Study On Carbide And Grain Refinement Mechanism Of M50 And M50Nil Steel During Multidirectional Forging

M50 steel has the advantages of high hardness under high temperature conditions and is currently an aviation bearing steel with excellent comprehensive performance.The carbide formed by the alloying elements in the M50 steel is an important organizational feature and is also a good guarantee of bearing performance.However,it is unevenly distributed.Excessively large size carbides can cause cracks in the bearing during service to cause failure.M50 Nil steel is a new generation of high-temperature bearing steel developed on the basis of M50 steel.After the surface hardening treatment,the steel has the characteristics of “hard internal toughness”,which greatly improves the fatigue life of the bearing.However,the grain size of the M50 Nil steel is large,which affects the performance of the forgings.Therefore,how to control the size and distribution of the carbides in the M50 steel and the problem of coarse grains in the M50 Nil steel improve the material properties and is a difficult problem in the preparation of high-performance high-temperature bearing steels.In this paper,the M50 steel and M50 Nil steel were multi-directionally forged.The effects of initial forging temperature and cumulative dependent variables on the microstructure of M50 steel and M50 Nil steel were studied.The mechanism of carbide fragmentation of M50 steel and the grain of M50 Nil steel were analyzed.At 1000? and 1100?,the multi-directional forging of M50 steel with cumulative strains of 1.2,2.4,3.6 and 5.4 were respectively performed.The carbides of forgings after multi-directional forging were significantly shredded.The size of the original M50 steel was 5~25 ?m,after deformation the carbide was fragmented to less than 10 ?m,and at the same time the initial forging temperature increased so that the particulate carbides dissolved significantly.When the initial forging temperature is 1100?,the crystal grains in the easy deformation region of the forged part are refined as the accumulated strain amount increases.When the cumulative strain amount increases to 5.4,the average grain size of the easily deformable zone is 3 ?m.At 1050?,1100?,1150? and1180?,the multi-directional forging of M50 Nil steel with a cumulative strain of 3.6 was found.After the forging,the grain size of the deformable zone of the forging significantly changed,when the M50 Nil steel was are maintained for a certain length of time only at these four temperatures without multi-directional forging,the grain size gradually increases as the heating temperature increases,and the grain size of the easily deformable zone after multi-directional forging reaches a minimum at 1100?.After heating at 1100? for 30 min,multi-directional forging of M50 Nil steel with cumulative strains of 0.6,1.8,3.6,5.4 and 7.2 was performed.The grain size of M50 Nil steel in the deformable zone showed a decrease trend with the increase of cumulative strain.