Optimization Of Alloy Composition And Properties Of High Speed Steel Fabricated By Powder Metallurgy

Spray forming can effectively produce high speed steel(HSS)with low cost,low alloy element segregation and uniform carbide.However,for the spray formed high speed steel,the heat treatment is still necessary to realize the combination of martensitic strengthening and carbide secondary hardening to further enhance its structure and mechanical properties.The heat treatment and strengthening of traditional cast high speed steel and powder metallurgy(PM)high speed steel have been studied.But there are still some unsolved mechanism problems.For example,few scholars focused on heat treatment and strengthening organization according to the microstructure characteristics of spray formed HSS,and the specific precipitation behavior of carbides.The complex transformation mechanism of carbide in tempering process is rarely reported.In this paper,the best heat treatment process of spray formed M42 is explored,and the influence mechanism of quenching process and tempering process on the microstructure,retained austenite and carbide transformation of spray formed M42 high speed steel is studied respectively.Considering that high-quality spray formed HSS is mainly composed of high V content at present,with high cost,a new composition strengthened by La is designed in order to achieve high mechanical properties and optimized structure,based on M2 HSS.The details and main conclusions are summaried as follows:(1)The effect of quenching and tempering heat treatment parameters on the transformation of carbide and size(M3C-M6C-M7C3)of M42 high speed steel was revealed,and the mechanism of retained austenite and carbide transformation on the secondary hardening of M42 high speed steel was further clarified.Suitable tempering process can effectively enhance the wear resistance of M42 high speed steel.The carbide dispersed in the matrix can effectively prevent the structure of M42 high speed steel from peeling off during the process of friction and wear.(2)The optimum heat treatment process of spray forming M42 high speed steel is determined as follows:quenching temperature 1180?,tempering temperature 540?,tempering for three times,one hour each time.Under the optimum heat treatment process,the hardness and bending strength of M42 high speed steel reached 67 HRC and 3115 MPa respectively.Compared with the traditional cast high speed steel and the normal pressure sintered powder metallurgy high speed steel,it has greater performance advantages.(3)By adding La to strengthen M2 high speed steel,the mechanism of refining martensitic grain of M2 PM high speed steel by La is clarified:rare earth elements are distributed at the martensitic grain boundary,forming dispersed RE alloy carbide,and the lattice distortion caused by La replacing Fe and the pinning effect of fine ceramic phase can promote the martensitic grain size from 50 nm when La is not added decreased to 13 nm with 0.6%La addition.The mechanism of strengthening the hardness and wear resistance of powder high speed steel by La refining martensitic grains and the homogeneous dispersion of rare earth alloy M6C carbide is revealed.The La strengthened M2 powder metallurgy high speed steel with a microhardness of 573 HV and a bending strength of 3298 MPa is obtained.