| Fine blanking is widely used in various industries.However,wear and fracture often occur in fine blanking,which seriously reduces the service life of dies and restricts the development of fine blanking.Powder metallurgy high speed steel（PM HSS）has the characteristics of fine grain and carbides,it has been widely used in fine blanking.But the relationship between microstructure and mechanical properties of PM HSS is not studied deeply enough.In this paper,a powder metallurgy high speed steel PM20 used in fine blanking was selected,the influence of microstructure evolution on wear and impact toughness during heat treatment was studied,which will provide a theoretical basis for improving the life of fine blanking dies.The main conclusions are summarized as follows:（1）The microstructure characteristics of carbides and martensite in PM20 were analyzed,the carbides in PM20 are spherical and distributed uniformly,the matrix is lath martensite,twin martensite and a small amount of retained austenite.With the increasing of quenching temperature from 1025°C to 1180°C,the volume fraction of carbides decreases from 15.76%to 10.48%,the grain coarsening occurs as the effect of pinning on grain boundary weakens.When the volume fraction of carbides decreases,the increased strength of carbides decreases by 20%,but the effect of solution strengthening increases.As a result,the compressive strength increases from 3225 MPa to 3533 MPa.（2）The effects of tempering temperature and cryogenic treatment on the microstructure and mechanical properties of PM20 were studied.With the increasing of tempering temperature from 530°C to 600°C,the volume fraction of carbides increases,part of carbides and martensitic lath coarsen,the compressive strength reduces from 3831 MPa to3125 MPa.Compared with traditional heat treatment,cryogenic treatment before tempering promotes the generation of nano-M6C,which can act as the nucleation core of carbides and promote the precipitation of carbides during tempering.Therefore,the volume fraction of carbides increases from 8.57%to 9.13%and 9.11%,when cryogenic at-196°C and-80°C,respectively.Cryogenic treatment refines the martensite block,reduces the carbon content of martensite.The compressive strength decreases from 3831 MPa to 3625 MPa and 3612 MPa.（3）The effect of microstructure on wear resistance of PM20 was studied.With the increasing of quenching temperature and decreasing of tempering temperature,the wear resistance increases.The samples which were cryogenic treated before tempering have better wear resistance.The samples show typical abrasive wear,accompanied by a small amount of fatigue spalling and oxidation wear.The stress on the friction contact surface is greater than the shear yield strength of PM20,which leads to the plastic deformation of martensite.VC can hinder the cutting of the matrix by abrasive particles,while M6C is cut by abrasive particles.The grain rotation and grain boundary migration driven by plastic deformation lead to the growth of martensite grains after sliding wear,while the carbides and segregation of solutes hinder the growth of grains,resulting in discontinuous grain growth.（4）The influence of microstructure on impact toughness,and the impact fracture characteristics of PM20 high speed steel were studied.The lower the quenching temperature is,the higher the tempering temperature is,the greater the impact toughness is.Cryogenic treatment at-196°C and-80°C before tempering,the impact toughness increases from46.1J/cm2 to 52.7 J/cm2 and 57.8 J/cm2,respectively.During impact fracture,the matrix of PM20 exhibits quasi-cleavage fracture and microplastic deformation,while the carbides exhibit cleavage fracture or fracture.The relationship between martensite and VC is semi-coherent,but not coherent with M6C.Microcracks are easy to occur in M6C or matrix interface.Austenite grain boundary can effectively prevent crack propagation. |
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