Study On Microstruture And Performance Of 4Cr4Mo3Ni5V2N Hot Working Die Steel

Precision mold is an important and basic component of high-end equipment manufacturing.With the equipment manufacturing become large-scale,complex and precise,the existing mold materials have been difficult to meet the new requirements of modern equipment manufacturing.It is urgent to carry out the design of element and preparation of new die steel.In this paper,a new type of medium Nickel and Nitrogen-containing 4Cr4Mo3Ni5V2N hot work die steel was studied.The effect of different heat treatment processes on the microstructure and properties of die steel was studied.The optimum heat treatment process was explored to give full play to the strengthening and toughening effect of alloy elements.The results of research have great significance for the composition and process design of hot work die steel containing Nickel and the development of high performance hot work die steel.The results of quenching and tempering process showed that the microstructure of4Cr4Mo3Ni5V2N hot work die steel was mainly lath martensite,retained austenite and undissolved carbides.When the quenching temperature was lower than 1100?,there was a lot of undissolved secondary carbides in the microstructure,resulting in low hardness.Increaseing the quenching temperature promoted the dissolution of carbides.When tempering temperature was below 300?,the morphology of martensite remained unchanged,cementite precipitated and hardness decreased.With the increase of tempering temperature,nano secondary carbides precipitated and secondary hardening occured.After tempering above 550?,the martensite coarsened obviously and the hardness decreased.The optimum quenching and tempering process of 4Cr4Mo3Ni5V2N die steel is quenching at 1150?and tempering at 525?,and the hardness can reach 60.1 HRC.The results of quenching-partitioning-tempering(QPT)process showed that the content of retained austenite could be increased by carbon-partitioning after quenching.With the increase of temperature of carbon-partitioning,the solid solubility of martensite decreased,the amount of M₃C carbide precipitated increased,and the content of retained austenite increased.Due to the consumption of carbon by M₃C carbide,the amount of nano secondary carbides reduced after tempering,and the hardness of die steel decreased slightly after quenching and tempering.Compared with the traditional quenching and tempering process,the impact toughness of QPT die steel increased by more than 20%.The results showed that the double quenching peocess refined grain and improved the uniformity of grain size,and promoted the matching of hardness and toughness of die steel.With the increase of first quenching temperature,the grain size decreased,the uniformity of grain size increased,and the hardness increased.The optimum double quenching process for 4Cr4Mo3Ni5V2N hot work die steel is:quenching at 1150?,tempering at 600?for 2h;then holding for 2h at 850? and cooling to 650? for isothermal annealing;finally,quenching at 1150?and tempering at 525?.