Effects Of Composition And Preparation Method On Microstructure And Properties Of Medium-entropy High-speed Steel

High-speed steel is widely used as a material for rolls,molds,and cutting tools due to its high hardness and wear resistance.This article is based on the composition design ideas of high-entropy(mid-entropy)alloys.Based on the composition of traditional M2(W6Mo5Cr4V2)high-speed steel,joint addition of alloy elements such as Co,Ni,Cu,Al,Ti,the effects of multicomponent components and preparation methods(broadband laser cladding,powder bed thickness,and electric arc furnace melting)on the microstructure and properties of mid-entropy high-speed steel were studied.Firstly,a C_0.95Mo₅W₆Fe_x(Cr VCo Ni Cu Al)_88.05-x mid-entropy high-speed steel coating with Fe content of 83,76,69,62 wt.%was prepared by laser wide band cladding.The study found that the solidification structure of the four-component coatings is similar to that of traditional high-speed steels,and they are all martensite,retained austenite,MC,and M₂C carbides.As the iron content decreases and the entropy effect of the alloy system increases,the residual austenite content in the coating gradually increases,and the type of carbides changes from M₂C to MC.The carbide content in the iron-containing 62 wt.%coating was significantly higher,and the total carbide content in the other three components was not significantly different.After three tempering at 530?,in addition to the large amount of retained austenite in the 62 wt.%iron-containing coating,the remaining austenite in the other three-component coatings can be completely transformed into martensite.However,in terms of carbide morphology,the carbides in the iron 83 wt.%coating are lamellar and have obvious network-like bulk distribution,while the carbides in the iron-containing 76 and 69 wt.%coatings are still distributed in a network,but the size is obviously small,the shape changes into a short rod shape,the distribution is relatively more diffuse,no obvious lamellar shape.The maximum tempering hardness of the coating is 821,842,787,and 680 HV_0.5 in order with decreasing iron content,and the crack arrestability gradually became better.It can be seen that the hardness of the three-component coatings with iron content greater than 69 wt.%is slightly higher or close to the hardness of 800 HV or more of the traditional M2 high speed steel.The anti-oxidation performance research shows that the anti-oxidation performance of coatings with different components is controlled by the two factors of alloy element content and the relative content of martensite and austenite.The order of high temperature oxidation resistance from high to low is:iron content of 76 wt.%is optimal,slightly higher than iron content of 83 wt.%and 69 wt.%component coatings,and the 62 wt.%iron-containing coating is significantly less resistant to oxidation due to the residual austenite resistance.The oxidation resistance is obviously poor.Comprehensive coating hardness,toughness and oxidation resistance show that the Fe content of 76 wt.%coating has the best performance.Secondly,the effect of powder bed thickness on C_0.95Mo₅W₆Fe₇₆(Cr VCo Ni Cu Al Ti)_12.05iron-containing 76 wt.%mid-entropy high-speed steel coating was studied,the found that the decrease of the thickness of the powder bed leads to an increase in the solidification rate of the molten pool,which can change the growth pattern of the reticulated carbides in the thick mid-entropy coating and obtain a significantly dispersed uniform carbide distribution.The maximum tempering hardness of thick and thin coatings(2.4mm and1.0mm)is 845 and 833 HV_0.5,respectively,the thin middle-entropy high-speed steel coating has a significantly better crack arrestability due to the more dispersed carbide distribution.The effect of C and Al content on the coating of thin-medium entropy high speed steel was further studied.The results show that when the C content increases to 1.10 wt.%,The coating hardness increases to 853HV_0.5,and the carbides can still maintain a uniform and even distribution.After the Al content in the thin coating was increased to 2.5 times,the carbide could still maintain the dispersion distribution,but the hardness did not increase after the Al content was increased,but decreased slightly.Finally,in order to expand the application field of medium-entropy high-speed steel,the vacuum arc furnace melting technology was used to prepare C_0.95Mo₅W₆Fe_x(Cr VCo Ni Cu Al)_88.05-xmid-entropy high-speed steel with Fe content of 83,76,69,62 wt.%.Research shows that the solidification structure of smelting mid-entropy high-speed steel is still mainly martensite,retained austenite,MC,M₂C and M₆C carbides.Compared with the mid-entropy high-speed steel with the same composition prepared by laser rapid solidification,a small amount of coarse M₆C carbides appeared.The maximum hardness of three tempering with the decrease of iron content is 825,841,816,761 HV_0.5,which is not much different from the tempering hardness 826HV_0.5 of M2 high speed steel prepared under the same melting conditions.The hardness change is consistent with the mid-entropy high-speed steel coating in laser rapid solidification,and the performance is best when the iron content is 76wt.%.In addition,the research found that C_0.95Mo₅W₆Fe₇₆(Cr VCo Ni Cu Al)_12.05 mid-entropy high-speed steel containing 76 wt.%Iron had fractured and spheroidized carbides after quenching at 1210°C,which achieved a uniform dispersion of the carbides.It is a phenomenon rarely reported in the traditional high-speed steel with conventional composition.According to preliminary analysis,it is believed that the entropy effect of mid-entropy high-speed steel promotes the partial melting and fracture of the reticulated carbide composed of MC and M₂C during high temperature heating.The dissolution of carbides during high-temperature quenching of mid-entropy high-speed steels provides a new way to avoid the formation of network carbides that are unfavorable to toughness in traditional high-speed steels.