Microstructure, microhardness and impact toughness of low pressure carburized PM steels. E. Santuliana, C. Menapace, A. Molinari, G.

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1 Metllurgi delle polveri Microstructure, microhrdness nd impct toughness of low pressure crurized PM steels E. Sntulin, C. Menpce, A. Molinri, G. Meli The effect of low pressure crurizing (LPC) on two low lloyed sintered steels processed ccording to different routes to otin different mounts of open porosity ws studied. In presence of n open porosity, overcrurizing promotes the precipittion of grin oundry crides in the Cr steel, nd the formtion of retined ustenite in the Cr free one. In presence of fully closed porosity LPC results in the formtion of homogeneous cse. Even slight mount of open porosity cuses overcrurizing. In sence of overcrurizing, impct toughness is typicl of rittle ehvior, s expected. However, n unexpected high toughness is mesured in the supercrurized steel contining retined ustenite. Keywords: steel, sintering, secondry tretments INTRODUCTION Low pressure crurizing (LPC) is incresingly used for cse hrdening of steel prts. 1, 2, 3 The min dvntge of this process, in comprison to conventionl gs crurizing, is the sence of oxidizing gents in the tmosphere, which prevents oxidtion of steel on heting 4 nd even during isotherml soking t the crurizing temperture in the cse of chromium steels. 5 In LPC the cron crrier gs is either cetylene or propne. These gses dissocite t high temperture ( C) nd in vcuum (15mr) nd produce cron nd hydrogen (cetylene) or cron, hydrogen nd methne (propne). 1 Another dvntge is the use of gs quenching insted of oil quenching, which results in n increse in the effective cse depth nd in the reduction of distortion of pieces 6. The Powder Metllurgy (PM) industry is looking t LPC with growing interest for two min resons. First, quenching oil remins entrpped in the open porosity, nd hs to e eliminted efore tempering for environmentl resons. Second, chromium steels hve een recently introduced in the mrket for the production of structurl prts, nd re incresingly used y prt mnufcturers. A crurizing process which does not use quenching oil nd does not contin oxidizing gents in the tmosphere might represent gret opportunity to improve wer nd ftigue properties of PM chromium steels. A criticism hs however emerged, due to the unfvorle comintion of the lrge surfce re of porous PM prts on one side nd to the very high crurizing potentil of LPC on the other. The result of this comintion is overcrurizing, which results in the precipittion of grin oundry crides nd the formtion of retined ustenite. 7, 8, 9 To overcome such limittion, surfce densifiction y rolling is used to eliminte porosity in the surfce lyers. 8, 9 In this cse, the microstructure of the crurized lyers ecomes homogeneously mrtensitic. An lterntive to rolling my e represented y shot peening, E. Sntulin, C. Menpce, A. Molinri Deprtment of Mterils Engineering nd Industril technologies, University of Trento (Itly) G. Meli Dn Itli Sp, Arco, Trento (Itly) which promotes the densifiction of the surfce lyers. 10 Moreover, high density prts (over g/cm 3 ) contin previling closed residul porosity, then their surfce re is comprle to tht of fully dense ones. These two lterntives solutions were investigted in the present work to prevent overcrurizing. Specimens were produced y three processing routes: ) cold compction nd sintering, tht results in medium density with previling open porosity nd shll represent the supercrurized reference; ) wrm compction nd sintering, which results in higher density with minly closed porosity; c) cold compction, sintering nd surfce densifiction y shot peening. Since the microstructure of the overcrurized cse depends on the chemicl composition of the steel, two different mterils, widely used in the industril production, were investigted: Cr-Mo steel nd Mo-Ni-Cu steel. The former is expected to form crides, the ltter should form minly retined ustenite. The present work ims t evluting the effect of different porosity in the surfce lyers on the microstructure nd microhrdness, nd on the impct toughness of the crurized steels. EXPERIMENTAL PROCEDURE Tle 1 reports the nominl composition of the powders (Högnäs AB, Sweden) used for the preprtion of the specimens, long with the percent of dded grphite. Chrpy rs (55x10x10 mm) were produced y cold compction nd wrm compction, to otin green densities round 6.8g/cm 3 nd 7.3g/cm 3, respectively. Sintering ws crried out t 1120 C in wlking em furnce with 30 minutes isotherml holding in N 2 /H 2 tmosphere. Density nd porosity were determined ccording to ASTM B328 nd re reported in Tle 2. %Cr %Mo %Ni %Cu %C DLH TAB. 1 Nominl composition (wt.%) of the powder investigted. Composizione nominle (wt.%) delle polveri. L Metllurgi Itlin - n. 4/

2 Mteril cold compcted wrm compcted DLH cold compcted DLH wrm compcted TAB. 2 FIG. 1 ρ ε ε open [g/cm 3 ] [%] [%] 6.9± ± ± ± Density (ρ), totl porosity (ε) nd open porosity (ρ open ). Proprietà degli ccii sinterizzti: densità (ρ), porosità totle (ε) e porosità pert (ε open ). Microstructure of the shot peened with the surfce densified lyer. Microstruttur dell cciio dopo pllintur. As expected, porosity is minly interconnected in cold compcted mterils, whilst the wrm compcted ones re significntly different. Despite of the sme sintered density, porosity is prcticlly closed in the Cr steel, whilst it is prtilly interconnected in the other one. The cold compcted d sintered specimens were shot peened, with the following prmeters: intensity 12 Almen, shot ASH230, coverge 150%. Figure 1 shows, s n exmple, the microstructure of fter shot peening. The tretment produces full dense surfce lyer round 100 µm thick, with only few isolted micrometric pores. Low pressure crurizing (LPC) ws crried out in n industril furnce. Two different tretments were investigted, chrcterized y three nd five oost-nd-diffusion cycles, respectively. The crurizing temperture ws 945 C nd gs ws cetylene. Tretment Boost + diffusion time (minutes) I II III IV V LPC LPC TAB. 3 LPC tretments. Rissunto dei trttmenti di cementzione di ss pressione. Quenching ws crried out with nitrogen flux t 6 r. Tempering t 180 C for 2 hours ws crried out fter LPC. The two tretments re descried in Tle 3. The microstructurl investigtion of the smples ws crried out y the Light Opticl Microscope nd the Scnning Electron Microscope, fter the metllogrphic preprtion nd etching (Nitl-Picrl). X-Ry Diffrctometry ws used to investigte the microstructurl constituents of the crurized lyers, using CuK rdition. The cron content of the crurized lyers ws nlyzed y LECO nlyzer. The verge content of 500 µm thick surfce slice ws nlyzed, to get n evlution of the cron pick-up fter the tretment. The HV0.1 microhrdness profile ws mesured on ll the specimens. Instrumented impct tests were crried out on three specimens of ech mteril. RESULTS AND DISCUSSION Cold compcted mterils Figures 2 nd 3 show the microstructure nd the microhrdness profile of the two mterils fter the LPC1 tretment. The microstructure of the crurized lyer is inhomogeneous in oth steels. Figure 4 shows the extensive precipittion of proeutectoidic crides t the prior ustenitic grin oundry in. Crides re recognized s chromium cementite y Kremel et l.7 Precipittion involves surfce thickness of round 100 m, where microhrdness is lower thn eneth. Figure 5 shows the XRD spectrum, elorted to otin the quntittive nlysis of the microstructurl constituents of the outer lyer of the crurized surfce (pproximtely 5 m thin): mrtensite (46 %), cementite ((Fe, Cr)3C, 49%), retined ustenite (3%) nd chromium cride M23C6 (2%). The microhrdness profile of in Figure 2 is quite shrp, ut microhrdness is low thn it could e expected, since grin oundry crides do not provide significnt contriution to hrdening, eing precipitted t the grin oundry. The Cr-free mteril does not revel crides in the metllogrphic nlysis. It contins quite lrge mount of retined ustenite (Figure 6), whose mount ws quntified to 25% y XRD nlysis (Figure 6). The microhrdness profile (Fig. 3) is influenced y retined ustenite, which is minly loclized in the outer prt of the cse where microhrdness is lower thn eneth. Microhrdness is still not very high. Both the grin oundry crides in nd the retined ustenite in DLH re due to overcrurizing. In presence of chromium, cron rects forming chromium cementite. 7 In sence of this element, cron dissolves into ustenite, decresing Mf- Ms tempertures promoting the stiliztion of ustenite t room temperture fter quenching. Both the results re undesirle from prcticl viewpoint, since crides cuse rittleness nd retined ustenite cn impir wer resistnce nd, if trnsformed to mrtensite y stress, dimensionl precision. The positive effect of overcruriztion on wer resistnce hs een reported in,11 ut in this cse crides re homogeneously distriuted nd not segregted t grin oundryies The LPC2 tretment ws not crried out on these specimens. Wrm compcted mterils Figures 7 nd 8 show the microstructure nd the microhrdness profile of the two wrm compcted mterils fter LPC1. Figures 9 nd 10 show the microstructure nd the microhrdness profile of the two wrm compcted mterils fter LPC2. The microstructure of is homogeneous without grin oun- 12 L Metllurgi Itlin - n. 4/2011

3 Metllurgi delle polveri FIG. 2 Microstructure () nd microhrdness profile () of LPC1 crurized cold compcted Microstruttur () e profilo di microdurezz () dell cciio compttto freddo e cementto con LPC1. FIG. 3 Microstructure () nd microhrdness profile () of LPC1 crurized cold compcted DLH. Microstruttur () e profilo di microdurezz () dell cciio DLH compttto freddo e cementto con LPC1. FIG. 4 Grin oundry precipittion in LPC1 crurized cold compcted. Precipitzione ordo grno nell cciio compttto freddo e cementto con LPC1. FIG. 5 XRD pttern of LPC1 cold compcted. Spettro XRD dell cciio compttto freddo e cementto con LPC1. L Metllurgi Itlin - n. 4/

4 FIG. 6 ) SEM microgrph nd ) XRD pttern of the cse of LPC1 cold compcted DLH. ) microgrfi SEM e ) spettro XRD dello strto superficile dell cciio DLH compttto freddo e cementto con LPC1. FIG. 7 Microstructure () nd microhrdness profile () of LPC1 wrm compcted. Microstruttur () e profilo di microdurezz () dell cciio compttto cldo e cementto con LPC1. FIG. 8 Microstructure () nd microhrdness profile () of LPC1 wrm compcted DLH. Microstruttur () e profilo di microdurezz () dell cciio DLH compttto cldo e cementto con LPC1. FIG. 9 Microstructure () nd microhrdness profile () of LPC2 wrm compcted. Microstruttur () e profilo di microdurezz () dell cciio compttto cldo e cementto con LPC2. dry crides. The microhrdness profile is etter fter LPC2, microhrdness eing higher nd depth greter thn fter LPC1. Contrrily, DLH contins retined ustenite fter oth the tretments, nd the microhrdness profile is still irregulr, with slight decrese towrds the surfce. The verge cron content is 0.44% in nd 1% in DLH. The ltter mteril, despite of wrm compction, is still overcrurized, whilst seems properly crurized nd the sence of crides is due to the low cron pick-up. The different response of the two mterils to the sme crurizing processes is due to the different chrcteristics of porosity. Despite of the sme density, Tle 2 shows tht porosity is lmost completely closed in, whilst there is still significnt mount of open porosity in DLH. In presence of 4.3% open porosity, the exchnge surfce re is significntly lrger thn the geometricl surfce of the prts, nd this cuses supercrurizing. The results re quite interesting from prcticl viewpoint, since they revel the role of porosity, given the density rnge otined y wrm compction. In prticulr, the reference to density is not relile, since two different mterils hving lmost the sme density my differ significntly in open porosity, resulting in quite different response to LPC. 14 L Metllurgi Itlin - n. 4/2011

5 Metllurgi delle polveri FIG. 10 Microstructure () nd microhrdness profile () of LPC2 wrm compcted DLH. Microstruttur () e profilo di microdurezz () dell cciio DLH compttto cldo e cementto con LPC2. FIG. 11 Microstructure () nd microhrdness profile () of LPC1 shot peened. Microstruttur () e profilo di microdurezz () dell cciio pllinto e cementto con LPC1. FIG. 12 Microstructure () nd microhrdness profile () of LPC1 shot peened DLH. Microstruttur () e profilo di microdurezz () dell cciio DLH pllinto e cementto con LPC1. FIG. 13 Microstructure () nd microhrdness profile () of LPC2 shot peened. Microstruttur () e profilo di microdurezz () dell cciio pllinto e cementto con LPC2. Surfce densified mterils Figures 11 nd 12 show the microstructure nd the microhrdness profile of the two mterils with densified surfces, fter LPC1. Figures 13 nd 14 show the microstructure nd the microhrdness profile of the two mterils with densified surfce, fter LPC2. The microstructure seems homogeneous in the two mterils, neither crides nor retined ustenite is oserved. The microhrdness profiles re regulr fter oth the tretments nd surfce hrdening is more intense fter LPC2. The sence of porosity on the densified surfce prevents overcrurizing. The verge cron content fter LPC2 in the two mterils is 0.41% nd 0.49%, respectively. Wrm compction vs. surfce densifiction nd the effect of se mteril The comprison etween mterils with the densified surfce nd those produced y wrm compction demonstrtes tht only surfce densifiction prevents overcrurizing in DLH, whilst in the two lterntive processing routes led to similr results. L Metllurgi Itlin - n. 4/

6 FIG. 14 Microstructure () nd microhrdness profile () of LPC2 shot peened DLH. Microstruttur () e profilo di microdurezz () dell cciio DLH pllinto e cementto con LPC2. Mteril wrm compcted shot peened DLH shot peened LPC 1 LPC 2 HV0.1 surf HV0.1 mx d 550 (µm) HV0.1 surf HV0.1 mx d 550 (µm) TAB. 4 Representtive prmeters of the microhrdness profile. Prmetri rppresenttivi dei profili di cementzione. Mteril DLH Impct energy (J) Processing route s sintered crurized s sintered crurized wrm compction cold compction nd shot peening wrm compction cold compction nd shot peening TAB. 5 Chrpy impct properties of s sintered nd crurized mterils. Proprietà d imptto degli ccii sinterizzti e cementti. FIG. 15 Lod deflection curves of the wrm compcted nd sintered nd DLH steels. Curve d imptto degli ccii e DLH compttti cldo. It my e interesting to compre the microhrdness profiles otined with the two lterntive routes in. Tle 4 reports the significnt prmeters of the microhrdness profile: the surfce microhrdness (HV0.1 surf ), the mximum microhrdness (HV0.1 mx ) nd the cse depth (d 550 ), expressed s the distnce, in micron, from the surfce t which 550HV0.1 is mesured. The results for the surfce densified DLH re reported, s well. There re slight differences in microhrdness, in prticulr fter LPC1, etween the two mterils, whilst cse depth is significntly greter in the wrm compcted steel. Here the effect of the residul surfce porosity cn e climed to justify the different depth. Even if not communicting with the internl pores, it might effectively increse the totl exchnge surfce re incresing the cron pick-up. Among the two steels, results in greter hrdness nd smller cse depth. The relevnce of this difference hs to e evluted y mens of ftigue nd wer tests. In the present work, Chrpy impct tests were crried out, to get n evlution of the emrittlement provided y LPC2. These results re reported in Tle 5. In the s sintered condition, hs higher impct toughness thn DLH ecuse of its ferritic/perlitic microstructure, which is more ductile thn the minly initic microstructure of DLH.12 This difference is clerly highlighted y the lod deflection curves reported in Figure 15. LPC cuses decrese of impct toughness in in oth cses, s it ws expected, since surfce hrdening decreses ductility. The effect of crurizing on DLH is different, since impct toughness decreses much less in the wrm compcted specimens nd increses in the cold compcted nd shot peened ones. In prticulr, the high impct toughness of the wrm compcted mteril is noticele. Figures 16 nd 17 compre the frcture surfces of wrm compcted nd DLH in the s sintered ( nd ) nd in the s crurized (c nd d) conditions. The frcture morphology of the cse ( nd c) nd of the ulk ( nd d) is shown. In s sintered frcture is highly loclized nd ductile oth 16 L Metllurgi Itlin - n. 4/2011

7 Metllurgi delle polveri c d FIG. 16 Frcture surfce of s sintered nd crurized wrm compcted : ) nd c) frcture morphology of the cse nd ) nd d) of the ulk. Superficie di frttur dell cciio pressto cldo dopo sinterizzzione e cementzione: ) morfologi dello strto superficile e ) del cuore. c d FIG. 17 Frcture surfce of s sintered nd crurized wrm compcted DLH: ) nd c) frcture morphology of the cse nd ) nd d) of the ulk. Superficie di frttur dell cciio DLH pressto cldo dopo sinterizzzione e cementzione: ) morfologi dello strto superficile e ) del cuore. in the cse nd in the ulk, nd remins very similr fter crurizing, with the presence of some smll clevges res in the cse. In s sintered DLH, frcture is still ductile nd loclized t the neck regions, ut fter crurizing it chnges significntly in the cse. Indeed, the ulk frcture morphology is still loclized nd ductile, whilst severl lrge frcture res with qusiclevge re oserved in the cse. They contriute to the spreding of deformtion in the ulk of the originl powder prticles, which is unusul in crurized lyer. Spreding of deformtion from the neck regions into the core of the prticles is promoted y strin hrdenle mtrix, contining retined u- stenite. Retined ustenite my contriute to toughness y either dsoring plstic energy or trnsforming into mrtensite, depending on its stility ginst the strin induced trnsformtion. The metllogrphic nlysis of the frctured specimen, just close to the frcture surfce, confirms the presence of retined ustenite, s shown in Figure 18. The toughening mechnism does not involve ny trnsformtion, since the high cron ustenite is quite stle. CONCLUSIONS The effect LPC on two low lloyed sintered steels processed c- L Metllurgi Itlin - n. 4/

8 FIG. 18 Microstructure of LPC2 DLH close to the impct frcture surfce. Microstruttur in prossimità dell superficie d imptto dell cciio DLH cementto con LPC2. cording to three different routes ws studied. The following lterntives were investigted: ) cold compction nd sintering, tht results in medium density with previling open porosity; ) wrm compction nd sintering, which results in higher density nd minly closed porosity; c) cold compction, sintering nd surfce densifiction y shot peening. In presence of n open porosity (vrint ) ll the mterils re overcrurized. The microstructure contins grin oundry crides in the Cr-contining mteril, nd retined ustenite in the Cr free. In presence of densified surfce (vrint c), LPC forms homogeneous cse, which is slightly hrder nd thinner in the Cr contining steel. The mterils processed y wrm compction give different results, since they develop fter sintering significntly different porosity. The Cr contining mteril hs fully closed porosity, whilst the other one still hs significnt mount of open porosity. Consequently, the cse in the Cr steel is homogeneous, whilst it contins retined ustenite in the other one ecuse of some overcrurizing. Impct tests were crried out to investigte the emrittlement provided y LPC. Impct toughness of the surfce densified steels is quite similr (13-16J), since oth the steels hve rittle ehvior. The wrm compcted steels re quite different. Whilst the Cr steel is lso rittle, the Cr free one possesses n unexpected impct toughness, due to the presence of retined ustenite, which should however hve negtive effect on wer resistnce. Work is in progress to confirm these hypotheses. This work confirms tht LPC cn e used to improve the surfce mechnicl properties of sintered steels, provided tht they do not contin lrge mount of open porosity. In principle, oth the mterils re suitle to LPC fter surfce densifiction. ACKNOWLEDGEMENTS The work ws crried out in the frme of the Högnäs Chir project, finnced y Högnäs AB, Sweden. REFERENCES 1. W. Grfen, F. Bless: Het Tret. Met., 2003, 3, W. Grfen, B. Edenhofer: Ipsen on Top, 1999, 2, H. Alten, F. Shrnk: Ger Technology, 2004, 2, G. Prrish, Crurizing, Mterils Prk OH, ASM H. Dnninger, C. Gierl, S. Kremel, G. Leitner, K. Jenicke-Roessler, Y. Yu.: Powder Metll. Prog., 2002, 2, S. Kremel, H. Dnninger, H.Alte, Y.Yu: Powder Metll. Prog., 2004, 4, S. Bengston, L. Forden: Adv. Powder Metll. Prticulte Mter, 2001, 6, S. Bengston, T.Mrcu: Adv. Powder Metll. Prticulte Mter, 2008, 6, S. Srits, C. Dogn, R. Vrol: Powder Metll., 1999, 42, M. M. A. Bepri, K. M. Shorowordi, J.: Mter. Process. Technol, 2004, , E. Sntulin: Proc. EuroPM 2009 Cong. nd Exhiition, Copenhgen, Denmrk, Octoer 2009, EPMA, vol. 1, Astrct Effetto dell cementzione in ss pressione sull microstruttur, microdurezz e resistenz d imptto di ccii sinterizzti Prole chive: cciio, sinterizzzione, trttmenti superficili L utilizzo sempre più diffuso di ccii ottenuti per metllurgi delle polveri (PM) in cmpo utomoilistico, h portto llo sviluppo di nuove polveri e ll utilizzo di trttmenti termochimici e meccnici l fine di umentrne le prestzioni. Fr le polveri di più recente formulzione possimo nnoverre le prelegte sso tenore di Cr e Mo, in grdo di fornire un microstruttur initico- mrtensitic in seguito sinter-hrdening. Il trttmento di cementzione in ss pressione (LPC) riveste sicurmente un ruolo importnte tr i trttmenti termochimici che si effettuno dopo sinterizzzione, l fine di migliorre lcune proprietà, come resistenz ll usur e ll ftic. L possiilità di utilizzre l cementzione in ss pressione, elimin l presenz di genti ossidnti in tmosfer ovvindo così ll ossidzione dell cciio. L ssenz di genti ossidnti rppresent un ottim opportunità per cementre nche ccii contenenti elementi in leg con elevt ffinità con l ossigeno, come per l ppunto il cromo. I gs più utilizzti nell LPC sono cetile e propno. Questi ultimi si dissocino l lt tempertur ( C) e in ss pressione (15mr), producendo cronio e idrogeno nel cso dell cetilene, e cronio, idrogeno e metno, per il propno. Il flusso di cronio risult mggiore utilizzndo cetilene piuttosto che propno, in qunto quest ultimo dissocindosi form metno, ovvero un gs inerte lle condizioni di tempertur e pressione previste dl trttmento di cementzione. Per l su decomposizione sono richieste temperture mggiori e vuoto spinto. 18 L Metllurgi Itlin - n. 4/2011

9 Metllurgi delle polveri Un ulteriore vntggio deriv dll possiilità di rffreddre con un flusso di gs piuttosto che temprre in olio: questo permette di ridurre le distorsioni del pezzo e l impregnzione dell porosità con l olio di tempr. Tuttvi, l cominzione sfvorevole tr l elevt re superficile degli ccii sinterizzti e l elevto potenzile di cronio reltivo l processo di cementzione in ss pressione induce un sovrcementzione. L conseguenz è l precipitzione di cruri ordo grno negli ccii contenenti cromo e l formzione di ustenite residu, in quelli in cui il cromo è ssente. Per ovvire quest limitzione è necessrio ridurre l porosità superficile degli ccii, promuovendo l densificzione superficile o l umento dell densità stess. In lettertur si trovno molteplici lvori che sfruttno l rulltur per promuovere l densificzione superficile, tuttvi un uon lterntiv può essere l pllintur. Proprio l vlidità di quest ultimo trttmento meccnico è stt studit in questo lvoro contrppost ll possiilità di umentre l densità medinte wrm compction. Sono stti investigti due differenti mterili, un cciio contente Cr-Mo e uno contenete Mo-Ni-Cu, prodotti ttrverso: presstur convenzionle con densità dopo sinterizzzione medio ss (), wrm compction e sinterizzzione con incremento dell densità finle () e pllintur sugli ccii con ss densità (c) l fine di promuovere densificzione superficile. E stto quindi studito l effetto del trttmento di cementzione sull microstruttur, sull microdurezz e sull resistenz d imptto di questi ccii. In presenz di porosità pert () il processo di cementzione in ss pressione induce supercementzione in tutti i mterili: l microstruttur è crtterizzt dll presenz di cruri ordo grno nei cmpioni contenenti cromo e dll formzione di ustenite residu negli ccii senz cromo. In presenz dell densificzione superficile (c) LPC form uno strto superficile mrtensitico omogeneo e sufficientemente indurito. Tuttvi quest ultimo h spessore inferiore quello misurto nei cmpioni compttti cldo con densità medio lt. Questo è dovuto proilmente l ruolo dell porosità superficile, l qule nche se non è comunicnte con quell intern, increment l superficie di scmio tr cciio e tmosfer, umentndo l rricchimento di cronio. Vi è un sostnzile differenz, livello microstrutturle, tr i due ccii con medio lt densità; quelli contenenti cromo presentno un microstruttur mrtensitic omogene, mentre dell ustenite residu è ncor presente nello strto di diffusione dei mterili senz cromo. L giustificzione ncor un volt risiede nel ruolo dell porosità superficile, sostnzilmente chius nei cmpioni contenenti cromo e prevlentemente pert negli ltri. Per qunto rigurd le proprietà d imptto dei due ccii cementti, si è riscontrto l infrgilimento indotto dl trttmento stesso e quindi dei vlori reltivmente ssi di energi d imptto. L Metllurgi Itlin - n. 4/