Effect Of Quenching-Partitioning-Tempering Process On Microstructure And Properties Of M50 Steel

M50 is a high-carbon high-alloy steel with excellent high-temperature hardness and wear resistance,which is widely used in the manufacture of tools,bearings and molds.However,with the development of advanced manufacturing and aerospace industries,the lack of toughness has become a major problem which limit s its use.The quenching-partitioning-tempering(Q-P-T)process as a novel approach was proposed in the last decades.The main purpose is to obtain partial austenite to improve the toughness by enhancing the stability of retained austenite.It emphasizes the strengthening effect of alloy carbides precipitated during tempering.In this present study,we utilize Q-P-T process to control quenching,partitioning and tempering parameters,thus improving the toughness of M50 steel under the premise of ensuring the hardness and strength.Meantime,based on the characteristics of the M50 steel chemical composition,the present work also investigated whether highcarbon steel with low Si or Al can achieve a relatively excellent combination of strength and toughness by means of the Q-P-T process,to provide practical and theoretical basis.The effect of austenitizing process on the microstructure of M50 steel has been investigated.The annealed microstructure owns plenty of carbides,which consists of M23C6,M6 C,M2C and MC.The carbides dissolution during austenitizing process will affect the initial carbon concentration of supersaturated martensite and the amount of retained austenite in the subsequent carbon partitioning process.After quenching at different austenitization temperatures,it was found that the absolute dissolution temperature of fcc-M23C6 and fcc-M6 C was higher than 1000 ? and 1100 ?,respectively,the absolute dissolution temperature of Hexa-M2 C and fcc-MC higher than 1160 ?.With the increase of austenitizing temperature,the gradual dissolution of carbides led to an increase in the carbon content in the matrix.The retained austenite content in the quenched M50 steel gradually increases,accompanied by the progressive coarsening of martensite.The effect of austenitizing time on the microstructure of M50 steel was also studied.The results show that the microstructure is less dependent on time.The related thermodynamics research of the typical Q-P-T process is based on the Fe-C binary alloy of constrained carbon equilibrium(CCE)model.The chemical composition of M50 steel was distinct from typical Q-P-T steel.Large amount of Cr and Mo in martensite and austenite,which is able to affect the carbon atoms activity coefficient.The CCE-Alloy(CCEA)model was constructed for the high-carbon high-alloy steels.The influence of austenitizing temperature on the equilibrium carbon concentration of retained austenite in M50 steel was studied by CCEA model.The actual measured carbon content were compared with the results calculated by CCE and CCEA model.The results show that the calculated results of the CCE A model are consistent with the actual measured changes in carbon content,and it can be used to guide the choice of austenitizing temperature during the actual heat treatment.As the key step of Q-P-T process,the partitioning process has an important influence on the final microstructure and properties of M50 steel.Therefore,the microstructure evolution of M50 steel during partitioning process has been studied.It is found that the carbon atoms diffuse into retained austenite when holding the quenched M50 steel at 250 ? ~ 400 ?.However,the carbon atom activity is relatively poor at the lower temperature,which resulting in low carbon content of retained austenite.On the other hand,due to the lack of Si/Al element in M50 steel,the precipitation of carbides is promoted during long-term preservation,which also leads to the decrease of carbon content of retained austenite and poor thermal stability.The in-situ HRTEM results indicate that the ?/? interface migrates during partitioning process,which also leads to a decrease of austenite content.Taking 300 ? as an example,the retained austenite content in the sample treated by the conventional quenching-tempering process is 2.0%.However,the retained austenite content increases to 7.5% treated by Q-P(300 ?,2 min)-T process,which caused by the excellent thermal stability generating during partitioning step.However,as the carbon partitioning time increases the content decreases to 2.7%(24 min).The stabilizing effect of retained austenite in M50 steel at other partitioning temperatures is approximately the same as 300 ?.Finally,the effect of microstructure on the properties of M50 steel w as studied.The results show that the toughness of M50 steel can be significantly improved by QP-T process without reducing its hardness and strength,and the strengthening and toughening mechanism under Q-P-T treatment was discussed as well.The microstructure of M50 steel treated by the traditional quenching-tempering(Q-T)process is mainly tempered martensite,retained austenite(2.0%)and dispersed secondary carbide.The room temperature impact absorption energy is 10.4 J,and the hardness is HRC 62.3,yield strength and tensile strength are 1894 MPa and 2377 MPa,respectively.After Q-P(300 ?,2 min)-T treatment,the retained austenite content of M50 steel reached 7.5%,and the impact absorption energy increased by 93% compared with the conventional process.However,the higher carbon content strengthened the austenite,and Fe3 C precipitated in the carbon partitioning process provides more nucleation sites for the precipitation of alloy carbides during the subsequent tempering process,resulting in the secondary carbides are more dispersed after the Q-P-T process.Therefore,the hardness and the strength of the material are not reduced than the conventional Q-T process.For Q-P-T samples with long-term carbon distribution and insulation,although the retained austenite content is approximately equal to that of the conventional process,yet it leads to the dislocation in martensite gradually recovers due to the long-term carbon partitioning.The deformation ability is enhanced,which results in the impact toughness that is still superior to the traditional Q-T sample.