By Hajime Suto* and Taku Murakami**

Transcription

1 On Gra an Aged Boundary Embrittlement 12-6-Fe Martensite By Hajime Su* Taku Murakami** Gra through Auger agg ment 375 occurred corresponded concluded gra segregion th 120 well creasg due spectroscopy. 20m orig most 300m agg age-hardeng heterogeneous demion. or On same 12-6-Fe gra h, se embrittlement rmo-mechanical gra facts was trement, quickly embrittle- accoustic emission above, stress concentrion demion studied described heterogeneous was proceeded Burst-type From This martensite temperure. embrittlement. age-hardeng or sured. controllg content an segregion it was is suppressed same ductility is creased. (Received Ⅰ. Mazch Introduction Various types maragg have been vestiged. Figure 1 is result obtaed Fioreen et al.(1), which reiion* etween notch- fl-tensilestrength Fe-18-8Co maragg s is shown when alloyed various elements impart age-hardenability 22, 1979) on m. As shown 1, beargs are very easilyembrittledaccompanyg gra fracture.tanaka et al.(2)suggested th this age-hardeng embrittlement was caused dislocionmotion which cut plelet bcc zones. Feng McMahon (3) poted out th segregions gra were reled embrittlement.but itis difficult consider th typicallymetallicelements havg high meltgpotssuch as weaken bondg strength gra same manner as metalloids, example, P S. presentpaper describes gra embrittlement an --Fe martensite more detail. Ⅱ. In Experimental present * Department Merials Science, Faculty Engeerg, Tohoku University, Sendai 980, Japan. ** Gradue School,Tohoku University,Sendai 980, Japan. Trans. JIM Steels se 2 vestige as embrittlement. were prepared knowledge Auger permed so vacuum S P m com- apply on a Table prepared improvements used shown Steels as almost chemical 3 were effects al. (1), 1 havg th is exame et s Feng carefully. order composition prepared position results alloy, same were 1 Relion between tensilestrength notch tensilestrength various types maragg (Floreen et al.). work, experimental 12-6-Fe Procedure gra 4 1. order 5 present ductility. spectroscopy Pa 1979 (S) was Vol. 20

2 366 Hajime Su Taku Murakami Table 1 Feng Chemical composition sused (wt%). et al.: 11.9, 5.75, 0.006C, 0.004P, beam 0.1mm diameter. peak height each element was normalized Fe 703eV peak height thisvalue iscalled "relivepeak height."figures 2 3 are conversion curves relive peak height. Figures 2 3 are conversioncurves relivepeak heights alloyconcentrion. apparent gra concentrionwas calculed from relive peak hight obtaedfrom 3 Conversion curve relivepeak height fracturedsurface, conversion curves vs content bulk. proportion gra area talfracturesurfacearea, con- sion was observed means accoustic emisvenience neglectg concentrion sion (). threshold level counter was controlled be 20mV. gradient segregionlayer. A more accure method calculion gra Ⅲ. Results Discussion concentrionhas been proposed aprevious paper(4). Figure 4 shows stress-stra curves demion behavior durg compres 1. It was proceeded 120 Conversion vs content curve bulk. relive peak height th brittle tergranular obtaed after da 4 apparent solid Stress-stra type. marks curves embrittlement agg transgranular changes gra durg 300m from shown changed 2 found remarkably mode dimples Figure agg Ar ion 375 fracture 5 shows a concentrions sputterg are da shown " 1"

3 On Gra Boundary Embrittlement an Aged 12-6-Fe near levels after 1 may ductile menu ion about important embrittlement 5 Change apparent concentrion 1 agg problem pot Feng et al. are diced dotted les. Although is slightlycreased agg, segregionremaed almost variant thisexperiment.in earlystage agg, very small proportion gra fracturesurface area tal fracturearea tends raisea problem on experimentalaccuracy. fracturemode 2 contags was similar 1 because itshigh content,but 3 contagp is easilyfracturedalong gra even as-quenched condition.ree segregionda are very reliable. dotted les 6 show results.both segregions as-quenched are small are creased rapidly agg 20m most. No change segregionoccurs even prolonged agg. surionlevel 3 isalmost equal th 1. ree, segregion 6 Change gra apparent concentrion 3 bulk 7. thickness is similar th s (7). It can be present results segregion affectg --Fe be vestiged shows Ar th is gra not an martensite. This must agg 8 ductility, from a different view. Figure enrich- fall removed zone facr rapidly were -Cr surface as shown from 367 creased 5nm segregion concluded be fracture temper-embrittled gra also range. sputterg, this Martensite pression decreased exceeded sted compressive count pro when stress. stress, pre-yieldg agg rapidly changes pro stage tensile hardness applied Tanaka com- stress et al. (2) th ductility s this type recovered over-scontag about 10% austenitemed reversed transmion grown particles over 30nm diameter.as is wellknown, burst type is caused heterogeneous demionsuch as tws discontuous slip.as shown 8, reisan correlion between counts loss ductility agg.phograph 1 (A) shows gracracks surface undulions heterogeneousdemionon side surface a compressedspecimen, (B) 7 peak tensityfrom an 1 as a function sputtered depth from fracturesurface.

4 368 Hajime Su Taku Murakami 8 Relionships hardness, shows among counts demion martensite ductility, agg tws pro med tensile yield-strength iscreased creasg resistivity glidemortion dislocionsunder sheargstress.on orh, strength brittle fracturealong graor cleavagestrengthismaly controlled cohesive strength between oms under tensilestress. It appears th cleavage strengthis generallylargerthan gra fracturestrengthdue ltice imperfections gra.ree, gra fracturemay be brought age-hardengout strengng cohesion between gra oms,, course,itwillbe promoted gra segregion metalloides or non-metallic clusionson. felydispersed coherentzones are found effectively crease yieldstrengthbut are easilycut dislocions. area a slipplane a zone is decreased dislocioncuttg, zone ismore more easilycut subsequent dislocions. Thus an avalanche-like demion is spped, stress concentrionmay promote gra fracture.a mechanism tw mion alloyshavg zones has been proposed Marchkowski etal.(6), thismay be applied thisfe-- alloy.of course, gra precipites tw-gra teractionalso or origs stress, lh mrix. Pho showg (A) Optical 2h. (B) demion micrograph Transmission tws. side surface a compressed micrograph 3% compressed 1 specimen 1 specimen,

5 On Gra Boundary Embrittlement stressconcentrion gra.from fdgsdescribedabove, we may conclude th orig controllggra embrittlement agg is stressconcentrion where heterogeneous demion is spped, although we cannot defitelydeterme which tws discontuousslipare more important. Figure 9 shows frequency burst type pre-yieldgstage decreases decreasgaustenitegrasize lowerg austenitizg temperure.this suggeststh gra sizerefementcan be used method improve ductility. an Aged 12-6-Fe Martensite 369 Figure 10 shows relion between counts strength three kds. Steel 1 was conventionally quenched s 4 5 were subjected hotswagg from 12 7mm diameter 1 pass followed direct quenchg. It was found th counts was remerkably decreased 9 Accoustic emission durg demion 1 havg variousgra sizes. 11 Stress-stra curves s 4 5 subjected rmo-mechanical trement. Pho. 10 Relionships counts counts decreasg among agg hardness, 460, re-mechanical pro showg stress, trement. 2 pressed straed lh Transmission up %, showg micrograph 460 small 30m tws com arrested

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