INFLUENCE OF DEEP CRYOGENIC TREATMENT ON THE PROPERTIES OF CONVENTIONAL AND PM HIGH SPEED STEELS

Transcription

1 Memorie >> Trattamenti termici INFLUENCE OF DEEP CRYOGENIC TREATMENT ON THE PROPERTIES OF CONVENTIONAL AND PM HIGH SPEED STEELS M. Pellizzari The influence of Deep Cryogenic Treatment (DCT) on the propertie of four wrought and PM high peed teel wa invetigated. Hardne and (apparent) fracture toughne Ka were meaured to highlight the poible influence of DCT carried out before and after tempering. Dry liding wear tet were carried uing a block on dic configuration. The propertie of the two wrought teel, HS6-5-2 (AISI M2) and HS (AISI M35), highlight a well defined influence of DCT. HS6-5-2 how a remarkable improvement in abraive wear reitance without any hardne increae. The mot promiing reult i obtained by carrying out DCT before tempering. The oppoite occur by HS , containing about 4.8%Co, whoe wear reitance decreae in any treatment condition including DCT. PM teel how a le ignificant change in propertie, ince thee are mainly controlled by the high amount of evenly ditributed primary carbide which are not influenced by DCT. A light increae in wear reitance wa oberved jut for HS A general worening wa oberved for HS , namely HS6-5-3 plu 8%Co. In the light of reult here preented cobalt eem to play a negative effect with repect to the low temperature conditioning of martenite. KEYWORDS: teel, phae tranformation, powder metallurgy, heat treatment, mechanical tet, tribology,propertie INTRODUCTION High peed teel (HSS ), taking the name from their ability to retain high hardne during cutting at high peed are known for over a century [1]. Their propertie are the reult of the complex interaction between hard primary carbide and a tougher metallic matrix, which i given by tempered martenite after heat treatment. The chemical compoition ha a prominent role in determining hardne, toughne and wear reitance. It primarily affect the olidification tructure in term of volume, type, morphology and ditribution of carbide. It alo affect the final hardne after quenching by acting on the partitioning of alloying between carbide and metallic matrix and on the amount of retained autenite. Finally it affect the econdary hardening repone of the teel during tempering at about 550 C. A typical compoition for HSS i that of AISI M2, HS 6-5-2, containing 6% tungten, 5% molybdenum, 2%vanadium M. Pellizzari Dipartimento di Ingegneria dei Materiali e Tecnologie Indutriali, Univerità di Trento and about 4% chromium. Mot tungten-molybdenum grade (AISI M) were developed tarting from thi bae compoition. The ue of cobalt in high peed teel i very important for hot hardne. Co doe not act a carbide former, but enter in olid olution into the matrix. It raie the olidu temperature, allowing higher hardening temperature to be ued and promoting carbide olution. Thi would promote higher amount of retained autenite after quenching, but Co decreae it tability allowing reduced amount of thi phae by increaing %Co [2]. Negative reult of cobalt addition are claimed to be reduced toughne and wear reitance [1]. The high peed teel propertie are alo dependent on the manufacturing proce. Due to the high content in carbon and alloying uch teel tend to form a highly interconnected eutectic carbide network during olidification, which can be only partially detroyed during following hot forging and rolling proce. Large carbide tring oriented along the direction of thermomechanical proceing are quite common in wrought teel. Thi poor carbide ditribution, along with the relative big ize of thee hard particle, i reflected in limited value for fracture toughne, o that their maximum hardne mut be kept below a critical level to avoid tool breakage or even microchipping phenomena during cutting [3]. The driving forge toward an improved carbide ditribution allowed PM High la metallurgia italiana >> ettembre

2 Trattamenti termici << Memorie Material W W PM PM HS AISI M2 M35 M3:2 - C Cr W Mo V Co Tab. 1 Nominal compoition of the teel invetigated (W=wrought, PM=Powder Metallurgy). Compoizione nominale degli acciai tudiati (W=forgiati, PM=Metallurgia delle Polveri). 3T 3TC C3T C2T Q + 3T Q + 3T + C Q + C + 3T Q + C + 2T Tab. 2 Lit of the heat treatment (Q=quenching, T=tempering, C=Deep Cryogenic treatment). Elenco dei trattamenti termici (Q=tempra, T=rinvenimento, C=trattamento criogenico). Speed Steel to appear in the early Since then everal different procee have been developed uch u everal new alloy uitable for PM production technique. The mall droplet produced during rapid olidification give rie to a very uniform carbide microtructure, which i retained even after Hot Iotatic Preing (HIP) and hot working operation. The conventional coare carbide egregation in conventional cat and forged high peed teel are thu avoided in PM HSS [4]. Heat treatment of quenching and tempering contribute in the definition of the final propertie of HSS tool. Autenitizing time and temperature are key parameter during quenching, becaue they control the final partitioning of carbon and alloying between the matrix and carbide and the grain ize, a well. Proper combination of tempering temperature and time mut be elected in order to achieve optimum econdary hardne, accompanied by a ufficient toughne. Secondary alloy carbide precipitate during heating, becaue of the diffuion of ubtitutional carbide forming element. Freh martenite form from detabilized retained autenite on following cooling to room temperature. In the pat the ue of ubzero cooling of tool wa propoed a a way to improve hardne and wear reitance. Subzero cooling of tool teel part ha been originally introduced in order to remove the retained autenite uually preent after quenching. Thi tranformation involve a hardne increae accompanied by a certain lo of toughne. However, the wear reitance, how a light improvement only, if compared to that obtained after a deep cryogenic treatment (DCT) carried out at lower temperature, i.e., in liquid nitrogen (-196 C) [5,6]. Thi claim for a different trengthening mechanim operating in thi temperature range. The mot reliable theory acribe thi experimental evidence to a low temperature martenite conditioning phenomenon involving the formation of carbon cluter, which repreent preferential nucleation ite for carbide precipitation during tempering [7]. A a conequence, the microtructure after tempering i more homogeneou, becaue econdary carbide reult maller and more evenly ditributed. In thi work the influence of DCT on the propertie of four wrought and PM high peed teel i invetigated. A tandard ubzero cooling wa introduced in the conventional heat treatment cycle before and after tempering. The influence of Co wa alo conidered with repect to hardne, fracture toughne and wear reitance. EXPERIMENTALS The compoition of the high peed teel invetigated i reported in Tab. 1. Two wrought teel, namely an HS6-5-2 (AISI M2) and HS (AISI M35) and two PM teel, HS6-5-3 and HS were elected. The firt grade wa already conidered in a previou publication of the preent author [8] and the reult will be recalled in the frame of the preent paper for purpoe of comparion with the new one. HS repreent a variant of the bae M2 containing about 5%Co, the element promoting hot hardne. PM HS6-5-3 can be conidered a the Powder Metallurgical variant of bae M2, added by a mall V addition. Finally, HS i the variant of the former grade containing about 8%Co. Primatic ample (10x10x55 mm 3 ) were extracted in the longitudinal direction from annealed bar. Standard vacuum heat treatment (3T, Tab. 2) followed, compriing autenitizing, ga quenching in 5bar nitrogen (Q) and triple tempering (3T), 2 hour each. Slow cooling down to about -180 C and oaking for 24 hour wa performed by uing a cryogenic proceor placed at the Univerity of Trento. Sample were cryogenically treated after and before tempering. In the firt cae (3TC) DCT followed tandard heat treatment. In the latter cae the treatment wa carried out between conventional hardening and tempering (C3T) and reducing the number of tempering cycle (C2T). An apparent fracture toughne Ka wa determined uing a procedure propoed by Lee et.al. to overcome the difficultie in introducing fatigue precrack to mall fracture toughne pecimen [11]. By high peed teel the major difficulty in introducing a fatigue precrack i given by the high brittlene of thee material. Notche depth with root radii ρ of 50μm were electro dicharge machined in tandard Charpy (10x10x55mm 3 ) pecimen. The ratio of notch depth to the pecimen width (a/w) wa et at 0.2. Static fracture toughne teting wa performed uing 10 ton capacity univeral teter (Intron). The pecimen were loaded in three-point bending at a crohead peed of 0.5 mm min 1 according to the ASTM E399. Dry liding tet were carried out uing a block on dic configuration. A counterpart dic (40mm external diameter, 10mm width) of ASP2052 alt bath quenched and tempered to 70HRC wa elected. The liding peed wa 0.68m/ec and a load of 200N wa applied for a total liding ditance of 3000m. Each tet wa interrupted each 18 ettembre 2008 << la metallurgia italiana

3 Memorie >> Trattamenti termici Fig. 1 Microtructure of a) HS and b,c) HS (Nital 2%) (SEM, BSE). Microtruttura dell acciaio a) HS e b,c) HS (Nital 2%) (SEM, BSE). 1000m to weight the ample by mean of a preciion balance (0.0001g). RESULTS In Figure 1 the microtructure of HS (Fig.1a) i compared to that of the PM HS6-5-3 (Fig.1b-c). In thi lat cae the higher amount and the better ditribution of primary carbide can be appreciated. A quantitative analyi wa carried out by analyzing XRD diffraction pattern uing the Rietveld method. The relative amount of MC (gray in Fig.1) and M6C (white in Fig1) carbide could be determined (Tab. 3). A expected, the highly V-alloyed grade diplay a higher MC content and a higher %vol of primary carbide, a well. The hardne value of the teel after conventional heat treatment (3T Tab. 2) evidence the influence of Co, allowing higher HRC and HV30 to be oberved. In order to evaluate the influence of DCT on the tempering behaviour the tempering curve for the four teel were determined. A already demontrated for HS6-5-2 [8] DCT hift the econdary peak hardne to lower temperature (ee Fig.2 and Tab. 4). Hence, in order to Fig. 2 The influence of DCT on the tempering curve of the invetigated HSS. Influenza del trattamento criogenico ulla curva di rinvenimento degli acciai rapidi tudiati. avoid a lo of hardne due to overtempering, cryogenically treated teel hould be tempered at a lower temperature. Thi alo explain the lower hardne diplayed by the teel DC treated following cycle C3T and even C2T in Tab. 2. Apparent fracture toughne The toughne tet of HSS material determined uing EDM notched pecimen alway evidenced a brittle fracture proce. In any cae no platic deformation could be appreciated during the 3-point bending tet carried out until fracture. Hardne Primary carbide HS HRC d HV d MC M 6 C CVP Tab. 3 HRC, HV30 and %vol primary carbide after conventional heat treatment 3T. Durezza HRC, HV30 e %vol di carburi primari dopo trattamento termico convenzionale 3T. Material T p (Q) 535 C 525 C 530 C 525 C T p (Q+C) 515 C 510 C 500 C 510 C T p (Q)- T p (Q+C) -20 C -15 C -30 C -15 C Tab. 4 Temperature of the econdary hardne peak for quenched (Q) quenched and deep cryogenically treated (Q+C) teel. Temperature del picco di durezza econdaria per l acciaio temprato (Q) e ottopoto ad ulteriore trattamento criogenico (Q+C). la metallurgia italiana >> ettembre

4 Trattamenti termici << Memorie Wear reitance Figure 4 diplay the influence of hardne on the wear rate of variou teel for all heat treatment variant conidered. Even if a trict correlation cannot be etablihed, the experimental point of teel not ubjected to DCT (red point in Fig.4) evidence a general decreae of the wear rate by increaing hardne. Thi i in agreement with the higher reitance veru abraion, the main damage mechanim operating during the tet. [8] However, higher wear rate are diplayed by the two wrought teel compared to PM teel at a given hardne level (Fig.4). Furthermore, conidering the experimental point of DC treated ample the above correlation between wear reitance and hardne tend to become le marked. It can be concluded that the microtructure alo influence the tribological behaviour of high peed teel. The mot remarkable reult i that of HS6-5-3 howing coniderably lower wear rate compared to HS6-5-2, having very imilar hardne and carbide content (Table 3). It i believed that thi reult can be explained looking at the finer microtructure of the PM grade. The more homogeneou primary carbide ditribution i reflected in a lower carbide mean pacing, cauing a more efficient protection of the ofter matrix from the abraion caued by the primary carbide in the ASP2052 counterpart material (Figure 5a) containing a big amount of hard MC. The preent explanation i upported by the SEM analyi of the worn urface. Fig.5b and 5c how that the abraion cratche are motly localized within the metallic matrix. The damage i more extended in wrought HS6-5-2 (Fig.5c,d), where big cratche can be appreciated even at low magnification. In thi material, and imilarly alo in HS , the uneven carbi Fig. 3 Apparent fracture toughne Ka v. HV30 hardne. Tenacità a frattura apparente Ka in funzione della durezza HV30. The apparent fracture toughne diplay a general decreae by increaing hardne (Figure 3). However, given a pecific hardne level lower Ka value were hown by wrought teel compared to thoe produced via Powder Metallurgy. Thi reult highlight the marked influence of primary carbide ditribution contrarily to previou invetigation aimed at etablihing a correlation between KIC and teel propertie.[ ] The decreae in tre triaxiality with increaing ρ caue a decreae in the tre (σyy) that would initiate fracture, o that the Ka value i generally higher than that of KIC meaured on precracked ample.[ ] Furthermore, ince the radiu of the platic zone ahead of the notch, i.e., rp, according to Eq.1 increae with notch radiu, the platicized region i higher in the cae of U-notched ample (ρ=50μm) compared to the pre-cracked one (ρ=0). Eq. 1 Eq. 2 for plane train tre intenity factor a eff = a + r p = effective crack length a = original crack length (notch depth, 2mm) λ(a eff ) = parameter depending on pecimen and crack geometry σ y = yield trength Fig. 4 Wear rate v. HV30 hardne. Tao di uura in funzione della durezza HV30. A a conequence the influence of primary carbide on crack nucleation i more likely to be oberved in U-notched pecimen, a demontrated by the lower Ka value exhibited by the two wrought teel, howing large carbide tring (Fig.1a). The dicued relationhip between Ka, hardne and microtructure i verified by conventionally treated teel (red point in Fig. 3) but, within the experimental catter, alo by all DC treated ample (dark point). With minor exception, the experimental point allow to conclude that DCT caue a general increae in toughne in Co-free grade, a decreae being alway oberved for the other two Co-containing grade here invetigated. Even if the influence of Co eem to be motly correlated to it influence on hardne, it i believed that i would alo exert an influence of microtructure. Thi aumption i confirmed by HS , the grade containing the highet %Co, where a marked drop of Ka i diplayed in correpondence of hardne value lower than that attained with conventional heat treatment 3T (red point). The ame, but to a lower extent, could be alo concluded for HS , the teel containing about 5%Co, howing contant Ka by hardne value lower than that of 3T. 20 ettembre 2008 << la metallurgia italiana

5 Memorie >> Trattamenti termici The influence of deep cryogenic treatment (DCT) on the propertie of four high peed teel ha been evaluated in thi work. Two of them, namely HS6-5-2 and HS , were elected among wrought grade, wherea the other two, HS6-5-3 and HS among PM grade. The teel were heat treated including DCT before and after tempering. DCT doe not ignificantly alter the hardne when carried out after conven Fig. 5 Microtructure of the a) counterpart ASP2052 HSS b) worn urface of PM HS6-5-3 c,d) worn urface of wrought HS Microtruttura a) dell antagonita ASP2052 b) dell acciaio PM HS6-5-3 uurato c,d) dell acciaio forgiato HS uurato. Fig. 6 Influence of different DCT cycle on the wear rate of HSS invetigated. Influenza dei differenti cicli di trattamento criogenico ulla reitenza ad uura degli acciai tudiati. de ditribution expoe the metallic matrix to heavy abraion. Thi i confirmed by the formation of quite big oxide patche on the worn matrix region, due to the local entrapment and intering of wear debri. Indeed, the cloely paced carbide in HS6-5-3 promote a very efficient protection of the matrix (Fig.5b). Hence, the wear rate reported in Fig. 4 are cloely related to the ize of abrading particle, i.e., the fine hard carbide of the counterpart HSS (Fig.1a) Looking in more detail at the influence of DCT it can be tated that the wear reitance i alway increaed in the cae of Cofree HSS (Fig.6). Thi reult wa already known for HS6-5-2 [8], where the improvement wa acribed to the low temperature conditioning of martenite, giving rie to a very fine precipitation of econdary carbide in the metallic matrix during tempering. The reult i now confirmed by the PM HS6-5-3, a well. However, the prominent role of primary carbide in the tribological contact and the lower expoition of the metallic matrix leave u to appreciate jut a lower increae in wear reitance for thi PM grade with repect to HS In any cae, both Co grade how lowered wear reitance after DCT. The mot deleteriou effect i diplayed by HS , containing the lower Co content but howing the pooret carbide ditribution, i.e., the larget influence of the metallic matrix to the wear behaviour. In the wort ituation (C2R) the wear rate i increaed of 40% with repect to the conventional heat treatment. CONCLUSIONS la metallurgia italiana >> ettembre

6 Trattamenti termici << Memorie tional tempering. Indeed, when carried out immediately after quenching, it alway caue an anticipation of the econdary peak hardne, o that a lower hardne i generally oberved. Lower tempering temperature mut be elected to avoid overtempering caued by carbide coarening. The meaured apparent fracture toughne Ka diplayed a ignificant influence of hardne and primary carbide ditribution. A general drop in Ka i oberved by increaing hardne. Wrought grade diplayed lower Ka than PM grade at a certain hardne due to the uneven ditribution of primary carbide, arranged in tring. DCT caue a general increae in toughne in Co-free grade, a decreae being alway oberved for the two Co-containing grade here invetigated. A general decreae of the wear rate by increaing hardne wa hown by conventionally treated HSS. The deciive role of primary carbide ditribution could be verified in protecting the ofter metallic matrix form the abraion caued by the hard carbide embedded in the counterpart high peed teel. From thi viewpoint the more homogenou ditribution in PM grade reulted in a remarkably higher wear reitance than wrought grade, even at comparable hardne level. DCT ignificantly influence the wear reitance of wrought teel, becaue of the prominent role played by the martenitic matrix. A marked increae wa hown by HS6-5-2, whilt a general worening wa diplayed by HS , thu highlighting the negative role of cobalt. The reult i confirmed by the two PM grade. While the Co-free grade HS6-5-3 diplayed a contained but defined increae in wear reitance, HS howed a general increae caued by DCT. In the light of reult here preented cobalt eem to play a negative effect with repect to the low temperature conditioning of martenite, even if an experimental upport to thi aumption wa not reported in thi paper. ACKNOWLEDGEMENTS The preent reult were obtained in the frame of a cientific collaboration between the Department of Material Engineering and Indutrial Technologie of the Univerity of Trento EFFETTO DEL TRATTAMENTO CRIOGENICO SULLE PROPRIETA DI ACCIAI RAPIDI FORGIATI E DA POLVERI Parole chiave: acciaio, traform. di fae, met. delle polveri, trattamenti termici, metallografia, prove meccaniche, tribologia, proprietà Nel preente lavoro i propone uno tudio dell influenza del trattamento criogenico (Deep Cryogenic Treatment, DCT) ulle proprietà di quattro acciai rapidi, due dei quali (HS6-5-2 E HS ) forgiati, gli altri due (HS6-5-3 AND HS ) ottenuti da polveri. Le analii HS e HS cotituicono una variante delle HS6-5-2 e HS6-5-3 con l aggiunta del 5% ed 8%Co, ripettivamente (Tab.1). Si è coniderato l effetto del trattamento criogenico (-180 C per 24 ore) eeguito ia prima (C3T, C2T) che al termine dei cicli di rinvenimento (3TC) (Tab.2). I trattamenti criogenici ono tati eeguiti impiegando un impianto frigorifero in grado di controllare la velocità di raffreddamento attravero PLC. La variante di trattamento con raffreddamento criogenico finale non modifica la durezza dei materiali in modo ignificativo. Realizzato ubito dopo tempra invece il trattamento caua un anticipo del picco di durezza econdaria (Fig. 2) per cui è neceario adottare temperature di rinvenimento inferiori a quelle impiegate convenzionalmente per evitare il calo di durezza dato da fenomeni di coalecenza dei carburi. La tenacità a frattura apparente (Ka) evidenzia una dipendenza ABSTRACT and Samputenili, a diviion of SAMP S.p.A., a company of the Maccaferri Indutrial Group. All people of Samputenili involved in thi reearch are greatly acknowledged for their kill and fruitful cooperation. REFERENCES [1] G. HOILE, High Speed Steel. Butterworth & Co., Ltd, The Univerity Pre, Cambridge, 1988, United Kingdom [2] P. GÜMPEL, E. HABERLING, 1983, Thyen Edelt. Tech. Ber., Spec. Iue, Vol.13. [3] S. KARAGOZ, H.F. FISCHMEISTER, Metall. Tran. 29A (1998) 205 [4] O. GRINDER, PM HSS and tool teel Preent tate of the art and development trend. Proceeding of 5th Int. Tooling conference, Leoben (Autria) 29 eptember-1 october 1999, 39 [5] E.A. CARLSON, Cold treating and cryogenic treatment of teel. ASM Metal Handbook IX Edition, [6] P. PAULIN, Mechanim and applicability of heat treating at cryogenic temperature. Indutrial Heating, augut 1992, [7] F. MENG, K. TAGASHIRA, R. AZUMA, H. SOHMA, Role of eta-carbide precipitation in the wear reitance improvement of Fe-12Cr-Mo-V-1.4C tool teel by cryogenic treatment. ISIJ Int., Vol. 34 (1994), N.2, p.205 [8] M. PELLIZZARI, A. MOLINARI, L. GIRARDINI, L. MALDARELLI, Deep cryogenic treatment of AISI M2 high peed teel. Proc. Of 7th International Tooling Conference, Tooling material and their application from reearch to market, Torino, Italy, 2-5 May 2006, Ed. M. Roo, M. Acti Grande, D. Ugue, Vol. 1, p Alo in Int. J. Microtructure and Material Propertie, Vol. 3, No. 2/3, 2008, [9] H.F. FISHMEISTER, L.R. OLSSON, Fracture toughne and rupture trength of high peed teel. In Cutting Tool Material, ASM,, pp , 1981 [10] LEE, BAIK-WOO; JANG, JAE-IL; KWON, DONGIL, Evaluation of fracture toughne uing mall notched pecimen, Material Science and Engineering, Vol. 334A, N. 1-2, pp , 2002 ignificativa dalla durezza e dalla ditribuzione dei carburi primari. Il valore di Ka diminuice all aumentare della durezza (Fig. 3). Gli acciai forgiati, a pari durezza, evidenziano valori di tenacità inferiori a quelli per polveri a caua della peggior ditribuzione dei carburi primari allineati in tringhe (Fig.1). Il trattamento criogenico produce un incremento generale di tenacità nei due acciai privi di cobalto, mentre un i oerva un calo nelle leghe contenenti tale elemento. La reitenza all uura, miurata attravero prove di triciamento a ecco contro un antagonita in acciaio rapito ASP2052 (70HRC), aumenta all aumentare della durezza nei materiali trattati convenzionalmente (Fig.4). La ditribuzione dei carburi primari i dimotra fondamentale per la protezione della matrice metallica relativamente più tenera dall abraione dei carburi primari dell antagonita (Fig.5). Da queto punto di vita la ditribuzione più fine ed omogenea negli acciai da polveri i riflette, a pari durezza, in una miglior reitenza all uura ripetto agli acciai forgiati. Il trattamento criogenico, dal canto uo, evidenzia un influenza ignificativa proprio ugli acciai forgiati, laddove la matrice metallica eercita un ruolo prevalente. Si ricontra un notevole incremento della reitenza all uura nell acciaio HS6-5-2, mentre una diminuzione è tata oervata nel HS , ribadendo il ruolo negativo del cobalto (Fig.6). Il riultato è confermato dagli acciai da polveri. Mentre la lega PM HS evidenzia un contenuto ma definito incremento della reitenza all uura, HS denota un generale deterioramento delle proprietà antiuura in eguito a trattamento criogenico. 22 ettembre 2008 << la metallurgia italiana