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Healing Behavior of Micropores in Powder Metallurgy 316L Stainless Steel during Hot Forging and Heat Treatment
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ９):８ ６ ２ Ｇ ８ ６ ８
He a l i ngBe ha v i o ro fMi c r opo r e si nPowd e rMe t a l l u r g y３１６L S t a i n l e s sS t e e ldu r i ngHo tFo r i ngandHe a tTr e a tme n t g Yi ng KAN１ ２ , Sh i Ｇ hongZHANG２ , L i ＧwenZHANG１ , Mi ngCHENG２ , Hong ＧwuSONG２ , Hong Ｇwe iLU３ ,
( １． Schoo lo fMa t e r i a l sSc i enc eandEng i ne e r i ng,Da l i anUn i ve r s i t fTe chno l ogy,Da l i an１１６０２４,L i aon i ng,Ch i na; yo ２． I ns t i t u t eo fMe t a lRe s e a r ch,Ch i ne s eAc ademyo fSc i enc e s,Shenyang１１００１６,L i aon i ng,Ch i na; ３．Fo r l an t,Angang He avy Ma ch i neCo ,Lt d ,Anshan１１４０４２,L i aon i ng,Ch i na) geP Ab s t r a c t:Thehe a l i ngbehav i o ro fmi c r opo r e si npowde rme t a l l u r P/M)３１６Ls t a i n l e s ss t e e ldu r i ngho tf o r i ng gy ( g andsubs e the a tt r e a tmen twa ss t ud i ed．Ther e su l t sshowedt ha tho tf o r i ngc animp r ovet hehomogene i t ft he quen g yo r es i z eandenhanc et her e l a t i vedens i t fma t e r i a li nva r i ngdeg r e eduet od i f f e r en tf o r i ngt empe r a t u r e s．Asar e Ｇ po yo y g su l to fde f o rma t i onandd i f f us i onbond i nga th i empe r a t u r e,t hei r r egu l a rpo r e s we r esphe r o i d i z edandf i na l l ght y t u r nedi n t os t ab l ei nne rg r a i npo r e s．Thec ompa r i s ono fc omp r e s s i onbehav i o rbe twe enP/Mandwr ough tdens ema t e Ｇ r i a l sha sshownt ha tt hepo r e sc ane i t he rbet heobs t a c l e so fd i s l o c a t i onmovemen to rbet henuc l e a t i ons i t e sa c c e l e r a Ｇ t i ngt her e c r s t a l l i z a t i ona c c o r d i ngt ot hed i f f e r enc eo fde f o rma t i ont empe r a t u r e s． y Ke r d s:powde rme t a l l u r t a i n l e s ss t e e l;mi c r o s t r uc t u r a le vo l u t i on;dens i f i c a t i onbehav i o r;mi c r opo r e gy;３１６Ls ywo
Powde r me t a l l u r P/M )s t a i n l e s ss t e e l sa r e gy ( t het i c a lc a n d i d a t e sf o ri n d u s t r i a la l i c a t i o n si n yp pp med i c a l,au t omo t i ve,ands t ruc t u r a lapp l i c a t i onsdue t ot he i renhanc edp r ope r t i e ssucha simp r ovedma t e Ｇ , , r i a lhomogene i t e f i ned g r a i ns i z e and ne a r Ｇne t y r shapec apab i l i t i chl e adst oc o s t Ｇ e f f e c t i vecompo Ｇ ywh [ ,] nen t s１ ２ ．Manyt e chno l og i e shavebe enapp l i edt o f abr i c a t et heP/M ma t e r i a l s,i nc l ud i ngho ti sos t a t i c r e s s i ng (HIP)andp r e s su r e l e s ss i n t e r i ng． Lowcos t p [ ３] i st he pr ima r f i to fp r e s su r e l e s ss i n t e r i ng ． y bene Howeve r,t heho ti s o s t a t i cp r e s s i ngt e chn i s queha somed i f f i cu l t i e ssucha spowde rpa ck i ngandd i ede Ｇ s i i c hw i l ls ub s t a n t i a l l n f l u e n c et h ef i n a ls c a l e gn,wh yi [] andshapeo fap r oduc t４ ．Pr e s su r e l e s ss i n t e r ed ma Ｇ t e r i a l sc anno tr e a cht hef u l ldens i t l l yandusua yhave [] apo r o s i t sh i s１５％ i nt hes i n t e r eds t a t e５ ． ya gha Fur t he rmo r e,t hep r e s enc eo fl a r to fpo r e s geamoun i n pr e s su r e l e s ss i n t e r i ng pa r t s wi l la c ta sde f e c t s, wh i chwi l lde c r e a s et heme chan i c a lp r ope r t i e so ft he [ ６] , ma t e r i a l ．Ont heo t he rhand s e c onda r rk i ng y wo t e chn i s( s u c ha spowd e rf o r i ng,powd e rr o l l i ng, que g
andpowde rex t rus i on)c ans i i f i c an t l et he gn yenhanc [ ７] , r e l a t i vedens i t o fm a t e r i a l s ． I n e n e r a l d e n s i f i c a Ｇ y g t i ono ft hep r e Ｇ a l l oyedpowde r sbypr e s sur e l e s ss i n Ｇ t e r i ng,f o l l owedbys e conda r rk i ngsucha scon Ｇ y wo ven t i ona lf o r i ng,i sane f f e c t i veme t hodf o rpr odu Ｇ g [] c i ngpa r t swi t henhanc edme chan i c a lp r ope r t i e s８ ． The r ea r e al o to f ma c r os cop i c cons t i t u t i ve mode l swh i cha r ef unc t i onso ft hevon Mi s e se f f e c Ｇ t i ves t r e s sandex t e rna lp r e s sur ef o rp r ed i c t i ngt he dens i t f P/M ma t e r i a l s dur i ng t he s e conda r yo y [ ９] wo rk i ngp r oc e s s．KuhnandFe r r a i ve Ｇ guson andDo [ ] l ue ta l １０ p r opos edy i e l dc r i t e r i ons,wh i chcons i d Ｇ e r edt hee f f e c to fhyd r os t a t i cs t r e s sont hey i e l dbe Ｇ hav i o ro fpo r ousma t e r i a l sby mod i f i ngt hevon Mi Ｇ y [ １１] s e sy i e l dl ocus．Fl e ck deve l oped a cons t i t u t i ve mode lbyt ak i ngbo t ht hel oad i ngpa t handr e l a t i ve [ ] dens i t n t oa c coun t．Yue ta l １２ and Madouand yi [ ] Leb l ond１３ de r i vedy i e l dc r i t e r i af o rpo r ousma t e r i a l s , bycons i de r i ngt hepo r e sa ssphe r e sande l l i s e r e Ｇ p spe c t i ve l r,t he r ea r ef ew de t a i l ed wo rks y．Howeve wh i chs t ud i edt hee f f e c t so fp r oc e s s i ngpa r ame t e r s
Founda t i onI t em: I t em Spons o r edbyNa t i ona lBa s i cRe s e a r chPr og r amo fCh i na ( ２０１２CB６１９６００) B i o r aphy: Yi ngKAN,Do c t o r; E Ｇma i l: a c cn; Re c e i v e dDa t e:June４,２０１３ ykan＠imr g Co r r e s i ngAu t ho r: Mi ngCHENG,Do c t o r,As s o c i a t eRe s e a r chFe l l ow; E Ｇma i l:mcheng＠imr a c cn pond
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
ont her e l a t i vedens i t c r o s t ruc t u r a levo l u t i on yandmi dur i ngho two rk i ngandsubs equen the a tt r e a tmen t． I nt hepr e s en two rk,un i ax i a lc omp r e s s i ont e s t and he a tt r e a tmen t we r ec a r r i ed ou tt os t udy t he he a l i ngbehav i o ro f mi c r opo r e sand mi c r o s t ruc t ur a l evo l u t i oni nP/M ３１６Ls t a i n l e s ss t e e la t８００,１０００ and１２００ ℃．Thee f f e c to f mi c r opo r e sont hede Ｇ f o rma t i onbehav i o ro fP/M ma t e r i a lwa ss t ud i edby compa r i ngt ha twi t ht hewr ough tma t e r i a l．
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１ Exp e r imen t a lPr o c e du r e Tab l e１showst hechemi c a lcompos i t i ono ft he ３１６Ls t a i n l e s ss t e e lpowde r sus edi nt h i swo rk．The r shaveanave r agepa r t i c l es i z eo f３９ ７ μm． powde F i １s howst hes c ann i nge l e c t r on mi c r o s c opy ( SEM) g mo r l ogyo ft hepowde r s． I tc anbes e ent ha tt he pho r t i c l e sa r esphe r i c a li nshape． pa The３１６Ls t a i n l e s ss t e e lpowd e r swe r emi xedwi t h
Tab l e１ Ch emi c a lc ompo s i t i ono f３１６Ls t a i n l e s ss t e e lpowd e r ma s s％ Cr
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Ni
１１ ３
Mo
２ １３
Mn
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P
≤０ ０４５
S i
≤０ ７５
S
≤０ ０３
C
０ ０２１
Fe
Ba l anc e
Thespe c imen wa squenchedby wa t e rt or e t a i nt he mi c r os t ruc t ur e whent hede f o rma t i on wa scomp l e Ｇ t ed．Thespe c imenswe r ecu tpa r a l l e lt ot heax i sand e l e c t r o l t i c a l l t chedi nt heso l u t i oncon t a i n i ng１０％ y ye oxa l i ca c i dand９０％ wa t e rf o rmi c r os t ruc t ur eobs e r Ｇ va t i on．He a tt r e a tme n twa sdon ei nv a c uuma t１２００ ℃ f o r１hus i ngt hef o r c imens． gedspe Themi c r os t ruc t ur ewa sobs e r vedunde ranop t i Ｇ c a lmi c r os cope．Thedens i t sme a sur edbyd r a i n Ｇ y wa ageme t hodus i ngane l e c t r on i cba l anc ewi t ht hea c Ｇ cur a c f０ ００００１g．Mo r et hant hr e et e s t swe r epe r Ｇ yo f o rmedf o re a chda t um po i n t．Thef o l l owi ng equa Ｇ , F i １ S EM r a h so f３ １ ６ Ls t a i n l e s s s t e e l o w d e r s t i o n sw e r eu s e dt oc a l c u l a t e t h ed e n s i t y g g p p m１ ( D ＝ １) w z i ncs t e a r a t eo f１％ i n ma s sf o r２handc ompr e s s ed m３ －m２ρ i n t oac l i nd r i c a ld i e wi t ha p r e s su r eo f１００ MPa． whe y st hed e n s i t st hes amp l ema s si na i r; r e,D i y;m１i ; Thec ompa c twa s５０ mmi nd i ame t e rand１３ mmi n m２i st hes amp l ema s si nwa t e r m３i st hema s so ft he he i twi t har e l a t i veg r e endens i t f０ ７１． gh yo st hed e n s i t fwa t e r． s amp l ec on t a i n i ngwa t e r;andρwi yo , Topr oduc easpe c imenf o rf o r i n t h e c o m a c t g g p ２ Re s u l t sandDi s c u s s i on wa ss i n t e r e da t１３６０℃f o r１hi nv a c uum． Du r i ngs i n t e Ｇ r i ngp r o c e s s,t het empe r a t u r ewa sr a i s edt o６００ ℃ a t ２ １ Mi c r o s t r u c t u r a le vo l u t i ono fP/M３１６Ls t a i n l e s s / , ar a t eo f５℃ mi nf o rd e ox i d i z a t i on． Th e nt h et emp e r a Ｇ s t e e ldu r i ngho tf o r i ng g t u r ewa sr a i s e dupt o１３６０ ℃ a tar a t eo f１０ ℃/mi n． F i ２showst hemi c r os t ruc t ur eo ft he３１６Ls t a Ｇ g Af t e rt ha t,t hema t e r i a lwa sma ch i nedi n t o１０mmi n d i ame t e rand１２mmi nhe i t,wi t har e l a t i vedens i Ｇ gh t f０ ８４． yo Theho tun i ax i a lc omp r e s s i ont e s twa sc a r r i ed ou tonaGl e eb l e３８００t he rmo Ｇme chan i c a ls imu l a t o r, wh i chc anbeus edt oc on t r o ld i sp l a c emen tandt emＧ r a t u r ea c cu r a t e l t a l umf o i l swe r eus edt ode Ｇ pe y．Tan c r e a s et hef r i c t i oni nf o r i ngp r o c e s s． g Th ef o r i ng wa sdon ea t８００,１０００,a nd１２００ ℃ g , , wi t ht het rues t r a i no f０ ４ ０ ８ and１ ２,r e spe c Ｇ t i ve l e et e s t swe r ec a r r i edou tf o re a chde f o rmＧ y．Thr a t i onc ond i t i ons．Thespe c imen wa she a t edt ot e s t F i ２ Mi c r o s t r u c t u r eo ft h e３１６Ls t a i n l e s ss t e e l g / s i n t e r e da t１３６０ ℃ f o r１h t empe r a t u r ea tar a t eo f５ ℃ sandhe l df o r１８０s．
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J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
i n l e s ss t e e ls i n t e r eda t１３６０ ℃f o r１h． I tc anbes e en t ha tt he po r e sd i s t r i bu t eun i f o rml hr oughou tt he yt r a i ns t ruc t u r e．Mo s to ft hei n i t i a lpo r e sa r esma l l e r g t han２５μm,ands omeo ft hepo r e sa r el a r ea n d i r Ｇ g r egu l a ri nshape． Be c aus eo ft hee f f e c to ff r i c t i on be twe ent he , endso fspe c imen and d i e s t hes t r a i no ft he de Ｇ f o rmedspe c imeni sd i f f e r en t,wh i chi sl a r s ti nt he ge c en t e randsma l l e s ti nt heend．The r e f o r e,t he mi Ｇ c r os t ruc t u r ei nt hec en t e ro fspe c imen wa sus edt o ana l et heevo l u t i ono fpo r e sa td i f f e r en ts t r a i nsand yz t empe r a t u r e s． Themi c r o s t ruc t u r eo f３１６Ls t a i n l e s ss t e e lunde r d i f f e r en ts t r a i nsa t８００ ℃ i sshowni nF i ３．I n g F i ３( a),t he r ewe r es omei r r egu l a rl a r r e sand g gepo
l i t t l esphe r i c a lpo r e s．Ast het rues t r a i ni nc r e a s edt o , , ０ ８ t hel a r r e sd i s appe a r ed andl ongp i Ｇ l i ke gepo pe r e sappe a r eda tg r a i nbounda r i e s．Theo r i en t a t i on po o ft he p i Ｇ l i ke po r e s wa s ma i n l r i cu l a rt o pe ype pend t he comp r e s s i on d i r e c t i on．When t he t rue s t r a i n r e a ched１ ２,t hes i z eo fpo r e sde c r e a s ed,andt he r a i n swe r es t r e t c he di n t ot hef l owl i n emi c r o s t r u c t u r e． g F i ４showst he mi c r os t ruc t ur eo f３１６Ls t a i n Ｇ g l e s ss t e e lde f o rmeda t１０００ ℃ und e rd i f f e r e n ts t r a i n s． I tc anbes e ent ha tt he r ea r el a r r e s whent he gepo s t r a i ni s０ ４．Ast hes t r a i ngoe supt o０ ８,t hemo r Ｇ l ogyo fpo r e sont heg r a i nbounda r r ene ck l a c e Ｇ pho ya , l i ke someo fwh i cha r ebr okeni n t osphe r i c a lpo r e s． Whent hes t r a i ni s１ ２,t hepo r e sa r ea l ls epa r a t ed andd i s t r i bu t edun i f o rml y．
F i ３ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t８００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
F i ４ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t１０００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
F i ５ showst he mi c r o s t ruc t u r eo fs t a i n l e s s g s t e e lde f o rmeda t１２００ ℃． I tc anbes e enf r omF i ５ g t ha tsma l lsphe r i c a lpo r e sandl a r r e scoex i s t ge po whent hes t r a i ni s０ ４．Howeve r,a lmo s ta l lt he r e sa r esphe r i c a l whent hes t r a i ni sl a r rt han po ge ０ ８．Be s i de s,t he r ea r es ome de f o rma t i on twi ns
wh i chc anc aus et heha rden i ng o ft he ma t e r i a la c Ｇ [ １４] co rd i ngt ot hep r ev i ouswo rko fRemy ． Fo rt hel a r es i z epo r e s,t he mov i ngd i s l o c a t i on s g a r ep i l eda r oundt hepo r e sdur i ngt hede f o rma t i on． The r e f o r e,t he r ea r es t r e s sconc en t r a t i on po i n t si n t hev i c i n i t fl a r r e s．Whent hepo r e sde f o rmed yo gepo
F i ５ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t１２００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
t oal ongp i Ｇ l i keshape,t hede s t ab i l i z a t i onoc cur s pe a tt hepo i n t swhe r et hes t r e s sc onc en t r a t i onr e a che s t heb r e akl imi to ft hepo r e． Be s i de s,al ongp i Ｇ l i ke pe r e wi l l be i n t e r s e c t ed by g r a i n and subg r a i n po , bounda r i e s wh i chc anr e su l ti nt hef o rma t i ono fr i d Ｇ sduet ot hed i f f us i onchanne le f f e c to fg r a i nand ge subg r a i nbounda r i e s．Theb r e akageo ft hel ongp i Ｇ pe l i kepo r eo c cu r sp r e f e r en t i a l l tt her i dge st ha tt ou Ｇ ya ched wi t ht he po r esu r f a c e．Asar e su l t,t hel ong i Ｇ l i kepo r eb r e aksi n t os eve r a ls e c t i onswh i chc an p pe b ed imi n i s h e dbyt h es ub s e e n td e f o rma t i ona ndhe a t qu t r e a tmen t．Thus,du r i ngt hede f o rma t i on,t hei n Ｇ c r e a s eo fg r a i nbounda r i e sandd i s l o c a t i onsc anp r o Ｇ mo t et heb r e akageo fp i Ｇ l i kepo r e s．F i pe g．６shows t hes chema t i cd i ag r am f o rt he evo l u t i on o f po r e s dur i ngc ompr e s s i on．
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shou l dbeno t edt ha tl a r r e s s i onde f o rma t i on gecomp , l e adst or educ t i onandco l l aps eo fl a r r e s and gepo t hus,imp r ove st hehomogene i t ft hepo r es i z eand yo d i s t r i bu t i on． ２ ２ Compr e s s i on beha v i o ro fP/M ３１６L s t a i n l e s s s t e e ldu r i ngho tf o r i ng g F i ７ shows t he comp r e s s i on behav i o ro f g wr ough t and P/M ma t e r i a l dur i ng de f o rma t i on r oc e s s． I tc anbes e enf r om F i ７t ha tt hede f o rma Ｇ p g t i onbehav i o r so ft hema t e r i a la r ed i f f e r en tduet ova Ｇ r i oust empe r a t ur e s．Fr om F i ３t o５,i ti sknown gs t ha tde f o rma t i onc ansubs t an t i a l l educ et hef r a c Ｇ yr t i onands i z eo fpo r e s,and mo r eimpo r t an t l n Ｇ y,i c r e a s et hed i f f us i onbond i ngve l oc i t fpo r e swhen yo t het empe r a t ur ei sr e l a t i ve l i he o t he r yh gh．On t hand,po r e sc anr educ et hef l ows t r e s so ft hema t e r i Ｇ a lt osomedeg r e e,a sshowni nF i ７．The r e f o r e,a l l g o ft heabove men t i onede l emen t sc anl e adt od i f f e r Ｇ en tde f o rma t i onbehav i o r s．
F i ６ S ch ema t i cd r awi ngf o re v o l u t i ono f g apo r edu r i ngho tf o r i ng g
Fu r t he robs e r va t i ono fF i ４( a)i nd i c a t e st ha ta g sma l lnumbe ro fr e c r t a l l i z edg r a i nsappe a ri nt he ys a r e a swh i cha r ef a rf r omt hel a r r e s．Du r i ngi n i Ｇ gepo t i a lc omp r e s s i on,t he ma t r i xde f o rmsnon Ｇ un i f o rmＧ l hede f o rma t i onc anno tbet r ans f e r r edt ot he y,andt ma t r i xa r oundl a r r e s． I ti swe l lknownt ha tdy Ｇ gepo nami cr e c r s t a l l i z a t i o no c c u r s i n３ １ ６ Ls t a i n l e s ss t e e l y whent hes t r a i nexc e edsac r i t i c a lva l ue．The r e f o r e, i tc anbec onc l udedt ha tt he ma t r i xi nt her e l a t i ve l y dens ea r e ade f o rmedf i r s ts i nc et he r ea r edynami cr e Ｇ c r t a l l i z ed g r a i ns．The ma t r i xa r oundl a r r e s ys ge po wi l lde f o rmonc et hef r a c t i onandt hes i z eo fpo r e si n r e l a t i ve l ea r e a sa r esma l lenough． ydens I tc anbes e enf r om F i ３( a),４( a),and５( a) gs t ha tt hepo r e sd i s t r i bu t eno tqu i t eun i f o rml y when , t hes t r a i ni ssma l l wh i l el a r f o rma t i onc animＧ gede r ovet hehomogene i t ft hepo r e si nt hema t r i x,a s p yo showni nF i ３( b),４( b),and５( b)． I ngene r a l, gs t hed i s t r i bu t i ono ft hepo r e si sa f f e c t edbyt hed i s t r i Ｇ bu t i ono fi n i t i a lpa r t i c l es i z e． I nt hep r e s en tc a s e,t he r e shaved i f f e r en ts i z e sbe c aus et hepa r t i c l es i z ei s po no tun i f o rm bu tf i t st hel og Ｇ no rma ld i s t r i bu t i on． I t
F i ７ S t r e s s Ｇ s t r a i nc u r v e sa td i f f e r en tt emp e r a t u r e s g
I ngene r a l,t he r ea r ewo rkha rden i ngandso f Ｇ t en i ng compe t i ngi nt he ho tde f o rma t i on p r oc e s s． Thede f o rma t i ont empe r a t ur eha sanobv i ouse f f e c t ont hef l owcur ve． I tc anbes e enf r omF i ７t ha tt he g f l owcur vet e s t eda t８００ ℃ exh i b i t scon t i nuouswo rk ha rden i ng wh i chi ss t r onga tt hee a r l d e f o r m a t i o n y s t ageandwe aka tt hel a t ede f o rma t i ons t age．HowＧ eve r,f l owcur ve st e s t eda t１０００and１２００ ℃ exh i b i t so f t en i ng a f t e rt he pe ak s t r e s s wh i chi sd i f f e r en t f r omt hecur vet e s t eda t８００ ℃． I ti sbe l i evedt ha tt her e l a t i onsh i twe ent rue pbe s t r e s sandt r u es t r a i ns a t i s f i e st heHo l l omone a t i on, qu ( σ＝Kεn ２) , ; ; whe r e σi st het rues t r e s s εi st het rues t r a i n Ki s s t r eng t hf a c t o r;andni st hes t r a i nha rden i ngexpo Ｇ nen twh i chr e f l e c t st heha rden i ngl eve lo ft hema t e Ｇ ( ) ( ) r i a la f t e ry i e l d i ng．Eq ２ t urnsi n t oEq ３ byt ak Ｇ i ngl oga r i t hmonbo t hs i de so fEq ( ２)．
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
８６６
( l n σ＝l nK ＋nl n ε ３) , anbec a l cu l a t edwi t ht he The r e f o r et heva l ueo fn c s l opeo fl n σＧ l n εcur veus i ngt hel i ne a rf i tme t hod． F i ８showst hel n σＧ l n εcur ve so fwr ough tand g / P M ma t e r i a l sdu r i ngt hec omp r e s s i onp r o c e s s． Ri s t hel i ne a r l tc oe f f i c i en t． I tc anbes e ent ha t ydependen , σＧ l n εcur ve wh i chi s t he r ea r etwos l ope sone a chl n s imi l a rt ot ha to ftwo Ｇ s ema t e r i a l s． I ti sno t i c e a Ｇ pha
b l et ha tt he r ea r en e a t i v es l op e sa t１０００and１２００ ℃ , g wh i c h me ant ha tt he ma t e r i a li sso f t en i ng a tt h i s s t age．Th i si si nag r e emen twi t ht heobs e r va t i ono f , mi c r os t ruc t ur ei nF i ４ i n w h i c ht h er e c r s t a l l i z e d g y r a i nswe r ef oundt obet he ma i nso f t en i ng me cha Ｇ g n i sm o f３１６Ls t a i n l e s ss t e e la t１０００ ℃．Howeve r, t he r ea r el i t t l er e c r t a l l i z edg r a i nsi nF i ５,wh i ch ys g me anst ha tt hes o f t e n i ng me c han i smi sg r a i ng r owt h．
( a)８００ ℃ ; ( b)１０００ ℃ ; ( c)１２００ ℃．
F i ８ l n σＧ l n εc u r v e so fwr ough tandP/M ma t e r i a l sdu r i ngc omp r e s s i onp r o c e s s g
T a b l e２s howst h ec omp a r i s ono fn v a l u e sb e twe e n P/M and wr ough t ma t e r i a l sa td i f f e r en tt empe r a Ｇ t ur e si nde t a i l． I tc anbes e enf r om Tab l e２t ha ta l lo f t henva l ue so fP/M ma t e r i a la r el a r rt hant hos eo f ge wr ough tma t e r i a la tt hef i r s ts t age．Att hes e cond , , / s t age howeve r t hen va l ue so fP M ma t e r i a la r e sma l l e rt hant ho s eo fwr ough tma t e r i a la t１０００ ℃． Theevo l u t i ono fnva l uei st her e su l to ft hec ompe t i Ｇ t i onbe twe enha r den i ngands o f t en i ngsubs t an t i a l l y c aus edbyd i s l o c a t i on． I ti swe l lknownt ha tt hed i s Ｇ
l oc a t i oni sf i r s t l i l edupt oape ak,andt hende Ｇ yp c r e a s edbydynami cr e c r s t a l l i z a t i o no rd n a m i cr e Ｇ y y cove r nt heho tde f o rma t i onp r oc e s s．Compa r edt o yi wr ough tma t e r i a l,t hemi c r opo r e si nt heP/Ms t a i n Ｇ l e s ss t e e limpedet hed i s l oc a t i on movemen tandl e ad t op i l i ngupo fd i s l oc a t i ona tt hebeg i nn i ngo ft heho t de f o rma t i onp r oc e s s．Ast hede f o rma t i ongo i ngon, r e sa r er educ edt oasma l ls i z e,wh i chc ane i t he r po
bet henuc l e a t i ons i t e sa c c e l e r a t i ngt her e c r t a l l i z a Ｇ ys t i ono rbet heimped imen t so fd i s l oc a t i onmovemen t．
Tab l e２ Compa r i s ono fn v a l u e sb e twe enP/Mandwr ough tma t e r i a l sa td i f f e r en tt emp e r a t u r e s /℃ Tempe r a t ur e ８００
１０００
１２００
na ts t age１ ０ ４７６６
０ １６５１６
０ １６５６９
P/M ma t e r i a l
na ts t age２
R
０ １６２２
０ ９９
－０ ０４５６９
０ ９９
－０ ２６８９６
２ ３ Re l a t i v ed e n s i t v o l u t i ono fP/M３１６Ls t a i n l e s s ye s t e e ldu r i ngho tf o r i ng g Ther a t eo fdens i t anbec a l cu l a t edu Ｇ ychangec s i ngt hef o l l owi ngequa t i oni nt heun i ax i a lc omp r e s Ｇ s i onpr o c e s s, ( d １－２ υp) d ε１ ４) ρ/ ρ＝－ ( ; whe r e,ρi st her e l a t i vedens i t d ε i st h ee f f e c t i ve y １ ; s t r a i ni nc r emen t υpi sPo i s s onr a t i owh i chi sde f i ned
０ ９９
na ts t age１ ０ ４２６６９ ０ ０９０５
０ １０７７５
Wr ough tma t e r i a l
na ts t age２
R
０ ０４７５７
０ ９９
－０ ２３５１２
０ ９９
－０ ０４１１４
０ ９９
２ ( υp＝０ ５ ５) ρ , I n gene r a l sma l ls t r a i nha rden i ngexponen tn wi l lde c r e a s et heva l ueo ft hePo i s sonr a t i owh i chr e Ｇ f l e c t st hef l ow behav i o ro fma t e r i a l s．Be s i de s,h i gh t empe r a t ur ede f o rma t i onc ansubs t an t i a l l c r e a s e y de t hen va l ueduet odynami cr e cove r cr e Ｇ yanddynami , c r s t a l l i z a t i o n ． F u r t h e r m o r e as m a l l n w i l l l e a dt o y mo r ehomogeneouss t r a i ni n P/M ma t e r i a ldur i ng f o rma t i on．Thus,t heexponen ti nEq ( ５)shou l d a st her a t i obe twe enr ad i a landax i a ls t r a i ns． I ti sbe Ｇ de b eaf u n c t i o n o ft e m e r a t u r e w h i c hc a na f f e c tt h e l i evedt ha tt he r ei sar e l a t i onsh i twe ent hePo i s Ｇ p pbe , l ueo fn．The r e f o r et henewr e l a t i onsh i twe en pbe sonr a t i oandt her e l a t i vedens i t r Ｇ va y．KuhnandFe gu [] s son９ pr opo s ed an emp i r i c a lequa t i on by un i ax i a l υpandρi a( T) ( υp＝０ ５ ６) compr e s s i onexpe r imen ta t３７１ ℃． ρ
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
８６７
whe r ea( T )i sapa r ame t e rwh i chc anbeexp r e s s ed ( ) a sEq ７ byf i t t i ngexpe r imen t a lda t a． a( T )＝１/[ １１ ５１３２４１３－１ ５３２６３８０８２× ( l og( ７) T )] ( ) ( ) Subs t i t u t i ngEq ６ i n t oEq ４ g i ve s d ρ ( ε１ ８) ( ) ＝－d １－ρa T ) ρ( Eq ( ９)c anbeus edt op r ed i c tt hedens i t fi Ｇ yo [] r onpowde ri nho tun i ax i a lc omp r e s s i onp r o c e s s９ :
d ρ ( ＝－d ε１ ９) １－ρ２) ρ( Fr om Eq ( ８),i tc anbec onc l udedt ha tun i ax i a l compr e s s i onc anno tmakef u l l ough tma t e r i a l se Ｇ y wr veni ft hes t r a i ni sve r a r e l a t i onsh i Ｇ yl ge．Ther p be twe ens t r a i nandr e l a t i vedens i t anbeob t a i nedby yc ( ) i n t eg r a t i ngEq ８ ． F i ９s howst h ec omp a r i s onb e twe e ne xp e r ime n t a l g andc a l c u l a t e dr e s u l tu s i ng Eq s ( ８)a nd ( ９)whe nt he i n i t i a l l e l a t i v ed e n s i t s０ ８５．I tc a nb es e e nf r om yr yi F i ９t ha tt he r ei st hes amet endenc twe ent he g y be c a l cu l a t edcu r veandexpe r imen t a lr e su l t si n wh i ch t her e l a t i vedens i t sc on t i nuous l nc r e a s i ng wi t h yi yi t hes t r a i n． F i ９a l s oshowst ha tt her e l a t i vedens i t g y i nc r e a s e swi t ht empe r a t u r ea tag i vens t r a i n．Ther e l Ｇ a t i vedens i t l u t i ono ft he ma t e r i a li nd i c a t e st ha t yevo t hedens i t ft hema t e r i a li sr e l a t i ve l i f t e rde Ｇ yo yh gha f o rma t i on．Thema t e r i a ldens i t si nc r e a s i ngbe c aus e yi o fc omb i nedt he rma land me chan i c a le f f e c t sdur i ng h i empe r a t u r ec omp r e s s i onp r o c e s s,wh i ch make ght l a r r e se s s en t i a l l o l l aps eandi nc r e a s et hed i f Ｇ gepo yc f us i onbond i ngve l o c i t fsma l lpo r e s． yo
F i ９ Re l a t i v ed en s i t v o l u t i ono fP/M３１６Ls t a i n l e s s g ye s t e e la td i f f e r en tt emp e r a t u r e s
Thec a l cu l a t edandexpe r imen t a lr e su l t sa r eno t , t i t a t i ve l i s t en ta ss howni nF i ９． I ts hou l d quan ycons g b eno t e dt ha tEq ( ９)wa se s t ab l i s he dt hr ought her e Ｇ , su l t so fi r onpowde r wh i chha sar e l a t i ve l t an t ycons va l uewhende f o rmedi nah i empe r a t ur er ange． ght Howeve r,i ti s be l i evedt ha tt het empe r a t ur ec an changet hes t r a i nha rden i ngexponen to f３１６Ls t a i n Ｇ l e s ss t e e ls i i f i c an t l r e f o r e,t he r ea r eobv i ous gn y．The d i f f e r enc e si nr e l a t i ve dens i t i e so f spe c imens de Ｇ f o rmeda td i f f e r en tt empe r a t ur e s． ２ ４ Mi c r o s t r u c t u r eandden s i t e sa f t e rhe a t ychang t r e a tmen t F i １０showst he mi c r os t ruc t ur e so fde f o rmed g ３１６Ls t a i n l e s ss t e e la f t e rhe a tt r e a tmen ta t１２００ ℃ f o r１h． I tc anbes e enf r om F i １０( a)and１０( b) gs t ha tmos to ft hepo r e sa r esphe r i c a l,andsomeo ft he
F i １０ Mi c r o s t r u c t u r eo f３１６Ls t a i n l e s ss t e e lf o r e da t１２００ ℃ a f t e rh e a tt r e a tmen twi t hs t r a i no f g g ０ ４ ( a),０ ８ ( b),and１ ２ ( c)
r e sa r ei r r egu l a ri nshape．Ca r e f u lobs e r va t i ono f po ( ) F i １ ０ c i n d i c a t e st h a t m o s to ft h e o r e si nt h e g p r a i na r esphe r i c a li nshapeandsma l li ns i z e． I tc an g bec onc l udedt ha tt hesphe r o i d i z a t i on o fi r r egu l a r r e so c cu r sdu r i ngt hehe a tt r e a tmen tduet ot he po e f f e c to fd i f f us i on and t he s t ab l e mo r l ogy o f pho r e si ssphe r i c a l． po F i １１showst hedens i t sa f t e rhe a t g y change t r e a tmen t． I tc an bes e ent ha t whent hes t r a i ni s
f i xed,t hei nc r e a s eddens i t ohe a tt r e a tmen t y duet wa sl a r e ri nt h e m a t e r i a lw h i c hi sc o m r e s s e da t g p l owt empe r a t ur e．Th i sr e su l tsugge s t edt ha tde f o rmＧ a t i ona tl ow t empe r a t ur es t o r ed mo r ede f o rma t i on ene r sd r i v i ngf o r c eo fpo r eshr i nkage． F i １１a l Ｇ gya g soshowst ha tt hedens i t s wi t hi nc r e a s i ng y change s t r a i na tt hes amede f o rma t i ont empe r a t ur e． I ngen Ｇ , e r a lt hed i s so l u t i ono fpo r e si nhe a tt r e a tmen ti sa k i ndo fd i f f us i onp r oc e s s,i nwh i cht hed r i v i ngf o r c e
８６８
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
F i １１ Den s i t e sa f t e rh e a tt r e a tmen t g ychang
ha rdne s schangea f t e rhe a tt r e a tmen t．I tshou l dbe no t edt ha tl ow t empe r a t ur ede f o rma t i onc ans t o r e mo r ede f o rma t i onene r hanh i empe r a t ur ede Ｇ gyt ght f o rma t i on．The r e f o r e,t he r ei st hel a r e s th a r d n e s s g changeo ft hema t e r i a lde f o rmeda t８００ ℃ a f t e rhe a t t r e a tmen t,wh i chc anr e l e a s et hes t o r edde f o rma t i on ene r t he rmo r e,i tc an beconc l udedt ha tno gy．Fur de f o rma t i onene r sr e l e a s eda th i empe r a t ur e gywa ght sucha s１２００ ℃ duet onoha rdne s schangeo ft he ma t e r i a l．Theha rdne s schange sa l soag r e ewe l lwi t h t her e l a t i vedens i t sa f t e rhe a tt r e a tmen t,a s ychange showni nF i １１． g
３ Conc l u s i on
i sc ruc i a l． Be c aus et hed i s l o c a t i onsa r ep i l edupi nt he sur r ound i nga r e ao fpo r e sdu r i ng de f o rma t i on,de Ｇ f o rma t i onene r anbes t o r edi nt hed i s l o c a t i ons, gyc wh i chc ano f f e rad r i v i ngf o r c ef o rd i f f us i onshr i nk Ｇ ageo fpo r e s． I ti sd i f f i cu l tf o rt heshr i nkageo fpo r e s l oc a t edi nag r a i nduet ot hel ow d i s l o c a t i ondens i t y sur r ound i ngt hei nne rg r a i npo r e s．Re s i dua ls t r e s si s r oduc eda f t e rc omp r e s s i onf o rt hes t r a i nssu r r ound Ｇ p i ngde f o rmed po r e sa r eno tun i f o rm．I ti sno t ab l e t ha tt hesha r o rne r so fade f o rmed po r eha st he pc l a r s tr e s i dua ls t r e s sa st he d r i v i ng f o r c e wh i ch ge l e adst ot hesphe r o i d i z a t i ono ft hepo r e．The r e f o r e, t hed r i v i ngf o r c e sa r ede f o rma t i onene r e s i dua l gy,r , , s t r e s s su r f a c eene r npu the a t wh i chsub Ｇ gy andi
I nt h i swo r k,t hehe a l i ngb e ha v i o ro fmi c r opo r e si n e rme t a l l u r t a i n l e s ss t e e ldu r i ngho tf o r Ｇ powd gy３１６Ls g i ngands ub s e e n the a tt r e a tme n tha sb e e ns t ud i e d． The qu r e s u l t si nd i c a t et ha tho tf o r i ngc ans i i f i c an t l g gn yimＧ r ovet hehomogene i t ft hepo r es i z eandhe l o p yo pt enhanc et her e l a t i vedens i t ft he ma t e r i a li nva r Ｇ yo i n d e r e ea c c o r d i n t od i f f e r e n tf o r i n t e m e r a Ｇ y g g g g g p t ur e s．Thei r r egu l a rpo r e sa r esphe r o i d i z edandf i na l Ｇ l urni n t or oundi nne rg r a i npo r e sduet ot hede Ｇ yt f o rma t i onand d i f f us i on a th i ht e m e r a t u r e ．T h e g p e f f e c to fpo r e sont hecomp r e s s i onbehav i o rc ane i Ｇ t he rbet heimped imen t so fd i s l oc a t i onmovemen to r bet henuc l e a t i ons i t e so ft her e c r s t a l l i z a t i o na c c o r d Ｇ y i ngt ot hed i f f e r e n c eo fd e f o rma t i ont emp e r a t u r e s．
２ ５ Ha r dne s schang e sa f t e rhe a tt r e a tme n t F i １２showst heHVha r dne s sevo l u t i ono ft he g ma t e r i a lbe f o r eanda f t e rhe a tt r e a tmen ta saf unc Ｇ t i ono fs t r a i n． I tc anbes e enf r om F i １２t ha tt he g ha r dne s so ft hema t e r i a la f t e rhe a tt r e a tmen ti sl owＧ e rt hant ho s eo ft he ma t e r i a l sa f t e rde f o rma t i onex Ｇ , c ep tt hema t e r i a lde f o rmeda t１２００ ℃ wh i chha sno
[ １] S．Bende r,V．Cha l i vend r a,N．Rahba r,S．El Wak i l,I n t．J． Eng． Sc i． ５３ ( ２０１２)６７ Ｇ ７３． [ i,Sc r i t a Ma t e r．６２ ２] J．Zou,GJ．Zhang,YM．Kan,T．Oh j p ( ２０１０)１５９ Ｇ １６２． [ ３] D．Buc eva c,B．Ma t ov i c,B． Bab i c,V．Kr s t i c,Ma t e r． Sc i．Eng． A５２８ ( ２０１１)２０３４ Ｇ ２０４１． [ ４] C G．Hj o r t h,J． I r onS t e e lRe s． I n t． １４ ( ２００７)Supp l． １,１２１ Ｇ １２５． [ ５] J．J a i n,AM．Ka r,A．Upadhyaya,Ma t e r．Le t t．５８ ( ２００４)
s t an t i a l l et hed i f f us i onc oe f f i c i en t． yenhanc
F i １２ HVha r dn e s se v o l u t i ono fP/M３１６Ls t a i n l e s ss t e e l g b e f o r eanda f t e rh e a tt r e a tmen t
Re f e r enc e s:
２０３７ Ｇ ２０４０． [ ６] H．Bo r t r öm,L．Nybo r I r onS t e e lRe s． I n t．１５ ( ２００７) gs g,J． Supp l． １,７０ Ｇ ７６． [ ７] A．Ra e shkannan,KS．Pandey,S．Shanmugam,R．Na r ay Ｇ j ana s amy,J． I r onS t e e lRe s． I n t． １５ ( ２００８)No ５,８１ Ｇ ８７． [ ８] T L．Nga i,YH．Kuang,YY．L i,J． I r onS t e e lRe s． I n t． １４ ( ２００７)Supp l． １,７７ Ｇ ８１． [ ９] HA．Kuhn,B L．Fe r on,Powde rFo r i ng,Un i ve r s i t f gus g yo Ca l i f o r n i a:Me t a lPowd e rI ndu s t r i e sFe d e r a t i on,Be r k e l e y,１９９０． [ １０] S M．Do r a i ve l u,HL．Gege l,J S．Guna s eke r a,J C．Ma l a s, J T．Mo r r ,I n t． J．Me ch． Sc i． ２６ ( １９８４)５２７ Ｇ ５３５． ganJ [ １１] NA．F l e ck,J．Me ch．Phys． So l i ds４３ ( １９９５)１４０９ Ｇ １４３１． [ １２] X M．Yu,W． Cheng,J．P． Chen,Theo r．App l． Fr a c t．Me ch． ５９ ( ２０１２)５７ Ｇ ６１． [ １３] K．Madou,J B．Leb l ond,J．Me ch．Phys．So l i ds６０ ( ２０１２) １０３７ Ｇ １０５８． [ １４] L．Remy,Me t a l l．Tr ans．A１２ ( １９８１)３８７ Ｇ ４０８．
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ９):８ ６ ２ Ｇ ８ ６ ８
He a l i ngBe ha v i o ro fMi c r opo r e si nPowd e rMe t a l l u r g y３１６L S t a i n l e s sS t e e ldu r i ngHo tFo r i ngandHe a tTr e a tme n t g Yi ng KAN１ ２ , Sh i Ｇ hongZHANG２ , L i ＧwenZHANG１ , Mi ngCHENG２ , Hong ＧwuSONG２ , Hong Ｇwe iLU３ ,
( １． Schoo lo fMa t e r i a l sSc i enc eandEng i ne e r i ng,Da l i anUn i ve r s i t fTe chno l ogy,Da l i an１１６０２４,L i aon i ng,Ch i na; yo ２． I ns t i t u t eo fMe t a lRe s e a r ch,Ch i ne s eAc ademyo fSc i enc e s,Shenyang１１００１６,L i aon i ng,Ch i na; ３．Fo r l an t,Angang He avy Ma ch i neCo ,Lt d ,Anshan１１４０４２,L i aon i ng,Ch i na) geP Ab s t r a c t:Thehe a l i ngbehav i o ro fmi c r opo r e si npowde rme t a l l u r P/M)３１６Ls t a i n l e s ss t e e ldu r i ngho tf o r i ng gy ( g andsubs e the a tt r e a tmen twa ss t ud i ed．Ther e su l t sshowedt ha tho tf o r i ngc animp r ovet hehomogene i t ft he quen g yo r es i z eandenhanc et her e l a t i vedens i t fma t e r i a li nva r i ngdeg r e eduet od i f f e r en tf o r i ngt empe r a t u r e s．Asar e Ｇ po yo y g su l to fde f o rma t i onandd i f f us i onbond i nga th i empe r a t u r e,t hei r r egu l a rpo r e s we r esphe r o i d i z edandf i na l l ght y t u r nedi n t os t ab l ei nne rg r a i npo r e s．Thec ompa r i s ono fc omp r e s s i onbehav i o rbe twe enP/Mandwr ough tdens ema t e Ｇ r i a l sha sshownt ha tt hepo r e sc ane i t he rbet heobs t a c l e so fd i s l o c a t i onmovemen to rbet henuc l e a t i ons i t e sa c c e l e r a Ｇ t i ngt her e c r s t a l l i z a t i ona c c o r d i ngt ot hed i f f e r enc eo fde f o rma t i ont empe r a t u r e s． y Ke r d s:powde rme t a l l u r t a i n l e s ss t e e l;mi c r o s t r uc t u r a le vo l u t i on;dens i f i c a t i onbehav i o r;mi c r opo r e gy;３１６Ls ywo
Powde r me t a l l u r P/M )s t a i n l e s ss t e e l sa r e gy ( t het i c a lc a n d i d a t e sf o ri n d u s t r i a la l i c a t i o n si n yp pp med i c a l,au t omo t i ve,ands t ruc t u r a lapp l i c a t i onsdue t ot he i renhanc edp r ope r t i e ssucha simp r ovedma t e Ｇ , , r i a lhomogene i t e f i ned g r a i ns i z e and ne a r Ｇne t y r shapec apab i l i t i chl e adst oc o s t Ｇ e f f e c t i vecompo Ｇ ywh [ ,] nen t s１ ２ ．Manyt e chno l og i e shavebe enapp l i edt o f abr i c a t et heP/M ma t e r i a l s,i nc l ud i ngho ti sos t a t i c r e s s i ng (HIP)andp r e s su r e l e s ss i n t e r i ng． Lowcos t p [ ３] i st he pr ima r f i to fp r e s su r e l e s ss i n t e r i ng ． y bene Howeve r,t heho ti s o s t a t i cp r e s s i ngt e chn i s queha somed i f f i cu l t i e ssucha spowde rpa ck i ngandd i ede Ｇ s i i c hw i l ls ub s t a n t i a l l n f l u e n c et h ef i n a ls c a l e gn,wh yi [] andshapeo fap r oduc t４ ．Pr e s su r e l e s ss i n t e r ed ma Ｇ t e r i a l sc anno tr e a cht hef u l ldens i t l l yandusua yhave [] apo r o s i t sh i s１５％ i nt hes i n t e r eds t a t e５ ． ya gha Fur t he rmo r e,t hep r e s enc eo fl a r to fpo r e s geamoun i n pr e s su r e l e s ss i n t e r i ng pa r t s wi l la c ta sde f e c t s, wh i chwi l lde c r e a s et heme chan i c a lp r ope r t i e so ft he [ ６] , ma t e r i a l ．Ont heo t he rhand s e c onda r rk i ng y wo t e chn i s( s u c ha spowd e rf o r i ng,powd e rr o l l i ng, que g
andpowde rex t rus i on)c ans i i f i c an t l et he gn yenhanc [ ７] , r e l a t i vedens i t o fm a t e r i a l s ． I n e n e r a l d e n s i f i c a Ｇ y g t i ono ft hep r e Ｇ a l l oyedpowde r sbypr e s sur e l e s ss i n Ｇ t e r i ng,f o l l owedbys e conda r rk i ngsucha scon Ｇ y wo ven t i ona lf o r i ng,i sane f f e c t i veme t hodf o rpr odu Ｇ g [] c i ngpa r t swi t henhanc edme chan i c a lp r ope r t i e s８ ． The r ea r e al o to f ma c r os cop i c cons t i t u t i ve mode l swh i cha r ef unc t i onso ft hevon Mi s e se f f e c Ｇ t i ves t r e s sandex t e rna lp r e s sur ef o rp r ed i c t i ngt he dens i t f P/M ma t e r i a l s dur i ng t he s e conda r yo y [ ９] wo rk i ngp r oc e s s．KuhnandFe r r a i ve Ｇ guson andDo [ ] l ue ta l １０ p r opos edy i e l dc r i t e r i ons,wh i chcons i d Ｇ e r edt hee f f e c to fhyd r os t a t i cs t r e s sont hey i e l dbe Ｇ hav i o ro fpo r ousma t e r i a l sby mod i f i ngt hevon Mi Ｇ y [ １１] s e sy i e l dl ocus．Fl e ck deve l oped a cons t i t u t i ve mode lbyt ak i ngbo t ht hel oad i ngpa t handr e l a t i ve [ ] dens i t n t oa c coun t．Yue ta l １２ and Madouand yi [ ] Leb l ond１３ de r i vedy i e l dc r i t e r i af o rpo r ousma t e r i a l s , bycons i de r i ngt hepo r e sa ssphe r e sande l l i s e r e Ｇ p spe c t i ve l r,t he r ea r ef ew de t a i l ed wo rks y．Howeve wh i chs t ud i edt hee f f e c t so fp r oc e s s i ngpa r ame t e r s
Founda t i onI t em: I t em Spons o r edbyNa t i ona lBa s i cRe s e a r chPr og r amo fCh i na ( ２０１２CB６１９６００) B i o r aphy: Yi ngKAN,Do c t o r; E Ｇma i l: a c cn; Re c e i v e dDa t e:June４,２０１３ ykan＠imr g Co r r e s i ngAu t ho r: Mi ngCHENG,Do c t o r,As s o c i a t eRe s e a r chFe l l ow; E Ｇma i l:mcheng＠imr a c cn pond
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
ont her e l a t i vedens i t c r o s t ruc t u r a levo l u t i on yandmi dur i ngho two rk i ngandsubs equen the a tt r e a tmen t． I nt hepr e s en two rk,un i ax i a lc omp r e s s i ont e s t and he a tt r e a tmen t we r ec a r r i ed ou tt os t udy t he he a l i ngbehav i o ro f mi c r opo r e sand mi c r o s t ruc t ur a l evo l u t i oni nP/M ３１６Ls t a i n l e s ss t e e la t８００,１０００ and１２００ ℃．Thee f f e c to f mi c r opo r e sont hede Ｇ f o rma t i onbehav i o ro fP/M ma t e r i a lwa ss t ud i edby compa r i ngt ha twi t ht hewr ough tma t e r i a l．
８６３
１ Exp e r imen t a lPr o c e du r e Tab l e１showst hechemi c a lcompos i t i ono ft he ３１６Ls t a i n l e s ss t e e lpowde r sus edi nt h i swo rk．The r shaveanave r agepa r t i c l es i z eo f３９ ７ μm． powde F i １s howst hes c ann i nge l e c t r on mi c r o s c opy ( SEM) g mo r l ogyo ft hepowde r s． I tc anbes e ent ha tt he pho r t i c l e sa r esphe r i c a li nshape． pa The３１６Ls t a i n l e s ss t e e lpowd e r swe r emi xedwi t h
Tab l e１ Ch emi c a lc ompo s i t i ono f３１６Ls t a i n l e s ss t e e lpowd e r ma s s％ Cr
１６ ４９
Ni
１１ ３
Mo
２ １３
Mn
１ ６０
P
≤０ ０４５
S i
≤０ ７５
S
≤０ ０３
C
０ ０２１
Fe
Ba l anc e
Thespe c imen wa squenchedby wa t e rt or e t a i nt he mi c r os t ruc t ur e whent hede f o rma t i on wa scomp l e Ｇ t ed．Thespe c imenswe r ecu tpa r a l l e lt ot heax i sand e l e c t r o l t i c a l l t chedi nt heso l u t i oncon t a i n i ng１０％ y ye oxa l i ca c i dand９０％ wa t e rf o rmi c r os t ruc t ur eobs e r Ｇ va t i on．He a tt r e a tme n twa sdon ei nv a c uuma t１２００ ℃ f o r１hus i ngt hef o r c imens． gedspe Themi c r os t ruc t ur ewa sobs e r vedunde ranop t i Ｇ c a lmi c r os cope．Thedens i t sme a sur edbyd r a i n Ｇ y wa ageme t hodus i ngane l e c t r on i cba l anc ewi t ht hea c Ｇ cur a c f０ ００００１g．Mo r et hant hr e et e s t swe r epe r Ｇ yo f o rmedf o re a chda t um po i n t．Thef o l l owi ng equa Ｇ , F i １ S EM r a h so f３ １ ６ Ls t a i n l e s s s t e e l o w d e r s t i o n sw e r eu s e dt oc a l c u l a t e t h ed e n s i t y g g p p m１ ( D ＝ １) w z i ncs t e a r a t eo f１％ i n ma s sf o r２handc ompr e s s ed m３ －m２ρ i n t oac l i nd r i c a ld i e wi t ha p r e s su r eo f１００ MPa． whe y st hed e n s i t st hes amp l ema s si na i r; r e,D i y;m１i ; Thec ompa c twa s５０ mmi nd i ame t e rand１３ mmi n m２i st hes amp l ema s si nwa t e r m３i st hema s so ft he he i twi t har e l a t i veg r e endens i t f０ ７１． gh yo st hed e n s i t fwa t e r． s amp l ec on t a i n i ngwa t e r;andρwi yo , Topr oduc easpe c imenf o rf o r i n t h e c o m a c t g g p ２ Re s u l t sandDi s c u s s i on wa ss i n t e r e da t１３６０℃f o r１hi nv a c uum． Du r i ngs i n t e Ｇ r i ngp r o c e s s,t het empe r a t u r ewa sr a i s edt o６００ ℃ a t ２ １ Mi c r o s t r u c t u r a le vo l u t i ono fP/M３１６Ls t a i n l e s s / , ar a t eo f５℃ mi nf o rd e ox i d i z a t i on． Th e nt h et emp e r a Ｇ s t e e ldu r i ngho tf o r i ng g t u r ewa sr a i s e dupt o１３６０ ℃ a tar a t eo f１０ ℃/mi n． F i ２showst hemi c r os t ruc t ur eo ft he３１６Ls t a Ｇ g Af t e rt ha t,t hema t e r i a lwa sma ch i nedi n t o１０mmi n d i ame t e rand１２mmi nhe i t,wi t har e l a t i vedens i Ｇ gh t f０ ８４． yo Theho tun i ax i a lc omp r e s s i ont e s twa sc a r r i ed ou tonaGl e eb l e３８００t he rmo Ｇme chan i c a ls imu l a t o r, wh i chc anbeus edt oc on t r o ld i sp l a c emen tandt emＧ r a t u r ea c cu r a t e l t a l umf o i l swe r eus edt ode Ｇ pe y．Tan c r e a s et hef r i c t i oni nf o r i ngp r o c e s s． g Th ef o r i ng wa sdon ea t８００,１０００,a nd１２００ ℃ g , , wi t ht het rues t r a i no f０ ４ ０ ８ and１ ２,r e spe c Ｇ t i ve l e et e s t swe r ec a r r i edou tf o re a chde f o rmＧ y．Thr a t i onc ond i t i ons．Thespe c imen wa she a t edt ot e s t F i ２ Mi c r o s t r u c t u r eo ft h e３１６Ls t a i n l e s ss t e e l g / s i n t e r e da t１３６０ ℃ f o r１h t empe r a t u r ea tar a t eo f５ ℃ sandhe l df o r１８０s．
８６４
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
i n l e s ss t e e ls i n t e r eda t１３６０ ℃f o r１h． I tc anbes e en t ha tt he po r e sd i s t r i bu t eun i f o rml hr oughou tt he yt r a i ns t ruc t u r e．Mo s to ft hei n i t i a lpo r e sa r esma l l e r g t han２５μm,ands omeo ft hepo r e sa r el a r ea n d i r Ｇ g r egu l a ri nshape． Be c aus eo ft hee f f e c to ff r i c t i on be twe ent he , endso fspe c imen and d i e s t hes t r a i no ft he de Ｇ f o rmedspe c imeni sd i f f e r en t,wh i chi sl a r s ti nt he ge c en t e randsma l l e s ti nt heend．The r e f o r e,t he mi Ｇ c r os t ruc t u r ei nt hec en t e ro fspe c imen wa sus edt o ana l et heevo l u t i ono fpo r e sa td i f f e r en ts t r a i nsand yz t empe r a t u r e s． Themi c r o s t ruc t u r eo f３１６Ls t a i n l e s ss t e e lunde r d i f f e r en ts t r a i nsa t８００ ℃ i sshowni nF i ３．I n g F i ３( a),t he r ewe r es omei r r egu l a rl a r r e sand g gepo
l i t t l esphe r i c a lpo r e s．Ast het rues t r a i ni nc r e a s edt o , , ０ ８ t hel a r r e sd i s appe a r ed andl ongp i Ｇ l i ke gepo pe r e sappe a r eda tg r a i nbounda r i e s．Theo r i en t a t i on po o ft he p i Ｇ l i ke po r e s wa s ma i n l r i cu l a rt o pe ype pend t he comp r e s s i on d i r e c t i on．When t he t rue s t r a i n r e a ched１ ２,t hes i z eo fpo r e sde c r e a s ed,andt he r a i n swe r es t r e t c he di n t ot hef l owl i n emi c r o s t r u c t u r e． g F i ４showst he mi c r os t ruc t ur eo f３１６Ls t a i n Ｇ g l e s ss t e e lde f o rmeda t１０００ ℃ und e rd i f f e r e n ts t r a i n s． I tc anbes e ent ha tt he r ea r el a r r e s whent he gepo s t r a i ni s０ ４．Ast hes t r a i ngoe supt o０ ８,t hemo r Ｇ l ogyo fpo r e sont heg r a i nbounda r r ene ck l a c e Ｇ pho ya , l i ke someo fwh i cha r ebr okeni n t osphe r i c a lpo r e s． Whent hes t r a i ni s１ ２,t hepo r e sa r ea l ls epa r a t ed andd i s t r i bu t edun i f o rml y．
F i ３ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t８００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
F i ４ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t１０００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
F i ５ showst he mi c r o s t ruc t u r eo fs t a i n l e s s g s t e e lde f o rmeda t１２００ ℃． I tc anbes e enf r omF i ５ g t ha tsma l lsphe r i c a lpo r e sandl a r r e scoex i s t ge po whent hes t r a i ni s０ ４．Howeve r,a lmo s ta l lt he r e sa r esphe r i c a l whent hes t r a i ni sl a r rt han po ge ０ ８．Be s i de s,t he r ea r es ome de f o rma t i on twi ns
wh i chc anc aus et heha rden i ng o ft he ma t e r i a la c Ｇ [ １４] co rd i ngt ot hep r ev i ouswo rko fRemy ． Fo rt hel a r es i z epo r e s,t he mov i ngd i s l o c a t i on s g a r ep i l eda r oundt hepo r e sdur i ngt hede f o rma t i on． The r e f o r e,t he r ea r es t r e s sconc en t r a t i on po i n t si n t hev i c i n i t fl a r r e s．Whent hepo r e sde f o rmed yo gepo
F i ５ Mi c r o s t r u c t u r eo fP/M３１６Ls t a i n l e s ss t e e ld e f o rme da t１２００ ℃ wi t hs t r a i no f０ ４ ( a),０ ８ ( b),and１ ２ ( c) g
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
t oal ongp i Ｇ l i keshape,t hede s t ab i l i z a t i onoc cur s pe a tt hepo i n t swhe r et hes t r e s sc onc en t r a t i onr e a che s t heb r e akl imi to ft hepo r e． Be s i de s,al ongp i Ｇ l i ke pe r e wi l l be i n t e r s e c t ed by g r a i n and subg r a i n po , bounda r i e s wh i chc anr e su l ti nt hef o rma t i ono fr i d Ｇ sduet ot hed i f f us i onchanne le f f e c to fg r a i nand ge subg r a i nbounda r i e s．Theb r e akageo ft hel ongp i Ｇ pe l i kepo r eo c cu r sp r e f e r en t i a l l tt her i dge st ha tt ou Ｇ ya ched wi t ht he po r esu r f a c e．Asar e su l t,t hel ong i Ｇ l i kepo r eb r e aksi n t os eve r a ls e c t i onswh i chc an p pe b ed imi n i s h e dbyt h es ub s e e n td e f o rma t i ona ndhe a t qu t r e a tmen t．Thus,du r i ngt hede f o rma t i on,t hei n Ｇ c r e a s eo fg r a i nbounda r i e sandd i s l o c a t i onsc anp r o Ｇ mo t et heb r e akageo fp i Ｇ l i kepo r e s．F i pe g．６shows t hes chema t i cd i ag r am f o rt he evo l u t i on o f po r e s dur i ngc ompr e s s i on．
８６５
shou l dbeno t edt ha tl a r r e s s i onde f o rma t i on gecomp , l e adst or educ t i onandco l l aps eo fl a r r e s and gepo t hus,imp r ove st hehomogene i t ft hepo r es i z eand yo d i s t r i bu t i on． ２ ２ Compr e s s i on beha v i o ro fP/M ３１６L s t a i n l e s s s t e e ldu r i ngho tf o r i ng g F i ７ shows t he comp r e s s i on behav i o ro f g wr ough t and P/M ma t e r i a l dur i ng de f o rma t i on r oc e s s． I tc anbes e enf r om F i ７t ha tt hede f o rma Ｇ p g t i onbehav i o r so ft hema t e r i a la r ed i f f e r en tduet ova Ｇ r i oust empe r a t ur e s．Fr om F i ３t o５,i ti sknown gs t ha tde f o rma t i onc ansubs t an t i a l l educ et hef r a c Ｇ yr t i onands i z eo fpo r e s,and mo r eimpo r t an t l n Ｇ y,i c r e a s et hed i f f us i onbond i ngve l oc i t fpo r e swhen yo t het empe r a t ur ei sr e l a t i ve l i he o t he r yh gh．On t hand,po r e sc anr educ et hef l ows t r e s so ft hema t e r i Ｇ a lt osomedeg r e e,a sshowni nF i ７．The r e f o r e,a l l g o ft heabove men t i onede l emen t sc anl e adt od i f f e r Ｇ en tde f o rma t i onbehav i o r s．
F i ６ S ch ema t i cd r awi ngf o re v o l u t i ono f g apo r edu r i ngho tf o r i ng g
Fu r t he robs e r va t i ono fF i ４( a)i nd i c a t e st ha ta g sma l lnumbe ro fr e c r t a l l i z edg r a i nsappe a ri nt he ys a r e a swh i cha r ef a rf r omt hel a r r e s．Du r i ngi n i Ｇ gepo t i a lc omp r e s s i on,t he ma t r i xde f o rmsnon Ｇ un i f o rmＧ l hede f o rma t i onc anno tbet r ans f e r r edt ot he y,andt ma t r i xa r oundl a r r e s． I ti swe l lknownt ha tdy Ｇ gepo nami cr e c r s t a l l i z a t i o no c c u r s i n３ １ ６ Ls t a i n l e s ss t e e l y whent hes t r a i nexc e edsac r i t i c a lva l ue．The r e f o r e, i tc anbec onc l udedt ha tt he ma t r i xi nt her e l a t i ve l y dens ea r e ade f o rmedf i r s ts i nc et he r ea r edynami cr e Ｇ c r t a l l i z ed g r a i ns．The ma t r i xa r oundl a r r e s ys ge po wi l lde f o rmonc et hef r a c t i onandt hes i z eo fpo r e si n r e l a t i ve l ea r e a sa r esma l lenough． ydens I tc anbes e enf r om F i ３( a),４( a),and５( a) gs t ha tt hepo r e sd i s t r i bu t eno tqu i t eun i f o rml y when , t hes t r a i ni ssma l l wh i l el a r f o rma t i onc animＧ gede r ovet hehomogene i t ft hepo r e si nt hema t r i x,a s p yo showni nF i ３( b),４( b),and５( b)． I ngene r a l, gs t hed i s t r i bu t i ono ft hepo r e si sa f f e c t edbyt hed i s t r i Ｇ bu t i ono fi n i t i a lpa r t i c l es i z e． I nt hep r e s en tc a s e,t he r e shaved i f f e r en ts i z e sbe c aus et hepa r t i c l es i z ei s po no tun i f o rm bu tf i t st hel og Ｇ no rma ld i s t r i bu t i on． I t
F i ７ S t r e s s Ｇ s t r a i nc u r v e sa td i f f e r en tt emp e r a t u r e s g
I ngene r a l,t he r ea r ewo rkha rden i ngandso f Ｇ t en i ng compe t i ngi nt he ho tde f o rma t i on p r oc e s s． Thede f o rma t i ont empe r a t ur eha sanobv i ouse f f e c t ont hef l owcur ve． I tc anbes e enf r omF i ７t ha tt he g f l owcur vet e s t eda t８００ ℃ exh i b i t scon t i nuouswo rk ha rden i ng wh i chi ss t r onga tt hee a r l d e f o r m a t i o n y s t ageandwe aka tt hel a t ede f o rma t i ons t age．HowＧ eve r,f l owcur ve st e s t eda t１０００and１２００ ℃ exh i b i t so f t en i ng a f t e rt he pe ak s t r e s s wh i chi sd i f f e r en t f r omt hecur vet e s t eda t８００ ℃． I ti sbe l i evedt ha tt her e l a t i onsh i twe ent rue pbe s t r e s sandt r u es t r a i ns a t i s f i e st heHo l l omone a t i on, qu ( σ＝Kεn ２) , ; ; whe r e σi st het rues t r e s s εi st het rues t r a i n Ki s s t r eng t hf a c t o r;andni st hes t r a i nha rden i ngexpo Ｇ nen twh i chr e f l e c t st heha rden i ngl eve lo ft hema t e Ｇ ( ) ( ) r i a la f t e ry i e l d i ng．Eq ２ t urnsi n t oEq ３ byt ak Ｇ i ngl oga r i t hmonbo t hs i de so fEq ( ２)．
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
８６６
( l n σ＝l nK ＋nl n ε ３) , anbec a l cu l a t edwi t ht he The r e f o r et heva l ueo fn c s l opeo fl n σＧ l n εcur veus i ngt hel i ne a rf i tme t hod． F i ８showst hel n σＧ l n εcur ve so fwr ough tand g / P M ma t e r i a l sdu r i ngt hec omp r e s s i onp r o c e s s． Ri s t hel i ne a r l tc oe f f i c i en t． I tc anbes e ent ha t ydependen , σＧ l n εcur ve wh i chi s t he r ea r etwos l ope sone a chl n s imi l a rt ot ha to ftwo Ｇ s ema t e r i a l s． I ti sno t i c e a Ｇ pha
b l et ha tt he r ea r en e a t i v es l op e sa t１０００and１２００ ℃ , g wh i c h me ant ha tt he ma t e r i a li sso f t en i ng a tt h i s s t age．Th i si si nag r e emen twi t ht heobs e r va t i ono f , mi c r os t ruc t ur ei nF i ４ i n w h i c ht h er e c r s t a l l i z e d g y r a i nswe r ef oundt obet he ma i nso f t en i ng me cha Ｇ g n i sm o f３１６Ls t a i n l e s ss t e e la t１０００ ℃．Howeve r, t he r ea r el i t t l er e c r t a l l i z edg r a i nsi nF i ５,wh i ch ys g me anst ha tt hes o f t e n i ng me c han i smi sg r a i ng r owt h．
( a)８００ ℃ ; ( b)１０００ ℃ ; ( c)１２００ ℃．
F i ８ l n σＧ l n εc u r v e so fwr ough tandP/M ma t e r i a l sdu r i ngc omp r e s s i onp r o c e s s g
T a b l e２s howst h ec omp a r i s ono fn v a l u e sb e twe e n P/M and wr ough t ma t e r i a l sa td i f f e r en tt empe r a Ｇ t ur e si nde t a i l． I tc anbes e enf r om Tab l e２t ha ta l lo f t henva l ue so fP/M ma t e r i a la r el a r rt hant hos eo f ge wr ough tma t e r i a la tt hef i r s ts t age．Att hes e cond , , / s t age howeve r t hen va l ue so fP M ma t e r i a la r e sma l l e rt hant ho s eo fwr ough tma t e r i a la t１０００ ℃． Theevo l u t i ono fnva l uei st her e su l to ft hec ompe t i Ｇ t i onbe twe enha r den i ngands o f t en i ngsubs t an t i a l l y c aus edbyd i s l o c a t i on． I ti swe l lknownt ha tt hed i s Ｇ
l oc a t i oni sf i r s t l i l edupt oape ak,andt hende Ｇ yp c r e a s edbydynami cr e c r s t a l l i z a t i o no rd n a m i cr e Ｇ y y cove r nt heho tde f o rma t i onp r oc e s s．Compa r edt o yi wr ough tma t e r i a l,t hemi c r opo r e si nt heP/Ms t a i n Ｇ l e s ss t e e limpedet hed i s l oc a t i on movemen tandl e ad t op i l i ngupo fd i s l oc a t i ona tt hebeg i nn i ngo ft heho t de f o rma t i onp r oc e s s．Ast hede f o rma t i ongo i ngon, r e sa r er educ edt oasma l ls i z e,wh i chc ane i t he r po
bet henuc l e a t i ons i t e sa c c e l e r a t i ngt her e c r t a l l i z a Ｇ ys t i ono rbet heimped imen t so fd i s l oc a t i onmovemen t．
Tab l e２ Compa r i s ono fn v a l u e sb e twe enP/Mandwr ough tma t e r i a l sa td i f f e r en tt emp e r a t u r e s /℃ Tempe r a t ur e ８００
１０００
１２００
na ts t age１ ０ ４７６６
０ １６５１６
０ １６５６９
P/M ma t e r i a l
na ts t age２
R
０ １６２２
０ ９９
－０ ０４５６９
０ ９９
－０ ２６８９６
２ ３ Re l a t i v ed e n s i t v o l u t i ono fP/M３１６Ls t a i n l e s s ye s t e e ldu r i ngho tf o r i ng g Ther a t eo fdens i t anbec a l cu l a t edu Ｇ ychangec s i ngt hef o l l owi ngequa t i oni nt heun i ax i a lc omp r e s Ｇ s i onpr o c e s s, ( d １－２ υp) d ε１ ４) ρ/ ρ＝－ ( ; whe r e,ρi st her e l a t i vedens i t d ε i st h ee f f e c t i ve y １ ; s t r a i ni nc r emen t υpi sPo i s s onr a t i owh i chi sde f i ned
０ ９９
na ts t age１ ０ ４２６６９ ０ ０９０５
０ １０７７５
Wr ough tma t e r i a l
na ts t age２
R
０ ０４７５７
０ ９９
－０ ２３５１２
０ ９９
－０ ０４１１４
０ ９９
２ ( υp＝０ ５ ５) ρ , I n gene r a l sma l ls t r a i nha rden i ngexponen tn wi l lde c r e a s et heva l ueo ft hePo i s sonr a t i owh i chr e Ｇ f l e c t st hef l ow behav i o ro fma t e r i a l s．Be s i de s,h i gh t empe r a t ur ede f o rma t i onc ansubs t an t i a l l c r e a s e y de t hen va l ueduet odynami cr e cove r cr e Ｇ yanddynami , c r s t a l l i z a t i o n ． F u r t h e r m o r e as m a l l n w i l l l e a dt o y mo r ehomogeneouss t r a i ni n P/M ma t e r i a ldur i ng f o rma t i on．Thus,t heexponen ti nEq ( ５)shou l d a st her a t i obe twe enr ad i a landax i a ls t r a i ns． I ti sbe Ｇ de b eaf u n c t i o n o ft e m e r a t u r e w h i c hc a na f f e c tt h e l i evedt ha tt he r ei sar e l a t i onsh i twe ent hePo i s Ｇ p pbe , l ueo fn．The r e f o r et henewr e l a t i onsh i twe en pbe sonr a t i oandt her e l a t i vedens i t r Ｇ va y．KuhnandFe gu [] s son９ pr opo s ed an emp i r i c a lequa t i on by un i ax i a l υpandρi a( T) ( υp＝０ ５ ６) compr e s s i onexpe r imen ta t３７１ ℃． ρ
I s sue９ He a l i ngBehav i o ro fMi c r opo r e si n３１６LS t a i n l e s sS t e e ldu r i ng Ho tFo r i ngand He a tTr e a tmen t g
８６７
whe r ea( T )i sapa r ame t e rwh i chc anbeexp r e s s ed ( ) a sEq ７ byf i t t i ngexpe r imen t a lda t a． a( T )＝１/[ １１ ５１３２４１３－１ ５３２６３８０８２× ( l og( ７) T )] ( ) ( ) Subs t i t u t i ngEq ６ i n t oEq ４ g i ve s d ρ ( ε１ ８) ( ) ＝－d １－ρa T ) ρ( Eq ( ９)c anbeus edt op r ed i c tt hedens i t fi Ｇ yo [] r onpowde ri nho tun i ax i a lc omp r e s s i onp r o c e s s９ :
d ρ ( ＝－d ε１ ９) １－ρ２) ρ( Fr om Eq ( ８),i tc anbec onc l udedt ha tun i ax i a l compr e s s i onc anno tmakef u l l ough tma t e r i a l se Ｇ y wr veni ft hes t r a i ni sve r a r e l a t i onsh i Ｇ yl ge．Ther p be twe ens t r a i nandr e l a t i vedens i t anbeob t a i nedby yc ( ) i n t eg r a t i ngEq ８ ． F i ９s howst h ec omp a r i s onb e twe e ne xp e r ime n t a l g andc a l c u l a t e dr e s u l tu s i ng Eq s ( ８)a nd ( ９)whe nt he i n i t i a l l e l a t i v ed e n s i t s０ ８５．I tc a nb es e e nf r om yr yi F i ９t ha tt he r ei st hes amet endenc twe ent he g y be c a l cu l a t edcu r veandexpe r imen t a lr e su l t si n wh i ch t her e l a t i vedens i t sc on t i nuous l nc r e a s i ng wi t h yi yi t hes t r a i n． F i ９a l s oshowst ha tt her e l a t i vedens i t g y i nc r e a s e swi t ht empe r a t u r ea tag i vens t r a i n．Ther e l Ｇ a t i vedens i t l u t i ono ft he ma t e r i a li nd i c a t e st ha t yevo t hedens i t ft hema t e r i a li sr e l a t i ve l i f t e rde Ｇ yo yh gha f o rma t i on．Thema t e r i a ldens i t si nc r e a s i ngbe c aus e yi o fc omb i nedt he rma land me chan i c a le f f e c t sdur i ng h i empe r a t u r ec omp r e s s i onp r o c e s s,wh i ch make ght l a r r e se s s en t i a l l o l l aps eandi nc r e a s et hed i f Ｇ gepo yc f us i onbond i ngve l o c i t fsma l lpo r e s． yo
F i ９ Re l a t i v ed en s i t v o l u t i ono fP/M３１６Ls t a i n l e s s g ye s t e e la td i f f e r en tt emp e r a t u r e s
Thec a l cu l a t edandexpe r imen t a lr e su l t sa r eno t , t i t a t i ve l i s t en ta ss howni nF i ９． I ts hou l d quan ycons g b eno t e dt ha tEq ( ９)wa se s t ab l i s he dt hr ought her e Ｇ , su l t so fi r onpowde r wh i chha sar e l a t i ve l t an t ycons va l uewhende f o rmedi nah i empe r a t ur er ange． ght Howeve r,i ti s be l i evedt ha tt het empe r a t ur ec an changet hes t r a i nha rden i ngexponen to f３１６Ls t a i n Ｇ l e s ss t e e ls i i f i c an t l r e f o r e,t he r ea r eobv i ous gn y．The d i f f e r enc e si nr e l a t i ve dens i t i e so f spe c imens de Ｇ f o rmeda td i f f e r en tt empe r a t ur e s． ２ ４ Mi c r o s t r u c t u r eandden s i t e sa f t e rhe a t ychang t r e a tmen t F i １０showst he mi c r os t ruc t ur e so fde f o rmed g ３１６Ls t a i n l e s ss t e e la f t e rhe a tt r e a tmen ta t１２００ ℃ f o r１h． I tc anbes e enf r om F i １０( a)and１０( b) gs t ha tmos to ft hepo r e sa r esphe r i c a l,andsomeo ft he
F i １０ Mi c r o s t r u c t u r eo f３１６Ls t a i n l e s ss t e e lf o r e da t１２００ ℃ a f t e rh e a tt r e a tmen twi t hs t r a i no f g g ０ ４ ( a),０ ８ ( b),and１ ２ ( c)
r e sa r ei r r egu l a ri nshape．Ca r e f u lobs e r va t i ono f po ( ) F i １ ０ c i n d i c a t e st h a t m o s to ft h e o r e si nt h e g p r a i na r esphe r i c a li nshapeandsma l li ns i z e． I tc an g bec onc l udedt ha tt hesphe r o i d i z a t i on o fi r r egu l a r r e so c cu r sdu r i ngt hehe a tt r e a tmen tduet ot he po e f f e c to fd i f f us i on and t he s t ab l e mo r l ogy o f pho r e si ssphe r i c a l． po F i １１showst hedens i t sa f t e rhe a t g y change t r e a tmen t． I tc an bes e ent ha t whent hes t r a i ni s
f i xed,t hei nc r e a s eddens i t ohe a tt r e a tmen t y duet wa sl a r e ri nt h e m a t e r i a lw h i c hi sc o m r e s s e da t g p l owt empe r a t ur e．Th i sr e su l tsugge s t edt ha tde f o rmＧ a t i ona tl ow t empe r a t ur es t o r ed mo r ede f o rma t i on ene r sd r i v i ngf o r c eo fpo r eshr i nkage． F i １１a l Ｇ gya g soshowst ha tt hedens i t s wi t hi nc r e a s i ng y change s t r a i na tt hes amede f o rma t i ont empe r a t ur e． I ngen Ｇ , e r a lt hed i s so l u t i ono fpo r e si nhe a tt r e a tmen ti sa k i ndo fd i f f us i onp r oc e s s,i nwh i cht hed r i v i ngf o r c e
８６８
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２１
F i １１ Den s i t e sa f t e rh e a tt r e a tmen t g ychang
ha rdne s schangea f t e rhe a tt r e a tmen t．I tshou l dbe no t edt ha tl ow t empe r a t ur ede f o rma t i onc ans t o r e mo r ede f o rma t i onene r hanh i empe r a t ur ede Ｇ gyt ght f o rma t i on．The r e f o r e,t he r ei st hel a r e s th a r d n e s s g changeo ft hema t e r i a lde f o rmeda t８００ ℃ a f t e rhe a t t r e a tmen t,wh i chc anr e l e a s et hes t o r edde f o rma t i on ene r t he rmo r e,i tc an beconc l udedt ha tno gy．Fur de f o rma t i onene r sr e l e a s eda th i empe r a t ur e gywa ght sucha s１２００ ℃ duet onoha rdne s schangeo ft he ma t e r i a l．Theha rdne s schange sa l soag r e ewe l lwi t h t her e l a t i vedens i t sa f t e rhe a tt r e a tmen t,a s ychange showni nF i １１． g
３ Conc l u s i on
i sc ruc i a l． Be c aus et hed i s l o c a t i onsa r ep i l edupi nt he sur r ound i nga r e ao fpo r e sdu r i ng de f o rma t i on,de Ｇ f o rma t i onene r anbes t o r edi nt hed i s l o c a t i ons, gyc wh i chc ano f f e rad r i v i ngf o r c ef o rd i f f us i onshr i nk Ｇ ageo fpo r e s． I ti sd i f f i cu l tf o rt heshr i nkageo fpo r e s l oc a t edi nag r a i nduet ot hel ow d i s l o c a t i ondens i t y sur r ound i ngt hei nne rg r a i npo r e s．Re s i dua ls t r e s si s r oduc eda f t e rc omp r e s s i onf o rt hes t r a i nssu r r ound Ｇ p i ngde f o rmed po r e sa r eno tun i f o rm．I ti sno t ab l e t ha tt hesha r o rne r so fade f o rmed po r eha st he pc l a r s tr e s i dua ls t r e s sa st he d r i v i ng f o r c e wh i ch ge l e adst ot hesphe r o i d i z a t i ono ft hepo r e．The r e f o r e, t hed r i v i ngf o r c e sa r ede f o rma t i onene r e s i dua l gy,r , , s t r e s s su r f a c eene r npu the a t wh i chsub Ｇ gy andi
I nt h i swo r k,t hehe a l i ngb e ha v i o ro fmi c r opo r e si n e rme t a l l u r t a i n l e s ss t e e ldu r i ngho tf o r Ｇ powd gy３１６Ls g i ngands ub s e e n the a tt r e a tme n tha sb e e ns t ud i e d． The qu r e s u l t si nd i c a t et ha tho tf o r i ngc ans i i f i c an t l g gn yimＧ r ovet hehomogene i t ft hepo r es i z eandhe l o p yo pt enhanc et her e l a t i vedens i t ft he ma t e r i a li nva r Ｇ yo i n d e r e ea c c o r d i n t od i f f e r e n tf o r i n t e m e r a Ｇ y g g g g g p t ur e s．Thei r r egu l a rpo r e sa r esphe r o i d i z edandf i na l Ｇ l urni n t or oundi nne rg r a i npo r e sduet ot hede Ｇ yt f o rma t i onand d i f f us i on a th i ht e m e r a t u r e ．T h e g p e f f e c to fpo r e sont hecomp r e s s i onbehav i o rc ane i Ｇ t he rbet heimped imen t so fd i s l oc a t i onmovemen to r bet henuc l e a t i ons i t e so ft her e c r s t a l l i z a t i o na c c o r d Ｇ y i ngt ot hed i f f e r e n c eo fd e f o rma t i ont emp e r a t u r e s．
２ ５ Ha r dne s schang e sa f t e rhe a tt r e a tme n t F i １２showst heHVha r dne s sevo l u t i ono ft he g ma t e r i a lbe f o r eanda f t e rhe a tt r e a tmen ta saf unc Ｇ t i ono fs t r a i n． I tc anbes e enf r om F i １２t ha tt he g ha r dne s so ft hema t e r i a la f t e rhe a tt r e a tmen ti sl owＧ e rt hant ho s eo ft he ma t e r i a l sa f t e rde f o rma t i onex Ｇ , c ep tt hema t e r i a lde f o rmeda t１２００ ℃ wh i chha sno
[ １] S．Bende r,V．Cha l i vend r a,N．Rahba r,S．El Wak i l,I n t．J． Eng． Sc i． ５３ ( ２０１２)６７ Ｇ ７３． [ i,Sc r i t a Ma t e r．６２ ２] J．Zou,GJ．Zhang,YM．Kan,T．Oh j p ( ２０１０)１５９ Ｇ １６２． [ ３] D．Buc eva c,B．Ma t ov i c,B． Bab i c,V．Kr s t i c,Ma t e r． Sc i．Eng． A５２８ ( ２０１１)２０３４ Ｇ ２０４１． [ ４] C G．Hj o r t h,J． I r onS t e e lRe s． I n t． １４ ( ２００７)Supp l． １,１２１ Ｇ １２５． [ ５] J．J a i n,AM．Ka r,A．Upadhyaya,Ma t e r．Le t t．５８ ( ２００４)
s t an t i a l l et hed i f f us i onc oe f f i c i en t． yenhanc
F i １２ HVha r dn e s se v o l u t i ono fP/M３１６Ls t a i n l e s ss t e e l g b e f o r eanda f t e rh e a tt r e a tmen t
Re f e r enc e s:
２０３７ Ｇ ２０４０． [ ６] H．Bo r t r öm,L．Nybo r I r onS t e e lRe s． I n t．１５ ( ２００７) gs g,J． Supp l． １,７０ Ｇ ７６． [ ７] A．Ra e shkannan,KS．Pandey,S．Shanmugam,R．Na r ay Ｇ j ana s amy,J． I r onS t e e lRe s． I n t． １５ ( ２００８)No ５,８１ Ｇ ８７． [ ８] T L．Nga i,YH．Kuang,YY．L i,J． I r onS t e e lRe s． I n t． １４ ( ２００７)Supp l． １,７７ Ｇ ８１． [ ９] HA．Kuhn,B L．Fe r on,Powde rFo r i ng,Un i ve r s i t f gus g yo Ca l i f o r n i a:Me t a lPowd e rI ndu s t r i e sFe d e r a t i on,Be r k e l e y,１９９０． [ １０] S M．Do r a i ve l u,HL．Gege l,J S．Guna s eke r a,J C．Ma l a s, J T．Mo r r ,I n t． J．Me ch． Sc i． ２６ ( １９８４)５２７ Ｇ ５３５． ganJ [ １１] NA．F l e ck,J．Me ch．Phys． So l i ds４３ ( １９９５)１４０９ Ｇ １４３１． [ １２] X M．Yu,W． Cheng,J．P． Chen,Theo r．App l． Fr a c t．Me ch． ５９ ( ２０１２)５７ Ｇ ６１． [ １３] K．Madou,J B．Leb l ond,J．Me ch．Phys．So l i ds６０ ( ２０１２) １０３７ Ｇ １０５８． [ １４] L．Remy,Me t a l l．Tr ans．A１２ ( １９８１)３８７ Ｇ ４０８．