Mechanical properties of compacted graphite cast iron with different microstructures

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1 Mechnicl properties of compcted grphite cst iron with different microstructures R.. Gregorutti* nd J. E. Gru Tensile strength, frcture toughness nd impct properties were evluted in compcted grphite (CG) cst iron with ferritic, perlitic nd usferritic microstructures. Ultimte tensile strengths for the ferritic nd perlitic smples were 337 nd 632 MP respectively. The ustempered smples showed significnt increment in the strength nd recording vlues between 675 nd 943 MP. The frcture toughness test reveled tht the stress intensity fctor K IC ws 34?0 MP m 1/2 for the ferritic CG iron, 39?7 MP m 1/2 for the perlitic nd between 51?0 nd 58?0 MP m 1/2 for the ustempered irons. On the other hnd, CG iron with ferritic mtrix exhibited the best impct properties with bsorbed energy of 33?3 J. The bsorbed energy of the perlitic CG iron ws the lowest, 14?3 J, while the ustempered smples showed vlues between 17?2 nd 28?4 J. Complementing these results, the criticl crck size ws lso nlysed. Keywords: Compcted grphite cst iron, Microstructures, Mechnicl properties Introduction Compcted grphite (CG) cst iron is prt of the cst iron fmily belonging to the Fe C irregulr eutectic system, chrcterised by n intermedite grphite morphology between the clssicl flke nd nodulr grphite irons (FG nd NG respectively). Mny studies hve been performed to elucidte the fetures of the solidifiction nd the grphite morphologicl modifiction in these mterils, 1 9 since the physicl, chemicl nd mechnicl properties depend on the grphite morphology, in ddition to the microstructure of the mtrix. In CG iron, the grphite is interconnected s in the cse of the FG iron but exhibiting compct shpe, with rounded tips. Studies performed to determine the compctness of the grphite prticles reveled tht the rtio between the length L nd the width d (L/d) is in the rnge of 2 10 in CG, while for FG, L/d is These morphologicl chrcteristics confer to CG n cceptble mechnicl resistnce, plsticity nd high therml conductivity. The convergence of these fetures promotes higher therml ftigue resistnce, 10,11 property tht llows the use of CG iron in components subjected to therml shocks. The industril pplictions of CG begn in the 1960s with the mnufcturing of components for hevy vehicles s truck, trctors, pssenger crs nd cylinder heds for lrge mrine diesel engines. 12 In the lst yers, the interest in this mteril hs been incresing, prticulrly in the utomotive industry for the production of cylinder blocks for high performnce diesel engines, since its better mechnicl properties compred with FG iron llow Lbortorio de Entrenmiento Multidisciplinrio pr l Investigción Tecnológic (LEMIT-CICPBA), Av. 52 s/n e/121 y 122, L Plt, B1900AYB, Argentin *Corresponding uthor, emil metlurgi@lemit.gov.r considerble reduction in the weight of this component. At the sme time, it ws reported the use of CG iron in rilwy brke discs in order to improve the brking performnce nd lifetime As in the cse of ustempered ductile iron (ADI), the mechnicl properties of CG irons cn be improved by mens of the ustempering het tretment, which could extend the use of this mteril to diverse industril pplictions. The ustempered microstructure is composed of ciculr ferrite nd high crbon ustenite, commonly clled usferrite, nd the kinetics of the trnsformtion ws studied in previous rticles by mens of the Johnson Mehl Avrmi formlism Considering tht CG iron is suitble for mnufcturing components, which re prone to crcking by therml ftigue, the present work nlyses the tensile strength, frcture toughness nd impct properties of this mteril with ferritic, perlitic nd usferritic microstructures, obtined by the corresponding het tretments. Experimentl Bse metl of composition 3?60C 1?80Si 0?10Mn 0?05Cu 0?027P 0?01S 0?040Cr 0?030Ni (in wt-%) ws used to produce CG iron in medium frequency induction furnce using the sndwich technique in ldle. Compcted grphite morphology ws obtined by dding 0?4%FeSiMgCeC lloy (8 10 Mg, Si, 0?7 1?5 C, 1 1?2 Ce, blnce Fe, in wt-%) to the liquid metl t 1450uC. Electrolytic Cu, FeMn (60 wt-%mn) nd FeSi (75 wt- %Si) were dded for Cu, Mn nd Si blnces respectively. The resulting chemicl composition in wt-% ws s follows: Fe 3?38C 2?56Si 0?48Mn 0?52Cu 0?014S 0?020P 0?035Cr 0?027Ni. Smples for mechnicl testing were obtined from 25?4 mm Y blocks (ASTM A- 395) cst in snd moulds. Ferritic microstructure ws ß S. Mney & Son Ltd. Received 7 June 2013; ccepted 25 April 2014 DOI / Y Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5 275

2 1 As cst microstructure obtined in snd mould obtined by nneling, heting up to 900uC nd cooling in furnce, while for the perlitic mtrix, the cooling ws performed in ir. Ausferritic microstructures were obtined by ustempering het tretment, ustenitising t 900uC for 1 h nd posterior, quenching in slt bth held t 300, 350 nd 400uC, in tht order, during 60 min. Opticl nd scnning electron microscopes were used for metllogrphic nlysis. As cst microstructure ws chrcterised by mens of Buehler quntittive imge nlyser with Omnimet Enterprise softwre. The retined ustenite in the ustempered smples ws mesured by X-ry diffrction using Philips P3710 diffrctometer with Cu K rdition. Single edge notched bend smple (ASTM E Stndrd) ws used for the frcture toughness test. Mechnicl properties were evluted through tensile tests on smples of 6?25 mm dimeter (ASTM E8-98 Stndrd) using n Instron mchine of 15 ton cpcity. Chrpy impct test ws performed on unnotched smples (ASTM A327M-91 Stndrd) using n AMSLER pendulum impct test mchine with cpcity of 300 J. Hrdness mesurements were performed by Brinell procedure using 2?5 mm dimeter steel bll nd lod of 187?5 kg. Results nd discussion Microstructurl nlysis Figure 1 shows the s cst microstructure mostly composed by ferrite nd smll quntities of perlite. The results obtined from the quntittive imge nlyser mde in 10 different fields reveled rtio of,80% ferrite nd,20% perlite. As cst microstructure depends on the crbon equivlent, the lloying elements nd the cooling rte. The crbon equivlent of the nlysed CG iron ws 4?24, close to the eutectic composition of 4?3. Thus, the greter mount of ferrite obeys the low mount of perlite stbilising lloying elements s Mn nd Cu, nd the low cooling rte produced by the snd mould. At the sme time s the ferrite/perlite rtio, it ws determined the mount of grphite prticles per squre millimetre (p mm 22 ) nd the compcted grphite percentge. Mesurements were mde in 10 different fields, nd the verge result ws 110 p mm 22, from which the percentge of compcted grphite ws in the rnge of 85 95% nd the nodulr grphite between 5 nd 15%. Ferritic nd perlitic microstructures, obtined by ferritising nneling nd normlising het tretments respectively, re shown in Fig. 2. The therml cycling of these het tretments differs in the cooling rte fter the ustenitising step. In the ferritising nneling, the cooling is performed in furnce becuse it should be slow enough to llow the diffusion of crbon from the ustenite to grphite prticles. In this condition, the ustenite remins crbon concentrtion close to the equilibrium nd will trnsform to ferrite following the stble Fe C phse digrm. Conversely, the cooling rte in the normlising het tretment is higher becuse it is crried out in ir. This fct reduces significntly the crbon diffusion rte, whereby the crbon toms will not hve enough time to diffuse to the grphite prticles. For this reson, the crbon content of the ustenite is greter thn the equilibrium concentrtion, leding to the eutectoid trnsformtion ustenite perlite. The high cooling rte in normlising lso promotes the formtion of fine perlite, s shown in Fig. 2b, where it cn be lso observed ferritic hlo round the compcted grphite, s occurs in NG iron, denoting tht the trnsformtion ws not complete. Ausferritic microstructures obtined by the ustempering het tretments re illustrted in Fig. 3. The usferritic microstructure is composed by ferrite nd high crbon ustenite. The ustempering trnsformtion begins with the nucletion of ferrite needles, which, 2 ferritic nd b perlitic microstructures obtined by het tretment 276 Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5

3 4 Schemtic illustrtion of phse reltionship s function of T At lower T, the nucletion rte of ferrite needles is greter becuse of the higher undercooling from T c,nd consequently, their growth is inhibited. This feture leds to finer nd more ciculr microstructure with smller res of ustenite, s illustrted in Fig. 3, corresponding to the smple ustempered t 300uC. In contrst, the increment of T reduces the nucletion rte of ferrite needles s consequence of the lower undercooling promoted. In such sitution, the fewer ferrite needles re ble to grow enclosing lrge res of ustenite, forming vst microstructure, s in the cse of the smple ustempered t 400uC shown in Fig. 3c. The vrition of the retined ustenite with the ustempering temperture ws mesured by X-ry diffrction, being the results recorded in Tble 1. t 300uC; b t 350uC; c t 400uC 3 Austempered microstructures during growth, reject crbon toms into the surrounding ustenite, stbilising this phse. The driving force for the trnsformtion is the undercooling generted between the ustenitising nd the ustempering tempertures (T c nd T respectively). At given T c, the reltion between ferrite nd high crbon ustenite vries with T, s indicted in the scheme of Fig. 4 reported by Byti et l., 21 in which the zc field is extended to the rnge of ustempering tempertures. Applying the lever rule t T 1, the mount of retined ustenite is defined by the reltionship AB=AC, while t higher T 2, the rtio is A B =A C. AsA B is greter thn AB nd A C is lower thn AC, it follows tht the percentge of retined ustenite increses with T, nd consequently, the mount of ferrite will be lower. Anlysis of mechnicl properties Tble 2 reports the vlues of the tensile nd hrdness tests for the different CG irons. It is well known tht the mechnicl properties of CG iron increse with the content of nodules. For this reson, the percentge of compct grphite corresponding to ech smple ws lso included. Ultimte tensile strength (UTS), yield strength (YS) nd hrdness for the ferritic nd perlitic CG iron re in greement with dt reported by Stefnescu et l. 22 The high vlues of UTS, YS nd prticulrly hrdness of the perlitic CG iron cn be ttributed to the smll perlite spcing, consequence of the fst cooling rte during normlising tretment. Ausferritic microstructures reported significnt increse in the mechnicl properties, noting tht UTS, YS nd hrdness increse s the ustempering temperture decreses. This behviour is becuse the Tble 1 Volume frction of retined ustenite Austempering temperture/uc Austenite/% Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5 277

4 5 Lod displcement curve of CG iron microstructure becomes finer nd more ciculr t lower T, s shown in Fig. 3. On the other hnd, the Young s modulus E, which indictes the cpcity for elstic deformtion of mterils, recorded rndom vlues with respect to the different mtrices, being in the rnge between 114 nd 133 GP. These results could be scribed to the rbitrry distribution nd the morphologicl chrcteristics of the grphite prticles, which re embedded in the metllic mtrix forming network. This grphite network genertes significnt discontinuity in the mtrix, ffecting the cpcity for elstic deformtion. Compcted grphite iron with ferritic mtrix showed the gretest ductility, with elongtion of 4?4%, while for the perlitic mtrix, the elongtion decreses significntly down to 1?0%. In the cse of the ustempered mtrices, the ductility ws directly relted to the content of retined ustenite. The microstructure obtined t 300uC recorded the lowest vlue of elongtion, 0?8%, s consequence of the lower mount of retined ustenite displyed in Tble 1, while t 350 nd 400uC, the elongtion increses up to 1?1 nd 2?8% respectively. It is noticeble tht the smple ustempered t 400uC nd the perlitic recorded close vlues of UTS nd YS. Nevertheless, the elongtion of the usferritic smple ws significntly higher, since the presence of ustenite in the microstructure confers greter ductility. Frcture toughness test Frcture toughness mesures the resistnce to extension of crck. This property ws evluted by mens of the liner elstic frcture mechnics, which define the stress intensity fctor K IC. This prmeter represents the inherent bility of mteril to resist progressive tensile crck extension. The greter the vlue of K IC, the higher the stress required to produce rpid propgtion nd the greter the resistnce of the mteril to brittle frcture. 23 The fctor K IC ws clculted using the following reltionship, given by ASTM E399 Stndrd 6 Stress intensity fctor K IC obtined for different mtrices K IC ~ P QS f B 3=2 Being f ~2 : 9 1=2z4 : 3=2z21 6 : 5=2 8 z37 : 6 7=2z38 : (2) 9=2 7 where is the length of the notch, B nd re the height nd thickness of the single edge notched bend smple respectively, nd S is the distnce between the support roll centres, S54. Figure 5 illustrtes the lod displcement curve obtined for CG iron tht shows the evolution of the crck extension becuse of the ction of the pplied lod. The lod displcement curve exhibits non-liner behviour, reson by which K IC ws clculted using the mximum lod criterion, ssuming tht P Q 5P mx, s reported in previous works performed in FG iron. 24,25 The results represented in Fig. 6 indicted tht ferritic CG iron reported the lowest vlue of frcture toughness, its K IC being 34?0 MPm 1/2. This property ws improved with the normlising tretment, since K IC for the perlitic iron ws 39?7 MPm 1/2, lthough the higher vlues were obtined by mens of the ustempering het tretment. The CG iron ustempered t 300uC exhibited the highest vlue of K IC, 58?0 MPm 1/2, while the vlues of smples ustempered t 350 nd 400uC were 54?0 nd 51?0 MPm 1/2 respectively. (1) Tble 2 Mechnicl properties obtined for different mtrices CG iron mtrix UTS/MP YS/MP E/GP d/% HB CG/% Ferritic Perlitic Austempered t 300uC Austempered t 350uC Austempered t 400uC Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5

5 7 Crck progress through grphite/mtrix interfce The results of the frcture toughness tests hve shown similr tendency to the mechnicl strength. Compring the vlues of the stress intensity fctor with those obtined in the tensile tests reported in Tble 2, it could be inferred tht the higher mechnicl strength, the greter frcture toughness, nd, consequently, the greter the energy required to propgte the crck. The metllogrphic nlysis of the res djcent to the frcture, illustrted in Fig. 7, revels tht crcks nuclete minly round the notch nd therefter propgte through the grphite/mtrix interfce. Chrpy impct test Chrpy impct test llows determining the toughness of mteril, in terms of the bsorbed energy becuse of the effect of dynmic lods. The results illustrted in Fig. 8 indicte tht CG iron with ferritic mtrix recorded the highest bsorbed energy, reporting vlue of 33?3 J. Perlitic CG iron showed the lowest toughness, being its bsorbed energy 14?3 J. The irons ustempered t 300 nd 350uC exhibited bsorbed energy of 17?2 nd 17?6 J respectively, while the ustempered t 400uC recorded n bsorbed energy of 28?4 J, close to the ferritic iron. The higher toughness with respect to the other ustempered smples is becuse its greter retined ustenite content, which confers mjor ductility. Toughness exhibits the sme tendency s ductility, reported in Tble 2, indicting tht the bsorption energy cpcity is greter s higher is the ductility. The nlysis of the frcture surfce illustrted in Fig. 9 reveled tht brittle clevge mechnism is prepondernt in ll the smples, s consequence of the smll totl plstic deformtion tht CG iron hs. At the sme time, decohesive rupture ws observed, since the crck propgtion occurs preferentilly through the grphite/mtrix interfce, s shown in Fig. 7. Nevertheless, some res with dimple rupture denoting ductile frcture were observed in the ferritic CG irons. In the perlitic nd in the ustempered CG irons, it cn be observed the fcets corresponding to the trnsgrnulr frcture. As mentioned previously, CG iron is suitble to mnufcture mechnicl components subjected to therml shock or therml ftigue. Under this operting condition, it is expected tht the prts fissure, compromising its 8 Chrpy bsorbed energy obtined for different mtrices performnce nd integrity. In previous work crried out in ADI, 26 it ws ssumed tht the criticl crck size is proportionl to (K IC /s yield ) 2, whereby (K IC /s yield ) is considered n dequte prmeter to estimte the reltive toughness of the mterils. The criticl crck size for ech mtrix is represented in Fig. 10, where it cn be observed tht the ferritic microstructure exhibits the highest vlue, s consequence of its greter toughness nd ductility reported in the Chrpy nd tensile tests. The mtrices with lower toughness nd ductility, s the perlitic nd the ustempered t 300 nd 350uC, showed considerble decrese in the criticl crck size. The usferritic microstructure obtined t 400uC, insted, displyed slight increse in this fctor. Conclusions Frcture toughness of CG iron with ferritic, perlitic nd usferritic structures ws evluted to complement the vilble dt concerning tensile nd impct properties. Ferritic mtrix showed the lowest stress intensity fctor K IC, indicting its lower resistnce to crck propgtion. K IC increses with normlising het tretment; nevertheless, the higher vlues were recorded for the usferritic structures, noting tht K IC increses s the ustempering temperture decreses. The results of the frcture toughness hve shown similr tendency to the mechnicl strength; consequently, the higher UTS, the greter K IC. Another prmeter of interest is the criticl crck size, mesured by (K IC /s yield ) 2, from which it could be estimted the reltive toughness of the mteril. CG iron with ferritic mtrix recorded the gretest criticl crck size with respect to the other smples. In the specific cse of ustempered CG irons, the higher criticl crck size ws obtined for the smple ustempered t 400uC, since its greter content of retined ustenite confers higher toughness nd ductility. The frcture surfce corresponding to the different mtrices ws lso nlysed, observing tht clevge is the min mechnism of frcture. At the sme time, decohesive rupture ws observed in ll smples, since the crcks propgte through the grphite/mtrix interfce. Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5 279

6 9 Frcture surfce corresponding to ferritic, b perlitic, c ustempered t 300uC, d ustempered t 350uC nd e ustempered t 400uC 280 Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5

7 10 Criticl crck size fctor Acknowledgement e thnk the Comisión de Investigciones Científics de l Provinci de Buenos Aires (CICPBA) for the finncil support to crry out the present reserch pper. References 1. I. Minkoff: The physicl metllurgy of cst iron, Chp. 5, Solidifiction of spheroidl grphite cst iron, ; 1983, Norwich, John iley & Sons. 2. A. Roviglione nd J. D. Hermid: From flke to nodulr: new theory of morphologicl modifiction in gry cst iron, Metll. Mter. Trns. B, 2004, 35B, I. Riposn, M. Chismer, L. Sofroni nd V. Brbie: Contributions to the study of the solidifiction mechnism nd of the influence of structure on the properties of compcted/ vermiculr grphite cst iron, in The physicl metllurgy of cst iron, (ed. H. Fredriksson nd M. Hillert), Vol. 34, ; 1985, New York, Elsevier Science Publishing Co. Inc. 4. D. M. Stefnescu: Solidifiction nd modeling of cst iron short history of the defining moments, Mter. Sci. Eng. A, 2005, A413 A414, D. M. Stefnescu: Modeling of cst iron solidifiction the defining moments, Metll. Mter. Trns. A, 2007, 38A, G. L. River, R. E. Boeri nd J. A. Sikor: Solidifiction of gry cst iron, Scr. Mter., 2004, 50, R. E. Boeri nd J. A. Sikor: Solidifiction mcrostructure of spheroidl grphite cst iron, Int. J. Cst Met. Res., 2001, 13, 5, G. River, R. Boeri nd J. Sikor: Reveling nd chrcterising solidifiction structure of ductile cst iron, Mter. Sci. Technol., 2002, 18, A. N. Roviglione nd J. D. Hermid: A new unidirectionl solidifiction method to study gry cst iron, Metll. Mter. Trns. B, 2002, 33B, K. R. Ziegler nd J. F. llce: The effect of mtrix structure nd lloying on the properties of compcted grphite iron, AFS Trns., 1984, 92, Y. J. Prk, R. B. Gundlch, R. G. Thoms nd J. F. Jnowk: Therml ftigue resistnce of gry nd compcted grphite cst iron, AFS Trns., 1985, 93, E. Nechtelberger, H. Puhr, J. B. von Nesselrode nd A. Nkysu: Cst iron with vermiculr/compcted grphite stte of the rt. Development, production, pplictions, Proc. 49th Interntionl Foundry Congress, Chicgo, IL, USA, April 1982, CIATF, Pper 1, S. Dwson: Compcted grphite iron mteril solution for modern diesel engine cylinder blocks nd heds, Proc. Conf. 68th orld Foundry Cong., Chenni, Indi, Februry 2008, The Institute of Indin Foundrymen, L. Guesser, P. V. Durn nd. Kruse: Compcted grphite iron for diesel engine cylinder blocks, Proc. Conf Congrès Le diesel: ujourd hui et demin, My 2004, Lyon, Frnce, Ecole centrle Lyon, Y. ng, F. Hu, H. Shi nd X. Fn: Appliction nd development of vermiculr iron to cylinder block cstings Modern Cst Iron, 2010, 06, C.-H. Lim nd B.-C. Goo: Development of compcted vermiculr grphite cst iron for rilwy brke discs, Met. Mter. Int., 2011, 17, (2), A. X. Pn: Study on the csting defects of vermiculr cst iron brke disc nd the countermesures, Adv. Mter. Res., 2012, , J. Desimoni, R. Gregorutti, K. Lneri, J. L. Srutti nd R. C. Mercder: Influence of the Mn content on the kinetics of ustempering trnsformtion in compcted grphite cst iron, Metll. Mter. Trns. A, 1999, 30A, K. F. Lneri, J. Desimoni, R. C. Mercder, R.. Gregorutti nd J. L. Srutti: Therml dependence of ustempering trnsformtion kinetics of compcted grphite cst iron, Metll. Mter. Trns. A, 2001, 32A, R. Gregorutti, K. Lneri, J. Desimoni nd R. C. Mercder: Study of the ustempering trnsformtion kinetics in compcted grphite cst irons, Metll. Mter. Trns. A, 2004, 35A, H. Byti, A. L. Rimmer nd R. Elliott: The ustempering kinetics nd processing window in n ustempered, low-mngnese compcted-grphite cst iron, Cst Met., 1994, 7, (1), D. M. Stefnescu, R. Hummer nd E. Nechtelberger: Compcted grphite irons, in ASM hndbook, 9th edn, Vol. 15, Csting, (ed. J. R. Dvis et l.), ; 1988, Mterils Prk, OH, ASM Interntionl. 23. J. D. Lndes: Frcture toughness testing, in ASM hndbook, 9th edn, Vol. 8, Mechnicl testing nd evlution, (ed. S. R. Lmpmn), ; 2000, Mterils Prk, OH, ASM Interntionl. 24. A. Glover nd G. Pollrd: Deformtion nd frcture of gry cst iron structures, J. Iron Steel Inst., 1971, T. Venktsubrmnin nd T. Bker: Frcture toughness of flke grphite cst iron, Met. Technol., 1978, 5, R. A. Mrtínez, R. E. Boeri nd J. A. Sikor: Impct nd frcture properties of ADI, compred with SAE 4140 steel, AFS Trns., 1998, 106, Interntionl Journl of Cst Metls Reserch 2014 VOL 27 NO 5 281