| As a kind of high temperature bearing steel,M50 steel has good high temperature hardness,high temperature basic fatigue property and high temperature dimensional stability.The traditional heat treatment process of M50 is quenching and three times high temperature tempering.Because of the high alloying element,the retained austenite can not be completely eliminated,and the change of the retained austenite results in the decrease of dimensional stability.The Q-P process proposed by Speer enables the diffusion of C elements from martensite to retained austenite to stabilize it.Cr,Mo and V in M50 steel are strong carbide forming elements,the precipitation of carbide will cause a strong competitive relationship to C elements.So,in this paper,a tempering treatment is first carried out to promote the precipitation of carbides,and also to decompose the retained austenite to form fresh martensite.And then,the carbon distribution treatment is carried out,the diffusion of C elements from fresh martensite to retained austenite making it stable.We call it Q-T-P.The microstructure of M50 steel after primary tempering were investigated.The effects of carbon distribution on the retained austenite and carbides and their effects on hardness and toughness were also investigated.First of all,the M50 steel after tempering has been studied.It is found that the retained austenite content is reduced from 18.24% to 1.32%.Lattice and twin martensite coexist,the tempering process makes the decomposition of austenite into martensite and cementite coexistence of high temperature bainite.Carbide forms are massive,spherical and needle-like.Carbides have two types,including MC and M2 C,analyzed by XRD.And then the carbon partition process was carried out.In the process of carbon partition,the amount of retained austenite decreases with the increase of temperature,and the rate of carbon partition is accelerated with the increase of temperature,too.At the same temperature carbon partition,the retained austenite content decreases with the carbon partition time,but a second increase in the retained austenite content occurs at a certain point in time,and this time point is advanced as the carbon partition temperature increases.After carbon partition,the types of carbides are still of MC and M2 C,but the content of M2 C carbides increases.The total content of carbides increases with the increase of temperature and the prolongation of holding time during carbon partition.The hardness,impact toughness and fracture toughness of the M50 steel after carbon partition treatment were tested.It is found that the hardness of M50 steel varies little after carbon partition at different temperatures and times.The impact energy of tempered M50 steel is 11.68 J.When the carbon partition parameter is 250℃ and 20 min,the highest impact energy is obtained.The impact energy is 17.42 J,which is 49% higher than that of the tempered sample.When the carbon partition parameter is 300℃ and 20 min,The impact energy is 17.42 J,which is 43% higher than that of the tempered sample.The impact energy increases little when the Carbon partition temperature is 350℃ and 400℃.The impact fracture surfaces of both tempered and carbon partition are typical brittle end cracks.After 250℃,20 min carbon distribution and 300℃,20 min carbon partition,the impact fracture fiber zone exists continuously,and the dimple distribution is uniform.when the carbon partition parameters are 350℃,20 min and 400℃,20 min,the fiber areas are not continuous,and there are large pieces of fragile carbides.The segregation of brittle elements was not found at the fracture microcracks after all carbon partition parameters treated.The fracture toughness of M50 steel after primary tempering is 17.79 MPa?m1/2,When the carbon partition parameter is 250℃,20 min,the fracture toughness of M50 steel is18.55 MPa?m1/2 When the carbon partition parameter is 300℃,20 min,the fracture toughness of M50 steel is18.55 MPa?m1/2 The fracture toughness is not much improved after carbon partition. |
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