical metallogra-phy: the "transformed" type which etches white using nital, and the "deformed" type whichetches dark. The term "transformed" was originally used to suggest that the phase transforma-tion from a (ferrite) to y (austenite) had taken place within the band during shearing, thoughmany investigations have failed to provide conclusive evidence that a phase transformation isnecessary to produce a wnite etching or "transformed" band. However, because of commonusage, thi. notation will be adhered to until conclusive evidence to the contrary is found.

This research program is aimed at studying the microstructural and mechanistic evolutionof adiabatic shear bands in high-strength steels. Specifically, the deformation processes andtheir relation to controlled microstructural constituents (carbides) have been examined throughthe use of optical microscopy, scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM). This paper examines the applicability of our testing techniques to studyinitiation mechanisms and presents some preliminary obseirvations.

EXPERIMENTAL PROCEDURE

In order to study the effects of microstructure on adiabatic shear initiation, both the microstruc-ture and the shear deformation must be controlled. The microstructures of VAR 4340 steel usedin the present study have been previously characterized by Cowie et al.1 '2 The controlled variableis the size and distribution of the carbides present in the tempered martensitic microstructure with-out changing the material's hardness (Rc52). This is accomplished by using different normalizingtemperatures (8459C, Z..50C, 10100°C, or 10-.00C for two hours) to produce each mierostructure.The range of possible griin refining carbide phases are redissolved and/or coarsened to differingdegrees at each normalizing temperature. These carbide distributions do not change during thefinal, short duration austertitization treatment at 845VC for 15 minutes (oil quenched). This is fol-lowed by a 20OPC temper for two hours. This produces microstructures with constant prior austcnlite grain size (ASTM 12) and hardness but with different carbide distributions.

The •ontrolled initiatioo and propagation o(f shear bnds octurred during split Flotpkinrvn co.mprc.,-stin bar testing using the experimental tchinique otf MeN"y c t a0, with a ihat-shaped spCcimcn. Theexperimental atTaagemcntt. sce•mnatic str=ss history, and s•mple F-ca-r.-try are sliowni ii Figurc 1. Usingthis type of test, it is possible to study microstnicture changes }_.k is a. d after the initiation of thebands. A 19 nun (0.750 in.) diameter H-opkinson bir w,• utilize, with a 152 umi (6 in.) tong strikertravliUg at a velocity of W.3 rs. Smear strain rates from 1& to 3 x l(? 5-1 are obtained.

L. cowtr% 1. 0. "Wt M~i, 17 of Sozvod ?Aas t mmu '%%, &.w~ a~di FwmJ T S~tjJtOXrdus.

2 Z. AtIN. - W. COVvIL% I. G'.. a W OUSON, Ct it, M2 I'h Ifiy cA~.tm tn i fa kyws See tJs, C;(*MMCnI IRcp~. 9_11 %SntWMatetuAh Tcc*.,ooy I,',Atty. Ws,.mtn. MA. "41. TR 1-2,, Januay IW7.

3, I!AAW.AN. K. IL. KtINZI-. It. 0, arhd MUT1R. I-. 'W mcIuc- .z I rrcvx emt 4ta.m: ýk V~tmarlh in shock- %.v2% ;trrJ itStntamR~te PftCnoaiWM in, hct*. N1 A. 4''ý~ Wo L F-. Muff. 1;s.t. Ilwaul Prws. Ncw York. Ohli.

A. MEY FR. L. W.. az4 MANWARING. S. Ariiw SvmghI a l7t~i f VLow .'tbdS#d1ev .~ivti c Loij. n %!rftupaA c'mtwmt xI. o' S&I& Waw. >xcd |itht.5.tr2;*.Rz.•a ? in . I h• .Muf. K. P M daatcri'. ?A N M"cvt. oit. MarN d l•CLtr. Itr.SV•amv. Yew,

¶'4 NW FlhC rmfi

side Cutaway

gample In cross v"

5.97 lAV!

Flgure ¶ Hopklnson bar with experimeiltal configizationand schematic stress hNstoy (a) WWd halr specimen 9e""i'(b d~eleopcil by "~eof Ot aL (seW R#eternc 3,4)_

Initial tests using this apparatus showed that ringing effects altered results during samplefailure. To eliminate this problem. a small copper disc (6.35 mm diameter and 0.5 mmn thick-ness) is placed in tmitween the striker bar and the input bar. This creatcs a trapezoidal(ramped) shaped input wave and elimi nates the ringing. Pulse shaping of this kind has beenutilized for dynamic testing of ceramics., Stress and displacement mcasurements were calcu-lated from the transmitted and reflected clastic waves in the bars. In order to Control (thedegree of deformation introduced. stop rings are placed around the sw~all cylinder of the

$. KMIM1ANDIM. Cs. atnd NIENAT-NASSFd( S. tf cvtnftýrnxs of Mnatsc Fmawun -of (Ott&r Ovit.-tir 1.qwri-vu :#ftJ(O wUr-& Pro-adliop of the, 10T*". rCX38 Mamb WI241,I~. Mvankt iut trr:ct4 n0.C% h. V 1. 1904Ž. P 44AU

"hat". With a range of stop ring lengths, it is possible to hait the shearing process at differ-ent stages of plastic deformation. From these interrupted samples, metallographic, SEM, andTEM specimens were produced enabling the observation of microstructure-defoi mation interac-tions and the development of bands.

TEM samples of the sheared regions were made by mechanically thinning the material toa thickness of less than 0.125 mm. Three millimeter discs were abrasively cut from thethinned material using a slurry disc cutter. The foils were electropolished in a 5% perchloric,95% methanol solution at -200C and 50 V. The shear band region polished preferentially butproduced insufficient electron transparent regions. Ion milling at 150 impact angle and 4 kVwas used to further thin the specimens for TEM examination. Both a 300 kV Philips CM-30and a JEOL 200 CX electron microscope were used.

RESULTS

Shear bands are easily produced in the hat specimens. Figures 2a and 2b show shearband development using optical microscopy in typical samples. The shear bands etch whit,: innital, typical of the "transformed" band type. Figure 2b shows the delormation zone beforethie "transformation" bands have formed in a sample with interrupted deformation.

Some specimens undergoing larger displacements with full shear band deformation ofitcnexhibited cracks along the shear bands; these cracks along shear bands are common. Sincetheir orientation with he original imposed shearing direction (see Figure 3) would close thecracks and not open them, i: is concluded that the cracks must have formed after theremoval of the imposed shear stresse,. It is aiso indicative of the increased hardness usuallyassociated with the shear band material after testing.

A typical stress-time diagram obtained by these tests is shown in Figure 4a. along withthe velocity of the input bar in Figure 4b; this sample was normalized at 845"C. After thesample reached its ultimate shearing strength (instability stress), the load drops until the sttp.per ring is impacted at about 2i5 ps. Note also that the velocity of the input bar remainssmoothly increasing at a moderate rate (fromn 4.4 m/s to 5.6 m/s) throughout the regime ofinterest (260 ps to 265 ps). during which the sample transitions from elastic deformation tostable. plastic flow to unstable plastic flow. An average displacement rate of 5.1 m/s was uscd

"•ek•.date thui avc:age strain rate of these tests.

Using the above mcntiioned displacement rate and tilert'ilotul dclOrma ion ;.ntrt ildthestimated from Figure Zb (5t prm). a strain rate before shear band formatitton oi ; 1.011 N105 "1 can be derived. The strain rate after shear band formation can be ;approximxtcd itone assumes that only the material within the shear band continues to deform and remains atconstant width: however. we have observed some widcning of srear bands a shearing has pro-

grc.sicd. Using a shear band width of 9 yim (see Figure Zb). a strain rate of5.64 x 103 5* 1 in the shear band is calculated.

Figure 2a Opflcal micrographs of shear bands produced using this -sample geomnetry.Samples were fromn the micmob~ucte norimalze at 845)"C.

' 0

VS..

~~~-~t 7z, sm-*#,.~.

Figure 2UWulsawje*~po*pot otedv~w fa'wkO *

r244

44it

Figure 3. Tensioni cracks produced in the specimens uponi unloading.Otigiriai loading direction is noW-. No~miaked al SV5C.

VTKS UV INE SAMI~

Tit. Sw~ag~sRO.S4

Fu4.TVpWa (a) shwa .SU"&l*w and (b) jnpt bar v ~cirIO cti~vf

From the stress-time and velocity-time curves, a stress displacement curve may be gener-ated. Both a typical engincefing shear stress and the corresponding true shear stress versusdisplacement curve are shownl in Figure S. The true stress values are valid only up to theinstability stress. These curves have a ljrTe line~ar region, and the stresses fall smoothly afterinstability. Earlier studies by M.eyer et al. 4displayed steeper drops after the ma~ximnum shearstresses were reached f'or a low alloy SLcej ýVHN 480) and a CrMoV steel of edustrength (ays 1300 MN~ at~ s'

2000

160o rT True

S1400M 1200

1w 10r00non

WO~J

WO

(150.001502

fllsglacerment. MMt~arnu~zd I1090 0C. 1 Hour

VAR (4W

PRguie S. Typica) anqowing sr1ww eSS-t( acKnnt and trwO She& SUOe3!,dLspLWAffamr

CWMve (!tto afsss Cu"v is Only *Cui3L up to the Po" (0 As"a4).

Fiturc 6 is a plot of the niaxinium engincering and true stresses rectwhed wt. a functimonifnormalizing tcnmperature. Virtuidly no changre is apparent im. the ruxiimuni shecar %Ircs&attained was indecndcnikn of the four microstructurcs tested. T'he same rnicrivitructures 'ser%orcviously studie at kiw mrtrii1 tates in pure iheor. and here. Ituo. no effect on tht: tnsiabtdity ltrcss¶ wal found.. Hwi.ever. signifi ant -Jifferc-nw betwcen the rakro~vitructurcs were notedAwhe" energy ahmsorption was considercd. 1Figure 7 ploits tthc energy ahuv~bcd heforc tins:'diittly(i) %Verus nortnalisiltg tempeirature relationship for a strain rate of Iva j notc the~cmrgy peak for the ttoC rm i~aliin lemperatute.

ieeirOR tImiuosoopv)Y WaSS USed to C~itn :hc transition hoe.-4'een the rnatrix and the shearha-kds.. Figurc P4a iS a SFAM microgra ph of a shear hand formed in the tmicf-.vro#ucturc aorma-l-mecd al 9215'C. FI.gure N4b "-thnw% tis hand nocar Its. tip. Note thre alicgnmen x. h are~t

Withs with thle ,.hearing diotuifetll ilong ti~c hand codgics and ihc akv-ntce (,( any remilv*Icgrain sitr-jturc within the hand at this nagnification. lwlines can be, dctCcicd pavallel to,the shear direction within the hand. Ficuic "),, Aibow a portion of the she-ir hand rce~~n

examined in the TEM. The microstructure appears mottled, unlike normal martensite. andthe presence of many moird fringes are noted. A Selected Area Diffraction Pattern (SADP)of this region is shown in Figure 9b. Although a small apertuie was selected (10 pm, whichilluminates a I pm diameter disc at the foil), a distinct ring pattern is observed indicating amicrocrystalline structure. This ring pattern is typical of "transformed" bands.6-10 The ringsdo appear somewhat broadened with the pattern indexing to be a-Fe(bcc). No fcc reflec-tions or any individual carbide reflections can be seen.

2•00

lw 01 maxinirtmn engineering suess

1 40. 4 maximum true stress• • 1700 ,:

2 %

13pU tso4 I

;'rraximum trua shea: strassquasi static

Soo 900 000 onormalizing temp, degj. C

Figure 6. M-aximum engmerng shear sresses and truest,'esses versus no~matizing temperatuge. Quasi-statc

la ts fromt Coww et aL (sýe Ret'ewa I amt 2).

I '10

41-

80.

r~tmania'fng toimpiralute, dUg. C

£ WrwC U absiofwj tia " veltp ~ &!J~ VV 41

V %k* T8.kt V ;k

IS V( V STkYWV .L td lT 1t - ,, ,, -

R WN'rTUL-t .CJ' • 1- a WV '& U A 0 .U t

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4 .

*.o".

Figure 8. SEM o0 a shear band kxmed In aU'arspecimen,

4e k

Figure 9a- TEM btgN helid macrgraph ci 1 Via Fgise Sb. SADl' using a WOpm apedtro,ban maleAl hin Ow N-%25 nlcosfrxtwo

Using dark field microscopy and centering the first bright ring along the optical axis. tileuicrocrystals can be individually illuminated. Figure 10 is such a dark field micrograph where

the crystallite size can be determine to range from 8 nm to 20 nm. The change from truemicrocrystalline structure to "normal" heavily deformed martensite away from the band wasgradual, agreeing with the works of Wittman et tl.8,9 SADPs as a function of distance awayfrom the center of the band (see Figures 9b and Ila through lid) illustrate the gradualtransition from a ring pattern to a typical spot pattern.

50 nm

Figure 10 TEM cark field centering the first srtog ring wich illuminatestthO mictfOC&*~aIS Sizes range ferom 8 rur to 20 rim tin uarneter

10

Flgve 11. Vwi~atkn ol saleded area difftactiv paIoms as a funcilon of "iance fromftIco'wt 04 the shwaband. Oistance from rt~w band r'o.,er Ls notodw on och patten.Note lbs coip~ulo to Figuto 9b from near Ow c"rOr t tOw band.

DISCUSSION/CONCLUSIONS

The primary purpose of this paper is to examine for VAR 4340 steel at Rc52 the applica-bility of using the hat-shaped shear specimens to study shear band initiation mechanisms andpresent preliminary observations on their formation. The hat specimens with tailored stress-pulse profiles and controlled amounts of deformation produced unfractured specimens at vari-ous stages of shear band formation (prior to band formation, shear band propagation, andshear band widening). This technique provides a (simple) way to produce shear bands in acontrolled manner qt relatively high strain rates; thus, this method permits experimental corre-lation between metallurgical variables and shear band formation in high strength steels.

The relative resistance of the four microstructures to unstable shear at high strain rates issummarized in Figure 7. It shows that the influence of microstructure can be important inthis regime of strain rates despite the intense heating that accompanies shear band formation.Since the principal variable which was studied, the carbide distribution, probably has little ifany effect on the material behavior after 4estabilization, it must affect the processes leadingto the destabilization of the deformation p ocess. The fact that one condition had higherenergy resistance even though all microstructures had equal hardness at this high strain rate issignificant.

These same microstructures have been examined by Cowie et al.1,2 at low to moderatestrain rates. In their study, the 9250C normalizing treatment also produced a peak in theresistance to unstable shear, but the tesistance to unstable shear had a minimum for 1010"•C,suggesting the presence of a second peak in the instability strain at low strain rates forhigher normalizing temperatures. Charpy tests over this same range of normalizing tempera-tures at room temperature and -40')C correlated in the sante manner; two peaks in absorbedenergy at room temperature, while the -400C Charpy results showed a single peak in tough-ness. This suggests that the increase in strain rate in the "hat" shear tests is analogous tothe decrease in temperature in the Charpy tests.

Examination by optical and electron microscopy of the hat spetirrfiens in various stages of'shear band formation and propagation has not yet shown any signtitant difference betweenthe four microstructures tested. The tips of the shear bands show the muartensite laths grad-ually aligning with the shearing direction which agree with the observations of Wittman cL al,.Note the absence of any observable grain structure within the shear band and tihe gradualchange from the band to the matrix.

The TEM examination of the shear hand showed the change from the internml shca t ha ndstructure to the matrix structure t) be gradual. This study has clearly resolved that the int-.rior of the shear band has an extremely fine microcrystalline structure with grain dialnctcrsranging from 1 nm to 2(1 nm. Previous studies had suggested that the crystallite size in"transformed" bands was between 100 nm to 1.000 nm. However, this new evidence clcarlyshows that microcrystals exist in these shear bands, which are an order of magnitude smallerthaon this. This accounts for the difficult resolution of the band in e~arlier SEM and TEMstudies. Foils that are many crystallites in thickness produce repeated scattering by subsc-qucnt crystalfites and mask their ttue structure, No austecritc or carbide rellcction\ Could ticdceteted in the diffraction patterns twken from the regions showing the extremely line micro

crystals. Since a small ctvstallite size within the shear band would enhance the stabilitv ol,austclnite that Was pres-ent. the comiplecte absence of austenite re•tction-s is signil'icant.

12

It suggests tiiat no transformation has occurred, and that the shear band is simply very heavilydeformed martensite, in which both the extremely fine grain size combined with the additionalcarbon in solution (from dissolved carbides) have increaseC the hardness of the residual (aftertesting) microstructure. The absence of carbides combined with the microcrystalline structurewould prevent preferential etching, creating the white ctching typical of "transformed" bands.

ACKNOWLEDGMENTS

The work was performed at the Univcrsity of California, San Diego (UCSD), and at theU.S. Army Materials Technology Laboratory, Watertown, MA. The work performed at UCSDwas supported by the U.S. Army Research Office (ARO) under Contract No. DAAL-03-86-K-6169, as part of ARO's URI center for the Dynamic Performance of Materials. The authorsare grateful to Mr. Jon Isaacs for valuable contributions in Hopkinson bar testing.

13

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