A medium manganese steel with high wear-resistance, strength and toughness has been produced with addition of a complex modifier (or refining agent) containing Nb, N, RE and Si-Ca. The results showed that the wear resistance, strength and toughness of the modified medium manganese steel are respectively 1.92 times, 1.45 times and 3.63 times as high as that of the referenced unmodified medium manganese steel. The plastic deformation characteristic involved in the wear mechanism of the modified medium manganese steel was investigated by means of plastic-elasticity calculation and TEM electro-microscopy. The relationship between wear resistance and yield strength of the steel was established. Since the wear volume W is proportional to the square of the loading and to the numbers of the abrasives, and inversely proportional to the square of the yield strength of the materials, the wear resistance can be substantially improved by the enhancement of yield strength of the materials. The calculation results generally agreed with the experimental results

XIE Jing-pei

YUAN Hai-lun

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194CHINA FOUNDRYVol.4 No.3Plastic deformation wear in modifiedmedium manganese steelYUAN Hai-lun1,  *XIE Jing-pei2,  WANG Ai-qin2,  WANG Wen-yan2,  WANG Cheng1(1. Huazhong University of Science and Technology,; 2. School of Material Science and Engineering, Henan University of Science andTechnology, Luoyang  471003, China.)Hadfield steel is a well known ferrous alloy for its superiortoughness, wear resistance and work-hardening abilityunder severe impact loading condition. However, a greatnumber of Hadfield steel castings containing high Mn areused under relatively non-severe impact service conditionsand their work-hardening ability can not be brought into fullplay.The ability of work-hardening of medium manganese steel(Mn7 steel), however, can be increased under non-severeimpact loading condition. In such case, the strain-inducedmartensite transformation and formation of high density ofdislocations have been identified in the work-hardening layer,and therefore, the wear resistance of the Mn7 steel has beenimproved remarkably [1-4]. Many experts in the research fieldhave studied the work-hardening mechanism, impacting wear,alloying treatment, heat treatment and abrasive wear [5-9].Recently, the authors modified the Mn7 steel grade using arefining agent, established the relationship between wearresistance and the yield strength of the modified Mn7 steel,and investigated the plastic deformation wear by means ofplastic-elasticity theory.1 Experimental details1.1 Chemical compositionsThe chemical composition of the referenced mediummanganese steel was 1.0%-1.4% C, 6%-8% Mn and 0.4%-0.8% Si. The composition of the complex modifier (or refiningagent) was 0.05%-0.3% Nb, 0.01%-0.07% N, 0.10%-0.15%RE and 0.2%-0.5% Si-Ca.1.2 SpecimensThe Mn7 steel melt was made in a 150 kg medium frequencyinduction furnace. The liquid steel was poured into Y-shapedmold. All the test samples were taken from the Y-block casting.The samples were roughly machined, heat treated and finished.The heat treatment was carried out in an electric resistancefurnace with a soak temperature at (1 05010), holding timeof 4 h, and then followed by water quenching.The mechanical properties of test bars were measured under aMTS-810 tensile testing machine. Impact toughness of testsamples were evaluated with an Oscillography impact machine,from which the crack forming energy (Wm) and crack extension(or propagation) energy (Wp) can be determined. The hardnessof test samples was checked using a HB-3000 Brinell Hardnesstester.1.3 Wear testingAbrasion wear testing was conducted in a ML-10 wearmachine. The testing condition and parameters included emerycloth abrasives, loading level at 50 N, specimen dimensionof Φ 4 mm  20 mm, dish rotation speed at 60 rpm, andfeeding rate at 4 mm per revolution. The weight loss of wearMale, born in 1965, Dr., Professor. His research interests includecomputer solidification simulation of the large casting, castingsurface composite, medium manganese austenitic steel and theeffects of RE on toughness of wear-resistance cast steel.E-mail: xiejp@mail.haust.edu.cn.Received: 2007-01-05;  Accepted: 2007-04-30*XIE Jing-peiAbstract:  A medium manganese steel with high wear-resistance, strength and toughness has been producedwith addition of a complex modifier (or refining agent) containing Nb, N, RE and Si-Ca. The results showed that thewear resistance, strength and toughness of the modified medium manganese steel are respectively 1.92 times, 1.45times and 3.63 times as high as that of the referenced unmodified medium manganese steel. The plastic deformationcharacteristic involved in the wear mechanism of the modified medium manganese steel was investigated by meansof plastic-elasticity calculation and TEM electro-microscopy. The relationship between wear resistance and yieldstrength of the steel was established.  Since the wear volume W is proportional to the square of the loading and tothe numbers of the abrasives, and inversely proportional to the square of the yield strength of the materials, the wearresistance can be substantially improved by the enhancement of yield strength of the materials. The calculationresults generally agreed with the experimental results.Key word:  modified medium manganese steel; plastic deformation; wear; yield strengthCLC number: TG142.72                      Document Code: A                      Article ID: 1672-6421(2007)03-194-04195Research & DevelopmentAugust 2007specimens was measured in a balancer with 1/10 000gaccuracy. The wear process and mechanism were studied bymeans of stress curves (calculated), stress interferometer,Leica-400 SEM and H-800 TEM.2 Results and discussions2.1 Effects of complex modifier on themicrostructureThe microstructure of the medium manganese steel dependson the compositions of C and Mn. The microstructure isaustenite above 900, and it undergoes eutectoidtransformation at 650 , that is γ  α+(Fe, Mn)3C. The as-cast microstructure of the steel is austenite + carbide + pearlite.Austenite can be obtained after holding at 1 050 for two hoursand water quenching. Generally, the grain size is coarse withoutmodification treatment (Fig. 1). After the modification, the grainsize is refined (Fig. 2) since NbN and Nb2C compounds canserve as heterogeneous nuclei for crystallization of austenite(Figs. 3 and 4) [10].2.2 Effects of complex modifier on themechanical propertiesIt can be seen in Table 1 that the yield strength of the modifiedmedium manganese steel is about 1.45 times as that of mediummanganese steel, and the crack forming energy (Wm) and crackextension energy (Wp) of the modified medium manganese steelare respectively 3.4 times and 3.3 times of that of the mediummanganese steel. There are two reasons for the enhancement ofstrength and toughness. On the one hand, the microstructure ofthe steel was refined by the complex modifier (or refining agent),and on the other hand, the boundary surface could be purifiedand the inclusions in the steel became spheroid [11].2.3 Effects of complex modifier on the wearresistanceDue to the enhancement of mechanical properties and highdensity of dislocations, wear resistance of the modifiedmedium manganese steel is 1.92 times as high as that ofmedium manganese steel (Table 2). More specifically, theyield strength, crack forming energy (Wm) and crack extensionenergy (Wp) were increased remarkably by the additionMicrostructure of medium manganese steelafter heat treatmentFig. 1100 µmMicrostructure of modified medium manganesesteel after heat treatmentFig. 2100 µmAs-cast microstructures of modified mediummanganese steel (SEM)Fig. 320 µmNbN and Nb2C in modified medium manganesesteel (TEM)Fig. 40.25 µmNbN Nb 2 C Table 1 Mechanical properties of regular and modified medium manganese steelsMedium manganese steelAlloy Tensile strengthMPaYield strengthMPaImpact toughnessJ.cm-2HardnessHBEnergy forcrack extensionJ.cm-2Energy forcrack formationJ.cm-25507963635104114920923318.260.522.877.5 Modified mediummanganese steel196CHINA FOUNDRYVol.4 No.3treatment with the complex modifier. Through the study onwear morphology, it was found that wear mechanism is mainlyplastic deformation wear (Figs. 5 and 6). Because plasticdeformation wear is a process from plastic deformation towork-hardening, crack formation and crack extension in theplastic deformation zone, the increase in yield strength andhigh density of dislocations are beneficial to the resistanceagainst plastic deformation (Figs. 7 and 8), and theenhancement in crack forming energy (Wm) and crackextension energy (Wp) leads to a higher resistance to cracking.Therefore, the wear resistance of the modified mediummanganese steel can be increased remarkably.3 The mathematic model and mech-anism of plastic deformation wearBecause the surface of specimens is always rough, wear takesplace between hard surface and soft surface. In order to imitatethe condition of wear, abrasive is chosen as one unit of thehard surface. The plowing is formed under the normal stressof abrasive with static loading. A part of material is pressedto the two side of the plowing, and the pressed material isworn out because of the repeated plowing, and the plasticdeformation wear takes place. For the analysis of stressdistribution in contacted area, one abrasive is generallyconsidered. When the abrasive is pressed into the matrix, theboundary of extruded part is considered as straight line AC(Fig. 9), and the vertical pressure can be calculated by meansof the elastic contact theory [12]:where θ is included angle between α line in AC edge and x axis,and the included angle between a line and AC, AB is  , theincluded angle between AC and x axis is γ-ϕ. σ is the averagestress in AEC area, k is the shear yield stress of the materials. InABD area:   . Because ξ is the content in the same aTable 2 Results of abrasive wearAlloyFixed abrasive with static loadingMedium manganese steelModified mediummanganese steel30.615.911.92Value of wearmgWear resistanceDislocation density in unmodified mediummanganese steel after wearFig. 70.2 µm20 µmMorphology of plastic deformation wear inmodified medium manganese steelFig. 620 µmMorphology of plastic-deformation wear inunmodified medium manganese steelFig. 50.2 µmDislocation density in modified mediummanganese steel after wearFig. 8(a) Slip-line field affected by abrasiveFig.9 Stress analysis in plastic deformation wear(b) Stress distribution in ABD area(1)197Research & DevelopmentAugust 2007line, and σ-2kθ =ξ, the following equation is inferred:From equation (2), σ can be calculated:σ is the average stress in AB line, while σn is the normal stress atany point, and σn=-K+σ . From Fig. 9(b):here σx=σy=σ, τx=k, σn=σ1=σ-kThe average compression stress in AB edge is p, that is σn=-p,from equation (4): p=k-σ =2k(1+ϕ).The pressing stress in unit length of Z axis is as follows:Here k is the shear yield stress of the materials, 2γ is the anglepressed into matrix, and k and 2γ can be measured. L and ϕ areunknown quantity.   The equation (5) can be rewritten:Here σs is yield strength: σs=2τs=2k. Because γ-ϕ is includedangle between AC line and x axis, L and ϕ can be obtained fromFig. 9(a)Because the material can not be compressed, the triangle OBGis equal to the triangle ACG:L is eliminated from equations (7) and (8), and ϕ can be written:Through calculation: ϕ =32.95=0.58rad.The wear volume for one abrasive in unit length is W:Before W is determined, h must be calculated from equation (7):If    ,Therefore:Equation (13) is used under the condition of one abrasive. Forpractical conditions, the number of abrasives is n, Therefore:Equation (13) can be rewritten as  , where ka is afunction of abrasive shape and plastic deformation angle ϕ. Underthe same wear condition, ka is a constant. Since wear volume Wis proportional to the square of the loading and to the number ofthe abrasives, and inversely proportional to the square of theyield strength of the materials, the wear resistance can be(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)increased remarkably from the enhancement of the yield strengthof the materials. The yield strength (σ) of the modified mediummanganese steel is 510 MPa (Table 1), while that of the referencedmedium manganese steel is only 363 MPa. According to theequation (13), the calculated wear resistance of the modifiedmedium manganese is about 1.92 times as high as that of themedium manganese steel, which matched very well with theexperimental results.4 Conclusions(1) Wear resistance of the modified medium manganese is 1.92times that of the medium manganese under two-body abrasivewear condition, and the wear mechanism is primarily plasticdeformation wear.(2) Wear volume W is proportional to the square of the loadingand to the number of the abrasives, and inversely proportionalto the square of the yield strength of materials, and therefore thewear resistance can be substantially improved by theenhancement of yield strength of the materials.(3) The yield strength, crack forming energy (Wm) and crackextension energy (Wp) of the steel can be greatly increased bycomplex modifiers.LI Shao-xiong, SHI Zhong-liang. The microstructurestransformation on the alloying surface of Mn13 and its wearresistance. Tribology, 1993, 13(3):201-207.HE Zhen-ming, JIANG Qi-chuan, FU Shao-bo. The relationshipbetween wear resistance and strain-induced martensitestrengthening in austenite medium manganese steel. ChineseJournal of Mechanical Engineering, 1988, 24(4):98-102.HE Zheng-ming, JIANG Qi-chuan, FU Shao-bo. Improved work-hardening ability and a wear resistance of austenitic steel undernon-severe impact loading condition. Wear, 1987, 120 (3): 305-319.XIE Jing-pei, ZHU Yao-min, WANG Xiao-ying. Thermodynamicanalyses of strain-induced martensite transformation in Fe-7Mn-1.2C alloy. Journal of Materials Science and Technology, 2000,16(4): 449-451.WU Guo-qing, ZHANG Xiao-feng, FANG Liang. The dynamicsimulation under the condition of two-body wear. Tribology, 2000,20(5):360-364.Rabinovic Z E. Friction and Wear of Material. New YorkPublishing House, 1965, 60-91.Fang L, Zhou Q. D. An explanation of the relation between wearand material hardness in three-body abrasion. InternationalConference on Wear of Material, Ed. By K. C. Ludema. London,1991, 513-520.QUE Jing-xin,WU xiao-li. Investigation on abrasive wear in boridinglayer. Metal Heat Treatment, 1992, (9):37-40. (in Chinese)Zum-Gahrk K H. Modeling of two-body abrasive. Wear, 1988,124(1): 87-103.XIE Jing-pei, WANG Wen-yan, WANG Ai-qin. Effects of Nb andN in medium manganese austenitic steel. Journal of Iron andSteel Research. 2002, (1):38-42. (in Chinese)XIE Jing-pei. Abrasive wear and strengthening-toughening inmodified medium manganese steel. Doctor dissertation in HarbinInstitute of Technology, 1997. (in Chinese)ZHUANG Yu-nian. Engineering Plastic Mechanics. Beijing: HighEducation Press, 1983:178. (in Chinese)[1][2][3][4][5][6][7][8][9][10][11][12]ReferencesThis work was supported by the Innovation Fund for Outstanding Scholar of Henan province, No.0621000600.

Plastic deformation wear in modified medium manganese steel

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