November 2010,Volume 4,No.1 1(Serial No.36) Journal of Materials Science and Engineering,ISSN 1 934・8959,USA Improvement to the Morphology of Carbides in Low Alloy White Cast Iron Deliang Meng 一,Guirong Peng ,Jianhua Liu and Ruijun Zhang j.StateKeyLaboratoryofMetastableMaterialsScienceandTechnology,Yanshan Universi Qinhuangdao 066004 China 2.School ofCivil Engineering and Mechanics,Yanshan University,Qinhuangdao 066004,China Received:April 12,2010/Accepted:April 24,2010/Published:November 25,2010 Abstract:Aimed at the problem that eutectic carbides in Jow alloy white cast iron are distributed with reticulation shape.the effect of heat treatment on morphology of eutectic carbides in low alloy white cast iron after deformation was studied.The results showed that the morphology and distribution of eutectic carbides in the cast iron were improved by high temperature heat treatment and hot deformation.and the morphology of eutectic carbides is changed from continuous reticulation into strip and block.When the cast iron after 20%hot deforrnation is treated at 970。C for 3 h.stripy and blocky carbides will replace reticular eutectic carbides.and the impact toughness is improved bv 88.56%. Key words:Low alloy white cast iron,morphology of eutectic carbides,hot deformation,heat treatment. 1.Introduction Low alloy white cast iron has high hardness and abrasion resistance,so it is popularly used in ore-mountain machine,metallurgy,construction and power industry as a bind of high—abrasive material but the heat treatment process after pre—deformation should be further perfected.In order to get the optimal heat treatment process,the effect of heat treatment parameters after pre—deformation OB the morphology of eutectic carbides in low alloy white cast iron was studied. 【l一2】.But its application is limited by its high brittleness resulted from the continuous reticulation 2.Experimental Materials and Methods The experimental material is low alloy white cast distributed carbides in it【3・5】.So researches about how to improve the morphology of eutectic carbides were paid great attention by materials researchers at home and abroad.There are lots of reports about how to improve the morphology of eutectic carbides,and iron,and its chemical composition(mass fraction%)is 2.14C,0.56Si,0.62Mn,1.78Cr,0.41Mo,0.030P, 0.034S.It was smelted in 250 kg medium frequency electric induction fumace by using pig iron,waste steel and chromium iron as raw materials,then cast into the main methods are plastic deformation【6—9], alloying,heat treatment and RE modification treatment, these methods have made some good results,and there are still some disadvantage.Recently,our experimental results show that heat treatment after pre—deformation can effectively ameliorate the morphology and distri- bution of eutectic carbides in low alloy white cast iron. cylinaraceous samples in dry sand mode.The samples were heated in RJX一8.1 3 electric resistance furnace to 950。C and kept at the temperature for 30 min.Then the samples undergo one—way compression deformation by air hammer of 400 kg,and their deformation amount is 20%.The deformation is calculated by the following , , formula: = — ×100%Where is the degree of associate DIr0fess r,research neld:metallic m ateriaIs.E-mail:gr8599@ysu.edu.cn. Corresponding author:Guirong Pcng(1 973一)、female, deformation, ,l is the height of samples without Improvement to the Morphology of Carbides in Low Alloy White Cast Iron deformation,lo is the height of samples with deformation.Then they were kept at the temperature of 820,870,920,970 and 1000。C for 4 h.Define the heating temperature according to the experimental results,then heat the samples at the definite heating temperature for 1,2,3,4 and 5 h,respectively,and then COOIin the air.Cut a 10 mmx 10 mmx55 mm impact no nick sample in the vertical direction from the original (a1 as-cast sample and the samples with optimum heat treatment. Test the impact toughness and hardness with ZBC一300B impact tester and HR.1 50A hardness tester. The finaI result iS the average of 3 times test.The microstructure and morphology and distribution of eutectic carbides of the samples were observed by Axiovert200MAT optical microscope and KYKY-2800B scanning electron microscope(SEM). 3.Results and Diseussion 3.1 Influence of Heating Temperature Oil the Morphology ofEutectic Carbides Fig.1 shows the microstructure of low alloy white cast iron before and after at different heating tempera- tures for 4 h.It can be seen that the microstructure of the cast iron iS composed of pearlite and eutectic carbides,and eutectic carbides are coarse and continu. OUS reticultaion distributed in the cast iron before heat treatment.There are micro.cracks inside the carbides due to 20%deformation.and in some areas reticular carbide is broken(Fig.2),after treatment at 820。C for 4h,the carbides are still reticulation distributed,and only a little reticular eutectic carbides dissolved and broken.With the increase of heating temperature,the amount of broken reticular eutectic carbides increased. At the same time fine grains of carbides precipitated from the matrix increased.When the temperature reached 970。C the character of reticular carbides almost disappeared.As shown in Table 1. j.2 Inlfuence of Heat Preservation 77me Oil the Morphology ofEutectic Carbides Through the observation to the morphology and (b)820。C (c)870。C (d 970。C Fig.1 Microstructures of samples with different heating temperature for 4 h. Fig.2 SEM image of specimen with 20%deformation. Improvement to the Morphology of Carbides in Low Alloy White Cast Iron 87 Table 1 Morphology and distribution of eutectic carbides in the cast iron at different heating temperatures for 4 h. Table 2 Morphology and distribution of eutectic carbides in the cast iron at 970。C with diferent holding time. Table 3 Mechanical properties of the cast iron. distribution of eutectic carbides in the cast iron which undergo different holding time at 970。C(Table 2).It can be seen that after heating preservation for 1 h,the reticular carbides dissolved and broken,but still kept the reticular feature.With the increase of heal preservation time,the carbides transformed from reticulation into strip and block.After heating preservation for 3 h,the reticular eutectic carbides disappeared and they distributed massively.With further increase of heat preservation time.there wasn’t obvious change in the morphology and distribution of eutectic carbides. 3.3 Mechanical Properties Table 3 shows the test result of impact toughness and hardness of low alloy white cast iron under different state.It can be seen that after heat deformation amount is 20%and at 970。C for 3 h,the sample’s impact toughness is 6.24 J.cm~.Compared with as-cast samples,the impact toughness improved by 88.56%, but the hardness has litle change.Therefore,proper heat treatment after pre-deformation can effectively improve the impact toughness of low alloy white cast iron 3.4 Discussion It is an inevitable phenomenon for reticular carbides melt down and transformed into mass at high tempera— ture[1 0—1 1].The weak section of reticular carbide and the place where crystal lattices distortion energy is higher is easy to break ta first.Coarse reticular carbides is stable and not easy to melt down at normal high temperature heat treatment conditions,so it is hard to change it’s shape【12].But the cast iron after deformation can engender cracks inside reticular carbides,increase weak section of carbides,engender crystal lattices distortion inside carbides and reduce the stability of carbides.The results lead to the reticulra carbides melt down at high temperature. So deformation can accelerate the reticular carbides molten down and transform to massive carbides during the following high temperature heat treatment.The melt down and transforming of reticular carbides during high temperature heat preservation process was performed through carbon atom diffusion and precipitation,so the higher the heat temperature is,the higher diffusion rate of carbon atom in austenite,the more eutectic carbides melt down and become massive carbide.So proper heat treatment to low alloy white cast iron after deformation can make eutectic carbides rfom continuous reticulation transform into strip and block,keep the continuity of matrix,mitigate stress concentration resulted from impacting,and enhance the impact toughness of the cast iron.But if the heat temperature is too high and the heat preservation time is too long,overheat and overburnt will appear,so the mechanical properties is deteriorated.From the view of saving energy resources,and integrated with the above—mentioned experimental results,the optimal treatment process to improve the morphology of eutectic carbides in low alloy wear resistant is at 970。C fnr 3 h after 20%defnFination. Improvement to the Morphology of Carbides in Low Alloy White Cast Iron 4.Conclusions Deformation together with high temperature heat treatment can effectively improve the morphology and distribution of eutectic carbides in low alloy white cast iron,make the eutectic carbides transform from reticulation into strip and block. When low alloy white cast iron after 20% deformation is treated at 970。C for 3 h.reticular eutectic carbides were almost replaced by stripy and blocky carbides,and the impact toughness of the cast iron was improved by 88.56%. References P.Hugger,B.Ben-Nissan,Development of a low melting point white cast jron for use jn composite alloy manufacture,Materials Forum 3l(2007)l6.23. f2】 A.H.Cai.Y.Zhou.J.Y.Tan.0ptimization of composition of heat.treated chromium white cast iron casting by phosphate graphite mold,Journal of Alloys and Compounds 466(2008)273-280. [31 M.S.Oi,Influence ofheat treatment on microstructure and mechanical properties of low alloy semi.stee1.Joumal of Materials Science and Engineering 3(20091 65.68. 【4】 Q.Ma,C.H.Wang,Impact—abrasion behavior of low alloy white cast irons Wear 209 f 19971 308.3 1 5. 【5】 D.L.Meng,J.H.Liu,R.J.Zhang,Normalizing treatment effects on carbide morphology and mechanical properties of Iow alloy wear resistant cast iron.Journal of Materials Science and Engineering 4(2o i O1 52-55. 【61 A.Bedolla-Jacuinde,S.L.Aguilar,B.Hemandez.Eutectic modification in a low.chromium white cast iFon by a mixture of titanium.rare earths.and bismuth:I.E et on microstructure.Journal of Materials Engineering and Performance 14(20051 l49一l57. [7J R.J.Zhang.J.H.Liu.Eflfect of rare earth on mechanical property and wear resistance of cast iron containing low alloy,Rare Metals 27(20081 140-143. 【81 L.M.Chang.M.Li.L.Liu.Properties of cross rolled low alloy white cast iron grinding ballk,Journal of Iron and SteeI Research l 4 f2007)47.5 1. 【9】 T.Xu.z.M.Guo.J.H.Liu.Influence of heat treatment on property of adding rare earth low chromium semi—steel, JournaI of Rare Earths 24 r2006)279.282. [10】 Q.Ma,B.C.Liu,Z.C.Wang,Breakup of eutectic carbide network of white cast irons at high temperatures.Jouma1 of Materials Science 30 f l 995)3383—3386. S.Q.Wang,Q.C.Jinag,Influence of trace element s and heat treatment OB carbide morphology and properties of CD22 stee1.Materials Science and Engineering A 264 fl9991 l72一I76. 【l2】 S.Q.Wang,Q.F.Guan,Q.c.Jiang,Study on carbide morphology modification of ledeburite high Cr die steel before forging.Iron and Steel 34 f l 999)46—50.