Report on EFFECT OF 1PFrEVIOUS i)EO1; U'i'XIOJq ONi THE CRLE,11 CTIARACT`RI-TICT` OF GR"D.3 0. _3OI XL.... 250:.i.D 1B5O ci'. by C. L. Clark A...;. hite Project Number 491-48 Report Number 1 for The Detroit Edison Company June, 1932

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';AFFIA.CT w,!'.'I F u.... D.,:I' I.... 0 I1 TII-E C-R P CHIALBACT.4T I"ITIC; ) F A' 2 G RI3CY' JL at: 600, 850 AiD 0G00~F. As a result of the large number of creep tests which have already-been conducted by the authors on various types of materials over a rather large tep)erature range, and from purely theoretical consideration s, they fe-l that the factors which may influence the ac tua ll or the observed creep characteristics of metallic materi 1 J,. ray be listed 8 s follows: Chemical Composition IIeat-Treatment Recrysta 1lization T.em:n)erature Grain Size -;,;el ting Practice Me thod of' n-f a c tulre s" Castiti'' l:,!rnc rcice Fa b brication Testing Procedure Previous De fo'r na ti o Many of these factors are overlaappin_- and it is difficult to consider one rwithout one or.o3re of the others. IEspecially is this;true of recrystallizatioln te's erature and chemical composition, and to a sonaowhat lesser extent, of grain size and bireat-treatnent, or of ethod of'uanufacture, heat-treatment, and previous deforrmation.

2. In previous reports, the aut}hors hjve considered ~in detail the affect of recrystallization temperature and grain size on thk resulting-: creep characteristics of a series of non-ferrous alloys, and a paper on this subject is being presented before the June 1932.meetin~ of the american Society for Testing Materials. Very little is available in the literature as to the effect of the various other factors, with the. possible exception of: chemical composition, and it is for this pur- pose that a series of investigations are being undertaken to throw additional light on these various factors. The work herein reported concerns the effects of previous deformation on the resulting creep characteristics and, insofar as the authors are avare, it is the first information which is available on this question. In addition to the creep tests, the hardness, the mstallographic structure and the x-ray patterns of the mnaterials have been determined both before and after the creep tests. The results obtained are presented in the following sections.

SU%'RY OF. CO UoCILUOITOS The results obtained from creep tests conducted at 600, 850 and 1000 F. on speciL ens of Grade B steel in the "as received" condition and after havi-nr been deform.ed 6 and 12 per cent, show the material in the "as received" condition to possess the mlaximum creep resistance at each of the temperatures considered.:Hardness tests and a metallographic and x-ray examination M.as conducted on each of the specimens, both' before and after the creep tests in an attempt to determine an explanation for the observed results.'The results of these tests, as well as of the creep tests, are sur!aarized,in Table I. The differences in the cresp characterist1cs at 600~. are observed to be very sli;zht,: but the nia{nitude of the d'ifferences increases as tlhe temnperature is increased. eo differences could be observed in the metallographic structures even though the examinations were made at 1000 diameters, Neither did the hardness values show any decided differences other than a slight increase at 600~F. folloled by a gradual softening ss the temperature was increased. A marked difference was obtained, honever, by the x-ray study.

?iable I Effec t of'revious Defr:.it Ors of 6 aid' 12:err Cent O r! The CreeL 7:esistancre, the }{rrne ss, adci tkUe e, ta llonr.- io ai.rd [-r; y:.trictures of Grade e Stel at 600, -0 f:3O:Ond iOO%~. C Cre 1 o Sl.arac teris tics Ter.i of T'est e:tress, 1000) ~Ttal YRate, per soct ell r:;rt'ress - tru ture___e treatrnent jeefj:r L/. iIn. Deforl.atior 10i) YTrs FoTer c-ite,-uer't elo;Traphi As Received -4.''nstrained 0* 6( Deformned. -.1 o visible setroin 1 12" Defor'ne d? r visible strain 1 As: TFeceived 60 2,0.oa 5 011.5, 2~O.0.0.str';ined. B_6 DIef~orYed 607 4,200. 010i76 1.9 C.: ~1.> 82.0 V o visije str9ir3 - 12 D efoxred CO.O 20.4, >,003.3 0150 C):,.3 91. " 22.0 -o visible strin 1efo e:00. -.s 2eceived 850 19,055 )',.. 90 72,0 Str.ined Q...e: -a 0 n;0 is?eceived lO0,, 00.0.0,003',90 0 1.122 7P.3 7$ 723 7T2.str:trined 0 6. Defor~ed 1002'> 5, OO0.w02% j,.,274 7..3 8:.:.5 74.6.o visi ble straiin ) e.o 12'' D0efo~red 1002' to,!000.020 ].,;323 70,3 *. 3 74.6 " is st;i?'....... __.. ~..... Un* ILstrained I; tra ined: 1- Sdic irhtly s tr lia ed i- Very sli:,htly strained

The x-ruty pattern of the is -n. ter'ial in the, "'as received"' condition s-ov/ ed the strucL tre tc be. fre fro-.train, and also that no marked charncies were prohlced in the strulcture by -the creep tests. The defor?;ed speci-Maens on the other hand, were observed to -o-osls s n. str. i ne. structure in the "as defor:n..ed coniition:f) nd th;is stru:lture,a,7s found to be unstabl.e.t uill of the;e,:e! tlres used in thie creep tests. On the basis -of tiheso fi'din~-t. i t. ir, t:,erefore, believed theat the observed differo-.ces in creep cbaractc ristics are due to the "'as received" -nateri'cl hievinrv a:ore sta.:,le structure at the te-z-')eratures emloyed inl the creep tests than has the defomed sr)eci:ein,. n ul.-A;t;oble struc-&ure implies that thk; crystals are in a greater i tate of mobility and will, ther:efore, defonn Jiore readly u nder ra? cmppliied stress than if tey wJere less hLobile.

The mnaterial chosen for this investiEation o:;as the ordinary commercial grade of Grade Y steel. It was obtained by The Detroit Edison Com.-.lany fro:m the IHational Tube Company in the form of a seamnless tube;;-ith a wall thickness of 3/4 inches. The chemical composition of this -r:terial is giiven in Table II.'Table II Che.lical Composition of Grade B Ieanmless [teel Pipe Desi nation Chemical Comp ositition, er C ent Carbon ancranese ilicon: ulnohur Phosphorus Grade B 0.408 o 917 ).21 031 014 The tensile properties at room temperature were as followss Table TII Tensile Prooerties at 8f90'Y of erade 13 &em-l-ss..teel Plipe Tensile Yield Prop. du cStrength Point Limit >1lonf:ation tion of!esig.n..tion Lbjq.In. qI n n;:in in 2 In. irea — S9pecified 62,000 35,000 3 25.0;As Received 90,300 43, 000 30,000 2. 0 4G6.4

7f The material w,'as.ub,,c'teJ to creep tests at 6'O, 350 and 10000F. in the "as received" condition and after having been plasticully defLormed 6 an d 12 per cent at roorn tem-neratuire orevious to the actutal creepo test. Tahe deformation was accomplished on machined, standard.50-5 inch diameter test secir-ens anr the deformaEtion referred to, that is, 6 and 12 per cent, was thlat which was obtained over the two-inch Egage section of the test piece. In other words., the deformation refers to the increase in length of the test piece rather than to its reduction in area. All the speciniens were subjected to metallocgraphic and x-ray examination, both before and after the creep tests at the various temperatures. The x-ray method of exarmination was used as this is more sensitive than the metallo-:2rapwhic test in detecting the presence of str,-ins within a metallic material. All the exaniinations were conducted on lon:-i tudina spec imens. Hiardness values were -also taken in all ca ses to deten.ine whether or not tlhe creep- test pIroduced any charne in the hardness of the oriE:inal specil- enS. In the creep tests. at the various terimperatures, only one load was used in eacl case and this load was so

selected that a oeefinite rate of (.r- c o' be cbtai.:e& The usrJ of a sinigle lo.d does not tmlke o3ss'l. - ieter:mi nation of the co:-n')le t- creo-, cG.Ltctcrio'ci s -.:c, -c J.iven te:lperatire but it;!s bel].ieved t'!'!at tb'Ijis.eti]-o 1v-/o uld clearly brinri out.ny 0( ifferences in creeSo, r:h:'act. ristics which _mirrht exist Jhe bo t ole ito "'e a t o.roviols Ief'-~lr. i which tVhe;;eci-Len hard under-r-orne. The results o f the c~reop tests, the 1,' rd ess rteatst arnd the metallorraphic and x-xTay e:amin ion a-i0::iven in the followin g7 se.cti orlns. Creep Tests. fis stated previously creep tests -'; ere c;ii tducted at 6)0, 850 and 10000F. on -Grade t3 steel in the "as rrcsived"' condition and after havrin,; been r>reovioua:y,;fjeorms&. j and n' peor cent at room temipera-tures. Tests at 000C - The c re tests at 600C -:ere conductt d on each of the thlree s:eciiens un'rder a s ress of 24, 200 pounds per square inch and tllie tests,:ere co:3tinule for 5.70 hours. The results w]l:ich were oilt- inled are:ivn ijn I jure 1>...

From. the figure it is evident thli. t; t 1l three specimens underwent the sai-le elastic deformation Ui)on1 tie a,-plication of the loa10, la t the-ai fter th e amnlount of deformation obtained varied.?he speci-nen in the "as recoived" condition suffered the least amount of> totl defor;lati:l,?,hitle that defo.rmed 12 oer cent:jossessed tihe b'reateste 7he differences were not, —'reat. for'hthat of the I'as receivea't soeci:i en was ~.00133I - inches per inc, r:t sc deformed 6 ocr cent was'0.001476 and thuc of th e speci'en de- omlied 12 oper cetit tlas.O0 1'- 50. /s for the: v'r ie rate t of Cre,-) oh t i ned for t e different.specinens duri:Tiv. t+)e latter part of tli test, it v;ill %je observed fromr the fi-,ulre thott whfiil" - the "ta:; r.ceive'" specinen was creepinw: at a rate of 0.013,-er cent )er 1300 hours, the correspondini values for the spd ci'ens defor:i.ed 6 aind 1.2 per cent, wtere o.0ln and C7.D.0 cer cent respectively. Again, the differences Ibet:;,ecn the three soecimens is 1not very.reat, but the "as received" specimen does possess the.ai:im.umn creep resistance, -and( thIe s eci:_en rwit'h toie?-reater amount of previous deformation, the low;est. On the b-tsis of t'he results at t is te'- oera-t.re, it mlay be said that previous deformation decre. ses tc a

I0. slig;ht extent the ability of this steel to resist creep, ut least wvhen stressed to a. load of L)4, 200 pounds 4er s..are inch. Tests at 9,0~J.1 The creep tests tt Oc-0.:-ere conducted on each of the three specimens under a stress of 19,055 pounds per square inch and the tests:;ere continued for periods from 5f3 hours to 6-3 hours. The results nhich were obtained are iiven in Figure'~. As was the case at 600IF., all three specimens undervwent practically the same elastic deforn.ation upon the application of the load, but thereafter the "'as receivedt' specimen suffered the least, and the more severely deform-ed specimen the greatest, elastic plus olastic defor-mation.:'Iie differences between the three specimens is much rmlore marked at this temperature than vras true Cat the te-rfl)erature )reviously considered. For ex.mple, the total elastic rlus plastic deformnations sufferedA by the "')s received," and the specilmens: deformed 6 and 12?per c3nt wiere, a-t t'.e end of 550 hours, 0.002450,.0,3 0410 and 0.005r010 inches per inch respec tively. iAlso, if the average rate of creep occurrin<- durii;, the latter portion of the test be considere, it will be foeund that while, in the case of t'e "as received"t

I. specimen, the rate of creep was 0.10 rer cent per 1000 hours, the corresponding values for the 6 and 12 per cent deformed specimens were 0.198 and 0.295 oer cent per 1000 hours. On the basis of the results at this tem oerature, it may, therefore, again be said that previous deformation decreases to a rather marked extent the ability of this steel to resikst creep, at least when the stress considered is' 19,055 pounds per square inch. Tests u-t 10000o. The creep tests at 10000F.?were conducted on each of the three specimens under a stress of i,0(}0 pounds per square inch and the tests Dwere continued for periods varying from 525 hours to 670 hours. The results V'; iCh were obtained are riven in Figure 3. As was true at the trio temperatures previously considered, the "as received"' material undergoes the least amount of elastic plus plastic defoxrmation, hile the more severely deformred sDecimien suffers the {-reatest amount. For exa3p-,le, the respective amounts of elastic -lus plastic deformsetion obtained during 525 hours with the "as received" and the 6 and 12 per cent deformed specirens were respectively 0.000980, 0.002600 and 0.003330 inches per inch respectively.

12. If th!e average rate of creep occurrin- (iuri rg the 1atter -:ortions of these tests be co sidere( i cr:ewhat different condition till be found to exi;-t tl-3n was true at the previously considered tempTeratures. in the other cases, the rate of creep incr assel -it icrc-siinf amounts of previous defor:ation. At tiis te erature,;owever, 1D30 F., it is fouind trhtt wh ile the:'as rec ivedd mate rial still possesses the lowest rate of cree 3, the Maximurm is possessed nozt by tihe specimmln doforned 12 ner cent, but by that defor ned per ce't. T:e;re rtes.... creep obtained are 0. 1i2,. 74, nd 0.323 per ce -1. er 1 00 hours. Th}oe curve represenrtins- the results obt:ine:d on the sneCei:reP deforied 12 i2 er cent, hof::wve, is ra'er irrei-ular and there are certain porti-ons wv:ich sho;-! mluch hig er creepT rate than the averae w-i.h- is rei)orte t. T.he irregularities in the curve are not felt to be du.e to experi:zental defects, but ratheer to marked recrystal 1iza tion -w-ichl fis v:i thout douib t occ:lrrin, vJithin the snecim-en. his )oi nt Vwill be, consideredl later in thel report.

The hardness of each soeciiraon.'s Cdet"e',icll both before aini after the creep tests at the v:riou,s tei)eratures, The Rock ell mIachine was usel, U;vd in the ci'se of the completed creeo test s eciLen,, rea ls,ier tt kzen both on the' shoulders and over tnhe:ife sect on, The results obtained are -ziven in Table Ifl It will be observed thfl't, in th e c:se of sEieci:.ens before being] subjected to the criep tests, the deform.ations employed results in only a very sliU.ht incre- xse in the hardness. For exar:pnle., the materi:l in t`he "as recei1ve d ori;ition possessed a Roce:rell',,,," Nardness cf 4 Tf:hile t1he hardness values of the specimens defor-ed 6 and 12 )er cenit were only 836,1 and 87.3 res-ectivel-. The creep test at 600-~ undr 3. stress of 21, 2s) pounds per square inch res ilted in a slia;t i cr eas in' —ness in the gage section of each of t>-e s, )eci ens. This condition is co.rm2onlyv observed v when c old v'-orkLe d n materi ls -ire siightly heated and is believed to bh due to In a: e-}ha rlenin&' effect, AAgLain, a t this ter perarture, the )-revio' sly defomr-ed soecimens are slihttl i hrder tharn the'cs rec. ived" one,

I L b Tab le IV Effect of'Previous Deforiuati.o and Creep e ests at Indicated Te1jDperat:lres The HIardness of Grade - r;teel Te-mpo of Ti -le of Roclkwell "~" -ardens"~ Creep,est Creep -est i/16"v I!: 3I, 3I) iT.Toad Condti on:e, r. Tours choulder ae hoider As Rieceived 84,. 6o: Deformed 6. 3. 1-' Deformed -7. 3 As 2eceived 600 570 82".5 7,0.,0O 6' IDeforrmed 600Q 570 06,3 91.2 ~2.0 2' De fo rme-i 600 570 6. 7 94. 1i3.3. ils R ece ved 850 67 7.0 0., 7. 0 6 — Deformed 850 583, 1.3 7.3 7 n. 0 12-: tefoled 850 6C3 PI.*3 3.7,0.3 As,Received 1000 775 77.2. 5 75 6 - Deformed 10)00 525 7?c.: F;2.5 74, 6 12; Deformed 1000 670 7- 2 8 3 76 3 Values are the avera-e f t;;east five rea ilnes,

The creep test at c50('". resulted in a slir-:' t softening of the specimens, us co-p ared to'the values obtained at 6000F., but the hardness values of the )reviously deformed sp)ecimen are still som-etiif-, t li- ger than what they were before bein:- subjected to the creeo test. in no case, however, isC th, observed chan;ge very:iarked, The creep test at 1$000'". has resilted in at further influencing of each of the s!ieci'lens nCid all of the values are now lower than uwha t they wvere before the creep test. Sven thouc:-h these tests were conducted for "25 hours or?wore, the hardness values of the:D)reviously deformred sDecimens are still hitcher than thc-t of the — aterial in the "as received" condition. Both of the deformed specLinens, however, possess the srame hardness.:l tallographi c xslina ti on. The metallographic structures of each of the three specimens were examined bJoth bef ore and after the creep tests at the various temperatures. The results obtained are showm in Photomicrographs 1 througth 12. In order to be h~ tter able to detect any structural changes, the examinationss,,ere w-ade nt a magnification of 1000 diameters.

1:t'om.75i cro-'3-t >bts I to *O ijnr4'3''' j.;' sotr t7e st u, Cture of the steel in,,e (as rec i ved ecoudi'i r, r',n -.tr h civiu beiih dort 6 "i 1 2 e- er ir ent ir,he, nification used no r c)-rlreCit;ble str;.:in hrdenin- dV (5et.oC;ab1e e-ven in the seci lenl d.efo..ued - th e srmQtet;?-ot. It.does apie:r, km;ever, (:.>-!t tVhe:ea3rlite i: th';e r-lo.r:.ei snecilmens is somewhat more di stinctly lrljins ted'then ~.]-,l it is in thte t"s receive t' c ndition.?ho touni cro gra ohs 4 to show the cor::osnond i. structures after t.e soecinons bad beten sub'',';Z ctt-< v... to xcr.,s, tests at' OOcF. for 57?0 hours.:o visilble strturol (,:nes have ocourred durin- tlhe creep tests a- t this terl';,.:r t-.re, Photomicrorae- hs 7 to 9 slow the struc tre;s ef er the spiecimens had been sub dected to creep tests at,-o[:0 ";.? for periods ranging from V.83 to 6G33 hours. Again, no visible structure chinge has oc curred.?hotomnicro.rhrls 1(0 to 12 < owr t'he strlluctures af ter th~e speciLen had been subjected to cree: tests alt.:;-'?-. or periods rangiwn from 52: to 670 -nours, fter this t0s4t, the rearlite of the steel in the "as received' c"otlit ion seemis to be somewhat less distinctly laninated than as true after the previouas tests, but the deformed speci-;:ens still ossess a distinctl> lamin ted structure.

ConSi idE r in~^ tle're r t- icytr A,'lC it'uay be slid tha.t no -'ir:uted struct;ral t.2lT:,es;:er- ro — d1ce(:, ei tbo r'tyr t:-;e (ifet7rr8t'io|. 2 eiid. } dor't.'troo~: teneratlro, or by the creeo testC s t<o.'..ih.t...e f tfterwards sub ected. It is f'et tit 1is i Fld no beo t-:en t -c r however, that n.(o cb.-nves.re nctr!';ll'y oc';rrnir, for nort'ins.ay th J t-akin:' lac v1 c re -. tt -L 5 order oe ryac.lrnitude, )thst the ocotil1o r:pbic e.inLti.-'t is nrot able to de tct they-. -ra y xamiV n tion. In order to dceter;nine whether or. not ch'"?:Ces wvre cctu0c-ly tekin2 nlace that could:ot be toen-:.e-de could-'e-t.: llo~cra..)bhic meOans, specillens of the ste,:i1 irn 1the'":-:s rec.;i,.d: co 1ditiob n cal:i'fter hv; L' heen efof -,le:' i 6.rW 18'01r cent n —ere subjected'tCo x-r.:y ex.inl tio, 1..th..e.-d i: the creep tes ts, it the va-rious tem o:era ures. eT he x-ray:+.eth.od of exLt!in'thion is -uch:_:ore sen-: tiv.= thon t;e'3etai -- gra:trp Iiic one, nd. on d i.r - ir. *,'tryI- eo I 1 I: t o be det cted by x-r?.y e:i Xna:ro i o eve;- t hou'' th;y ar e V,'y siinTbt. The results obtsained are s?:own in 7 i o-ures 4. tnd -o

'7~*~1 6 Fi ure 4 X-Ray Patterns of Grade B Steel _rfore and After Creep Tests t 600F. = "As Received" K- Deformed', L= Deformed 2f G = "As Received" Creep lest at 60OF. H - 6: Deformed, Creep Test At 600cF. I - 12% Defornmed, Creep Test at 600F.

I. F i gure 5 X-Ray Patterns of Grade B Steel After Creep Tests at 850 and 10000F. D = As Received, Creep Test at 8500F. E 6% Deformed, Creep Test at 8500F. F = 12', Deformed, Creep Test at 850OF. A = As Received, Creep Test at 1000~F. B = 6% Deformed, Creep Test at 1)000F. C 12% Deformed, Creep Test at 1000 0F.

From- these two fi.,zures it is evident that certain changes have occurred within the specimen due both to the deformation at room ternperature and to the creep tests to:which the speci ens iwere afterrrrds subjected, -It wi:ll be observed that with the rmateri-l in the "as received'; condition, x-roy doublets are distinct -ndi sharply defined, thus denotirng the absence of;:ny atppreciable strain within the material. In the case of the specimhens-:which:have been deformed, however, the doublets are not distinctly 1defined, thus denoting'the presence of stra-in. Ti th the specimefi.s subjected to the creep tests at 6000~., the doub lets are again clearly defined in the case of the asas received' m aterial and indistinct in the case of those which were originally deformed. The doublets of the deformed specimens, however, are-not quite as indefinite as they were in the "as deformled" condition. This Sho ws two facts: first, that the stress emoloye, thr:t is, 24,200 pounds, weis not sufficient at this tem oerature to produce appreciable str-cin hardeniing wtithin the "as received" specimen and second, that the temroserature and stress conditions vwere:not suitable to produce very much recrystallization in the previously'deformed sp ecimens.

VWith the specimens subjected to the creep tests at:850~F.,- the doublets obtained with the tas received" material are again clear and distinct. Those from the previously deformed specinlens are much clearer and more distinct than what they were after the 600CTi. test, but are not as definite as those obtained from the ma terial in then "as received" condition. This indicates thnst some recrystallization has occurred at this temperature, but that the recrystallization ha s not been complete..After the creep tests at 1000%F., the x-ray doublets obtained from all three specimTens are exactly similar, and are,very: sharp and distinct. This indicates that sufficient recrystallization has occurred to remove all the strains which were originally produced, by the deformations of 6 and 12 per cent at room temperature. EXLAI ATI-ON OF RESULTS Since it is a recognized fact that cold-working increases the strength characteristics of steel at room temperature, some metallurgists have accepted the belief that such will also be the case at elevated temperatures. In fact, one metallurgist has questioned whether or not the so

20. called first stage of creep could not be eli Ainjated thr>:h previouls workin. and thus the trnount of' Ceforratio, occurI ring durinf the cree) test could be considlerLj:,l- re. uced. The results herein presented indiccate tll::t coldvworklng decreases a steel's ability to withstand crsep,;t least at temperatures of 600, 850 and 1000~F., and with the loads in question, and it is believed tha.t these findings are consistent with what should be epccted from theoretical considerations. As a general rule it may be said thet the ~material w hich possesses the.most stable structure at a ny iriven temperature will also possess the greatest creeo resistance at, that same temperature. If a metal's structure is unstable it is implied that the molecules or crystals are. in a more mobile condition and,ossess a tendency to change into a form:. which will make them more stable. Because of this rreater mobility, any applied stress should cause the crystals to deform to a -greater extent then would be tbre case if they were already in a stable coi dition. In oter'.ords, if a metal's- structure is in a strained condition, the crystals possess a greater tendency to move, and so cran be Mnore easily displaced by an apoli-ed stress.

21 o If now the x-ray patterns of the mraterials are referred to, it will be seen that while the structure of the' "as received" material is the same both before and after the creep tests, those of the defonred specimens are chianged by the -creelp tes ts, even by the ones conducted at 600c F. The relati-onship between the structure and the creep results is, therefore, in agreement with the explanation outlined above. The- time-elongation curves obtained durinr the creep tests, and shown in Figures 1 throu.,h 3, also indicate the "as received"' m'aterial to possess the more stabl- structure. It will be observed theat especially at 600 ain 8 50c'. the curve for the ftas received," naterial is considerably more regular and uniform than are those obtained for' the previously deformed specilmens. It is felt that this differ-nce is due to the fact that in the case' of the "as recei-ved" m3aterial, the def0raation is entirely due'to the op liedi stress, while in the case of the previously derormed speci.ens, the observed changes are due to the applied stress plus structural changes which are occurring at rmore or less irregzulsr time intervals. The question now arises as to wh ether or not the observed structuiral changes at 6003~. are "iue to actual

recrystallization within the metal, or to other changes. This question is one which is receiving considerable attention at the present time and on wihich there is as yet no agreement. It is felt by many that because of the great sensitivity of the x-ray, the observed changes are due to slight rearrangements which may occur at tem peratures considerably. below the recrystallization t r). -eratures. Others feel that this first observed change, no matter how slight it is, should be considered as the lowest recrystallization temperature. If this latter view be accepted, it will be necessary to decidedly lower the temperatures which have for-,erly been selected as representing the lowest temperature of recrystallization. For example, on the basis of the work herein recorded, it will be necessary to place the lowrest temperature of recrystallization of Grade B steel below'00~0. C O i CL US I O NS Creep tests were undertaken at 600, B50 and 10 0"F. on specimens of Grade B steel in the "as received" condition, and after having been oreviously deformed 6 and 12 per cent. Attemptts were made to explain the above results throu]-h a

23. comparison of the hardness values, the metallopraphic structures, and the x-ray patterns of the specimens both before and after the creep tests. The following conclusions we re reached. Cre__ Tests. The creep test results showed the "as received" material to possess the nmaximum creep resistance at the three temperatures which were considered and the most severely deformed specimens to possess the least..oreover, the differences between the "as received"' and the deformed specil ens were more marked at.950 and 1000~F. than what they wvere at 600~F. Hardness Tests. The hardness tests conducted on the specimnens before and after the creep tests showed very little difference in hardness to have been produced either by the initial deformation or by the creep tests. The creep tests at 600eF. produced a slight increase in hardness, while those at 850 and 1000F. caused a softening. In no case, however, were the differences very marked.

24.?-etallographic Sxanrination. The metallographic examination showed no appreciable structural changes to be ca:used either by the initial deformations of G and 12 per cent or by the creep tests, even though a magnification of 1300 diameters was ermployed. X ra Examina tion, The x-ray iatterns of the "as received" miaterial showed it to be practically free fron initial strains and also to undergo no marked changes during the creep tests. The patterns of the S and 12 per cent deformed soeci-nens show the materials to oossess a strttined structure before the creep tests and that the creep tests at 600%C. produced a slight change in the oridfinal structure -nd a pronounced changed in the 8500F. and 100 00. tests. In other words, the structures of the strained specimens were tending to return to a stable condition at 6000F. Fnd R50QF. an, had returned to such a state at 1000OF. Explanation of Observed Results. It is believed that the superior creep resisting ability of the "as received" ~ateri al at 600,. C:0 and 1000~1F. is due to the fact that its structure at these temperatures

is in more stable conirition th'nan are those of tre s'oecimnens defor':ed G and 132 per cent. Bec'iuse of this condition, the crystals of tie'-U"as received"' r-;mteri-l are in. a less mobile condition anId.ill not, th,3refore, oje as free to miove unde-r an applield stress as will be the case with,.,~e tywo de-ormed specimens.

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No2.:4 -Graie B Steel Creep Specimen at 600~F. Tested in ise Received Cordi-tion Toad: 2,20( Tb. /:i.In. 1Ti0 O570 IDours No. 5 Grae t ee No rad B Steel GCreep Specimen at 600~F. Creep Specimen at 6000~F. Previously Deforned 6/ Previously Deformed 12< Loed: 24,200 Lb./Sq. In.. Load: 24,200 Lb./Gq. In. Time: 570 Hours Time: 570 Hours X100. D XlOOOD

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