Evolution of graphite phase morphology during graphitization process in hypereutectoid

Transcription

1 1 Introduction Evolution of grphite phse morphology during grphitiztion process in hypereutectoid steels Grphite is one of the min phses in the structure of most cst irons whose shpe, morphology nd distriution hve significnt effect on the mechnicl properties of cst irons. In this respect we cn mention the improvement in tensile strength nd impct strength of ductile iron with sphericl grphite in comprison with similr irons such s gry iron with lmellr grphite. In present reserch, the presence of grphite in the mtrix of hypereutectoid steel is studied fter performing the grphitiztion trnsformtion from mrtensite structure using SEM (Scnning electron microscope) imges nd EDX (Energy Dispersive X-ry) qulittive nlysis. In ddition, the effect of csting process nd the ddition of grphitizing lloying elements on the morphology, size nd distriution of grphite during the grphitiztion process in cst hypereutectoid steels is studied in comprison with commercil ones. Seyyed Amin Rounghi, Pym Shyesteh nd Ali Rez Kini-Rshid, Mshhd, Irn Mnuscript received 6 My 2010; ccepted 10 My 2010 S. A. Rounghi, P. Shyesteh, A. R. Kini-Rshid, Deprtment of Mterils Engineering, Ferdowsi University of Mshhd, Mshhd, Irn Grphitiztion in steels is done during het tretment from the solid structure. During this diffusion trnsformtion, the semi-stle cementite phse is decomposed into stle grphite nd ferrite phses [1-7]. Grphite is minly formed in steels from mrtensitic or cooled-rolled structure t 600 to 700 C [4-6, 8]. In nture, grphite is one of the est solid luricnts [9]. The presence of this stle cron isotope in iron lloys especilly cst irons results in the improvement of wer nd mchining properties of these mterils [10, 11]. The ovementioned properties especilly the mechnicl properties of cst irons re severely ffected y morphology nd the distriution type of grphite in their microstructures. In this respect, stndrds re defined y ASTM institute with which grphite in cst irons is clssified ccording to its distriution nd morphology [12]. A customry process for chnging the morphology of grphite in cst irons is spherodizing which cuses the spherodizing the grphite prticles y dding spherodizing elements such s mgnesium nd cesium to the melt of gry iron leding to the formtion of ductile iron [13]. Bsed on relevnt literture [12], mechnicl properties especilly the strength nd wer resistnce of these cst irons re considerly higher thn gry irons. According to the considerle effect of grphite's morphology on mechnicl properties of cst irons, there is similr expecttion out grphitized steels in which the investigtion of morphology nd the distriution of grphite in them is less mentioned y reserchers. For this reson ttempts hve een done to prepre steel y lloying the steel vi csting using grphitizing lloying elements such s silicon. So fr, there re some reports tht in this cse, the grphite phse formed would e totlly different from grphitized structures in common commercil steels during lter het tretment with respects to its morphology, distriution nd size. 2 Experimentl procedures In this reserch, the grphite formed during het tretment in commercil hyper-eutectoid steel (with the composition mentioned in Tle 1) is compred to hyper-eutectoid cst steel. In order to produce the steel y csting, 15 kg of CK45 steel ws chrged grdully into the induction furnce. The grnulr grphite ws dded to the melt long with Ferro-silicon (contining 75 wt % silicon) efore pouring the melt. The ddition of grnulr grphite ws done in order to increse the cron content of steel nd therefore producing hyper-eutectoid steel. In ddition, Ferro-silicon ws dded in order to decrese the period of the grphitiztion trnsformtion. After the ddition of the ove-mentioned mterils to the melt, pouring the melt ws performed within wet snd molds. Then the solidified prts were rejected from the mold. The chemicl composition of the cst steel ws determined y quntometric nlysis ccording to Tle 1. Cst specimens were put inside continer full of cst iron's filings in order to prevent oxidtion nd decrurizing nd then were held t 1100 C for 18 hrs for homogenizing. After this period, they were cooled in ir. 2 Interntionl Foundry Reserch/Giessereiforschung 62 (2010) No. 3

2 In order to form mrtensitic structure, the homogenized prts were ustenitized t 900 C for 20 min long with similr specimens prepred from CK100 commercil steel fter eing coted with nti-oxide/crurizing coting Crostop. Then quenching ws done immeditely in wter t mient temperture. For grphitiztion, specimens were put inside continer full of cst iron's filings fter re-coting with Crostop. Cst nd CK100 specimens were held t 670 C for 20 hrs nd 60 hrs respectively. In order to investigte the microstructure using metllogrphic technique, ll of the prts were sectioned in order to prepre microscopic imges from their centrl regions fter surfce preprtion nd etching with 2 % nitl. Some of SEM, EDX nd mpping imges were prepred in Rzi Metllurgy Reserch Center, Tehrn/Irn, using TES- CAN mchine nd for prepring other imges the Scnning Electron Microscope model 1450 VP mde y Zeis Germny from the Centrl Lortory of Ferdowsi University of Mshhd ws used. 3 Results nd discussion Figure 1: Microstructure of: ) commercil steel fter nneling t 670 C for 60 hrs; ) of cst steel fter nneling t 670 C for 20 hrs Figure 1 indictes the grphitized structure of the pre mentioned steels. In order to confirm the presence of grphite, EDX nlysis ws performed from grphite prticle in the commercil nd cst steels seprtely (Figures 2 nd 3). For identifying the distriution of lloying elements, mpping nd EDX line scn were crried out from grphite nodule in the cst steel (Figures 3 nd 4). As seen, the cron pek is identified y EDX in the plce of the grphite phse (Figures 2 nd 3). The percent of other lloying elements such s chromium nd mngnese is low in these regions due to the presence of cron in pure stte in grphite. But ccording to mpping nd EDX line scn imges, reltive even distriution of these elements is oserved in other res. In ddition, sed on mpping nlysis (Figure 4) nd the findings of other reserchers [14], the proility of the concentrtion of these elements in residul sphericl crides in the mtrix is strengthened. As seen in Figure 1, the grphite prticles formed in the cst steel re more sphericl thn commercil ones. Also the size of these nodules is very igger thn the commer- Figure 2: SEM imge from commercil steel fter grphitiztion with point scn: ) cksctter electron imge of grphite prticle; ) point scn nlyze from this prticle Tle 1: Chemicl composition of commercil nd cst steels Steel Chemicl elements, wt % C Si S P Mn Ni Cr Mo Cu Al CK As-cst Interntionl Foundry Reserch/Giessereiforschung 62 (2010) No. 3 3

3 Figure 3: Scnning electron microscopy (SEM) imge from cst steel fter grphitiztion with EDX line scn: ) cksctter electron imge of grphite nodule; ) EDX line scn nlyze from this prticle cil steel, ut theyhve less nd more limited distriution. It should e mentioned tht the grphite formed in the cst steel re identicl to the grphite of cst iron (out 10 µm) [15, 16] forming specil structure in tht steel. Bsed on the reserches performed [14, 17, 18], the ddition of luminum nd oron to the composition of the steel results in the formtion of nitride precipittes which their structures re the sme s grphite. Therefore, the presence of these nitride precipittes in the steel mtrix increses the nucletion sites of grphite leding to their finl fine structure. Since grphitiztion periods re not considered identicl in these two steels nd the chromium content of the cst steel is higher thn the cst one, ccording to the effect of this element on the stility of cementite nd the decrese in the grphitiztion driving force [19], the decrese in the grphitiztion period in the cst steel my not e judged definitely (due to the ddition of silicon in comprison with the commercil steel). The surfce re percent of grphite in the cst nd commercil steels were clculted using opticl microscopic imge nlysis s 2.2 % nd 3.4 % respectively. Figure 5 shows SEM imges of grphite prticles in oth of these steels. By comprison these imges with ASTM stndrds in reltion to grphite's morphology in cst irons [12], the shpe of grphite in the commercil steel my e considered similr to tempered grphite in mllele iron (type III). On the other hnd, the shpe of grphite in the cst iron is identicl to the sphericl grphite in ductile iron (type I). It hs een demonstrted lower Mn/S rtio in cst irons leds to formtion of more sphericl in tempered grphite prticles [12]. So fr, ccording dt listed in Tle 1, Mn/S rtio for cst nd commercil steels cn e clculted nd respectively. One cn generlize the role of Mn/S rtio in cst iron for grphitic steels. In nother word, the high difference in the morphology of grphite in these steels my e due in Mn/S rtio. 4 Conclusions 1. After grphitiztion, the shpe of grphite prticles in the cst nd commercil steels re similr to their morphologies in ductile (type I) nd mllele (type III) iron respectively. 2. Grphite prticles in the commercil steel re very finer with more distriution in comprison with the sme prticles in the cst steel. 3. Grphite prticles in the cst steel re identicl to the prticles in cst irons from size nd distriution point of view. The uthors of the pper pprecite Prof. Dr. Ahd Zet nd Prt Szn Co. for prepring some of the light opticl microscopic imges. In ddition we thnk Mr. Ghsem Is- Adi-Bozchelouie for performing some of the tests. 4 Interntionl Foundry Reserch/Giessereiforschung 62 (2010) No. 3

4 Literture [1]Andrew, J. H.; Lee, H.: Effect of cold-work on steel, section IV-effect of high-speed deformtion on steel. J. Iron Steel Inst. 165 (1950), p [2]Smith, G. V.; Royle, B. W.: Compensted temperture prmeters for correlting creep nd creep rupture dt. Trns. ASME 78 (1956), p [3] Rosen, A.; Tu, A.: The kinetics of grphitiztion in steel t sucriticl tempertures. Act Metll. 10 (1962), p [4] Sueyoshi, H.; Sueng, K.: Effects of pre-tretment on the grphitiztion ehviour in hypoeutectoid low lloy steel. Rev. Soc. Jp. Met. (1978) no. 42, p [5] Smuels L. E.: Light opticl microscopy of cron steels. ASM, Metls Prk, Pp [6] He, K.; Brown, A.; Brydson, R.; Edmonds, D. V.: Anlyticl electron microscope study of the dissolution of the Fe3C iron cride phse (cementite) during grphitistion nnel of cron steel. Journl of Mterils Science 41 (2006) no. 16, pp [7] Bidsh, V. I.; Prikhod ko, A. I.: Grphitiztion of low-cron steel during spheroidizing nnel. Met. Sci. Het Tret. 29 (1987), p [8]Neri, M. A.; Colás, R.; Vltierr, S.: Grphitiztion in high cron commercil steels. J. of Mterils Eng. nd Performnce (1998) no. 7, p [9]Pierson, H. O.: Hndook of cron, grphite, dimond nd fullerenes properties, processing nd pplictions. Pulished y Willim Andrew Inc., P [10] Ktym, S.; Tod, M.: Mchinility of medium cron grphitic steel. Journl of mterils processing technology 62 (1996) no. 4, pp [11] Klus, H.: Comprtive investigtions into the mchinility of grphite-contining cst ironse mterils. Giesserei 52 (1965), pp [12] ASM Interntionl, Hndook Committee. Properties nd selection: irons, steels, nd high-performnce lloy. ASM Interntionl (1998), pp [13] ASM Interntionl, Hndook Committee. Ductile iron, Csting. ASM Interntionl, [14]He, K., e..: An electron microscopic study of spheroidl grphite nodules formed in mediumcron steel y nneling. Act Mterili 55 (2007), pp [15]Lux, B.: On the theory of nodulr grphite formtion in cst iron. AFS Cst Metls Reserch Journl 8 (1972), p [16] Purdy, G. R.; Audier, M.: The physicl metllurgy of cst iron. Mter Res Soc Symp Proc, 1986, 34. Pp d Figure 4: SEM imge from grphitized cst steel with mpping: ) secondry electron imge of grphite nodule; ) mpping nlyze from this prticle; c) cron K α ; d) chrome K α ; e) mngnese K α c e [17] Bnerjee, K.; Venugopln, T.: Development of hypoeutectoid grphitic steel for wires. Mterils Science nd Technology 24 (2008), pp Interntionl Foundry Reserch/Giessereiforschung 62 (2010) No. 3 5

5 Figure 5: SEM imge from grphitized steels: ) commercil steel; ) cst steel [18]Iwmoto, T.; Murkmi, T.: Br nd wire steels for gers nd vlves of utomoiles-eco-friendly free cutting steel without led ddition. JFE GIHO (2004), no. 4, p [19] Zhukov, A.: News in grphitiztion theory. Thermodynmics of grphitizing iron lloys. Metl Science nd Het Tretment 26 (1984), pp Interntionl Foundry Reserch/Giessereiforschung 62 (2010) No. 3

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