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Effect of K/Na on microstructure of high-speed steel used for rollsYichuan Pan *,Hua Yang,Xiangfa Liu,Xiufang BianThe Key Laboratory of Liquid Structure and Heredity of Materials,Ministry of Education,Shandong University,73Jingshi Road,Jinan 250061,ChinaReceived 29September 2003;accepted 3December 2003AbstractIn the present work,the effect of K/Na on microstructure of high-speed steel (HSS)used for rolls was investigated utilizing Hi-scope video microscope (HSVM)and electron probe microanalyser (EPMA).As-cast microstructure of the alloy is mainly composed of pearlite matrix,M 7C 3,M 2C and MC eutectic carbides.The carbides are connected or placed next to each other to form a network along grain boundaries.After K/Na modification,the morphology,size and distribution of carbides change greatly.The carbide network tends to break,and all carbides are refined and distributed homogeneously in the matrix.The mechanism of K/Na modification on microstructure of the alloy is also discussed.D 2004Elsevier B.V .All rights reserved.Keywords:K/Na;High-speed steel;Modify;Microstructure;Carbides;Grain boundaries1.IntroductionHigh-speed steel (HSS),widely used for tools,is char-acterized by excellent hardness,wear resistance and high-temperature properties.Recently,HSS has been applied to roll materials in order to make rolled plates to have homogeneous thickness and uniform surface during hot rolling,thereby leading to enhanced surface quality of rolled plates and extended roll life [1–4].In addition,HSS rolls fabricated by centrifugal casting are being increasingly used due to their high cost performance [5–7].However,alloy elements segregate seriously during solidification,so eutec-tic carbides often form a network along grain boundaries in as-cast microstructure,which seriously deteriorates service performance of the rolls.So,it is important to improve morphology,size and distribution of eutectic carbides in the microstructure.Modification is one economic and effective method [8–12].Recently,alkali metal element K/Na has been found to have a broad influence on microstructures and properties of many Fe–C alloys,such as spherical graphite cast iron,gray cast iron,ductile cast iron,high manganese steel,etc.[13–16].They are regarded as such important elements as rare-earth elements,which can greatly affect ferrous met-als.However,it is difficult to add K/Na into an Fe–C alloy melt,since K/Na is easily oxidized and has a low boiling point and density.For example,the boiling point of K/Na is below 900j C,while the melting point of iron or steel is above 1000j C.So,we have prepared SG modifier,the composition of which is Fe–15K/Na–10Si–2C (in wt.%).SG modifier takes the form of black powder and would be baked before addition into Fe–C alloy melts.In the present work,the microstructure of HSS alloy modified after addition of SG modifier was investigated.It was found that the morphology,size and distribution of eutectic carbides greatly improved.2.Experimental proceduresThe alloy studied in this work conforms to high-speed steel used for rolls,the composition of which is Fe–2.0C–5.0V–9.0Cr–2.5Mo–2.0W–1.0Si–0.9Mn (in wt.%).Different amounts of SG modifier were added into the alloy,as listed in Table 1.About 1kg of alloy was0167-577X/$-see front matter D 2004Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2003.12.005*Corresponding author.Tel.:+86-531-8395414E-mail address:riverpan@ (Y .Pan)./locate/matletMaterials Letters 58(2004)1912–1916prepared in quartz crucible in main frequency furnace at 1600j C.After being held for15min,the melts were cooled to1500j C and poured into a metal mould to obtain ingots of F30Â55mm in size.The modifier was sprinkled on the melt surface during pouring process. After casting,the ingots were austenitized at1000j C, water quenched and double-tempered at575j C.The head face of each ingot was polished and etched in potash prussiate solution,and the microstructure was investigated using KH-2200Hi-scope video microscope (HSVM).The phase and microstructure identification was carried out by JXA8840electron probe microanalyser (EPMA).3.ResultsThree main solid–liquid reactions proceed during so-lidification of the melts:(1)L!y,ferrite firstly precip-itates from the melts;(2)L+y!g,a peritectic reaction occurs between ferrite and the melts,which results in austenite;(3)L!g+Le,a eutectic reaction occurs between carbon and alloying agents such as W,Mo,V, Cr in the retained melts,and ledeburite comes into being [17].The ledeburite is composed of eutectic carbides and austenite.There are mainly three kinds of eutectic car-bides,which are identified as MC,M7C3and M2C carbides according to their respective composition.Al-though MC carbides precipitate before traditional eutectic reaction,the precipitation temperature of which is20j C higher than eutectic temperature,they are also included into eutectic carbides in the present study.When the castings are cooled to room temperature,the austenite turns into pearlite.After heat treatment,the pearlite changes into tempered martensite.Fig.1shows microstructures of the four alloys.We can see that the carbides show gradually change in morphology,size and distribution as the content of K/Na increases in the alloy.Alloy1was unmodified,in the microstructure of which massive coarse carbides and slender carbides are connected to each other to form a network along grain boundaries,and some spot-like car-bides are also distributed in the matrix(Fig.1(a)).Alloy2 was modified by0.07wt.%SG modifiers,and it is seen that massive carbides disappear,slender carbides tend to break and some square carbides are also found to be distributed,but carbide network remains along grain boundaries(Fig.1(b)).In the microstructure of alloy3,Table1Chemical compositions of the alloys investigated(wt.%)Alloy code SG modificator(wt.%)High-speed steel(wt.%)10Bal.20.07Bal.30.10Bal.40.15Bal.Fig.1.Optical images showing the eutectic carbides in(a)alloy1,(b)alloy2,(c)alloy3(d)alloy4.Etched in potash prussiate solution.Y.Pan et al./Materials Letters58(2004)1912–19161913which contains 0.10wt.%SG modifier,carbide network breaks up and square carbides grow up (Fig.1(c)).As the content of SG modifier reaches 0.15wt.%in alloy4,the microstructure becomes completely different from that of alloy1.Carbide network has vanished and all carbides are refined and distributed homogeneously in the matrix (Fig.1(d)).Further identification of carbides in the microstructure was carried out utilizing EPMA,as is shown in the following.Fig.2a shows secondary electron image (SEI)of carbides in alloy1.It is seen that massive coarse carbides and slender carbides form a network along grain boundaries,and a small amount of petal-like or square carbides are distributed in the microstructure.Elementary maps of C,V ,Cr,W and Mo in the image are shown as Fig.2b–f ,respectively.Quantitative analysis of carbides is shown as Table 2.Based on the qualitative andquantitative analysis,the massive coarse carbides are identified as M 7C 3and M 2C carbides.The carbide forming elements of M 7C 3carbides are mainly Cr and small quantity of V ,while that of M 2C carbides are mainly W and Mo.The slender,petal-like and square carbides are identified as MC carbides,which mainly consist of V andC.Fig.2.Electron probe microanalysis (EPMA)of eutectic carbides in alloy1:(a)SEI showing the eutectic carbides;(b)C-X-ray image;(c)V-X-ray image;(d)Cr-X-ray image;(e)W-X-ray image;(f)Mo-X-ray image.Table 2Quantitative analysis results of carbides in alloy 1Secpt.Metallic element (at.%)Eutectic typeV Cr Mo W Fe carbide Black region ED 81.23 4.63 1.83 1.53Bal.MC Gray region ED 7.1034.69 3.74 1.17Bal.M 7C 3White regionED4.6110.2633.0328.41Bal.M 2CY.Pan et al./Materials Letters 58(2004)1912–19161914EPMA analysis has also been carried out on eutectic carbides in alloy4,as shown in Fig.3.Quantitative analysis of carbides is shown as Table 3.It can be seen that M 7C 3,M 2C and MC carbides are all refined and distributed homogeneously in the microstructure,and carbide network vanishes.4.DiscussionIn general,there are mainly two ways to modify microstructures of iron or steel.One is to remove impurity from the melts,and the other is to add modifying elements to the melts.K/Na can act as modifying ele-ments,because they can purify the melts of Fe–C alloy.[S]and [O]are the main impurity in Fe–C alloy,while K/Na can strongly deoxidize and desulfurate the melts.As Fe–C metallurgy and crystallization theory indicate,surface ten-sion of the melts will increase if [S]and [O]are removed and the melts are purified.In addition,this will lead to an increase in the nucleation free energy of the crystalline phase,so that it may need more energy to form crystal nucleus.As a result,crystal nucleation occurs at lower temperature,liquid line during solidification falls down and the degree of supercooling of eutectic reactionisFig.3.Electron probe microanalysis (EPMA)of eutectic carbides in alloy4:(a)SEI showing the eutectic carbides;(b)C-X-ray image;(c)V-X-ray image;(d)Cr-X-ray image;(e)W-X-ray image;(f)Mo-X-ray image.Table 3Quantitative analysis results of carbides in alloy 4Secpt.Element (at.%)Eutectic typeV Cr Mo W Fe carbide Black region ED 74.77 4.860.91 1.58Bal.MC Gray region ED 9.4639.57 5.21 1.49Bal.M 7C 3White regionED12.4613.2436.0828.31Bal.M 2CY.Pan et al./Materials Letters 58(2004)1912–19161915enhanced.All the above have resulted in modification of the microstructures of high-speed steels.All carbides are re-fined and distributed homogeneously in the microstructure, and carbide network along grain boundaries breaks up completely.The properties of rolls are determined by various microstructural factors such as:(1)kind,morphology, volume fraction and distribution of carbides;(2)charac-teristics of the matrix;and(3)solidification cell structure made up of carbides[1,2].It is impossible to meet all the properties of the rolls at the same time,but it is necessary to satisfy those according to characteristics of the rolls and their rolling environment.As to HSS rolls used on plate finish rolling mills,fracture toughness and wear resistance are very important properties.During rolling process of the rolls,microcracks often initiate in and proceed along hard carbides distributed in microstructures.In alloy1where most carbides are connected in network,fracture easily occurs under low stress intensity factor levels and shows cleavage fracture mode on the fracture surface.On the contrary,in the microstructure of alloy4where all carbides are refined and carbide network breaks up,the fracture path is accompanied by a considerable amount of plastic deformation and crack blunting when passing through the matrix,and by an increased amount of ductile fracture on the fracture surface, which may enhance service performance of the rolls.5.Conclusions(1)In the microstructure of unmodified HSS alloy, massive M7C3,M2C carbides and slender MC carbides are connected to each other to form a network along grain boundaries,and a small amount of square MC carbides are also distributed in the matrix.(2)After K/Na modification,the morphology,size and distribution of carbides change greatly.M7C3,M2C and MC carbides are all refined and distributed homogeneous-ly in the microstructure,and carbide network breaks up completely.References[1]K.C.Hwang,S.Lee,H.C.Lee,Mater.Sci.Technol.A254(1998)282.[2]K.C.Hwang,S.Lee,H.C.Lee,Mater.Sci.Technol.A254(1998)296.[3]J.C.Werquin,J.C.Caillaud,in:R.B.Corbett(Ed.),Rolls for theMetal Working Industries,Iron and Steel Society,Warrendale,PA, 1990,p.4,ch.[4]W.H.Betts,H.L.Baxter,in:R.B.Corbett(Ed.),Rolls for theMetal Working Industries,Iron and Steel Society,Warrendale, 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