Effect Of Heat Treatment On Microstructure And Properties Of Selective Laser Melted 24CrNiMo Steel

Selective laser melting(SLM)is currently considered as one of the most concerned and rapidly developed laser additive manufacturing,which has been widely used in the preparation of metal parts with high precision and complex structures.As a low alloy steel with high strength,24 CrNiMo steel has been extensively utilized in the preparation of high-speed rail brake discs due to its satisfactory thermal stability and low cost.This study was based on the national urgent demand for optimizing the service life of the brake disc,the preparation process and posttreatment of SLM-forming 24 CrNiMo low alloy steel were optimized,in order to achieve the manufacturing theory and principle of high-quality and high-properties rail brake discs.The process parameters of SLM-forming 24 CrNiMo steel was firstly optimized in this paper,and the influence of laser energy density on the forming quality and mechanical property of the specimens were comprehensively investigated.The specimens formed in this experiment possess excellent density and surface quality,and the melting-solidified structure of the SLMforming 24 CrNiMo steel was mainly divided into two areas: melting pool area and re-melting area.However,the re-melting area gradually expanded with the laser energy density gradually increased,which is not conducive to the microstructure uniformity and performance stability of the specimen and accumulated excessive thermal stress,thereby bringing about adverse effects on the forming quality and mechanical properties of the material.When the process parameters were 320 W and 950mm/s,due to the lower laser energy density,the forming specimen possessed the optimal forming quality and mechanical properties.The tensile strength and yield strength of the specimen respectively were 1343 MPa and 1249 MPa,as well as the elongation was13.50 %.Subsequently,conventional quenching and isothermal quenching were used to optimize the structure and properties of the SLM-forming 24 CrNiMo steel.According to the phase transition point of the steel,two quenching temperatures(800 °C and 860 °C)and four isothermal temperatures(200 °C,250 °C,300 °C and 350 °C)were set in this paper.With the increase of quenching temperature,the martensite laths were widened and the parallelism between the laths decreased.Therefore,with the quenching temperature was 800 °C,the specimen possessed better strength-ductility.The tensile strength,yield strength and elongation of the specimen were respectively 1931 MPa,1374 MPa and 5.53 %,and the fracture mechanism was quasi cleavage fracture.Then,the specimens were subjected to isothermal quenching experiment.When the isothermal temperature was higher than the starting point of martensitic transformation(Ms),the microstructure of the specimen was mainly composed of lath martensite and upper bainite.When the isothermal temperature was lower than Ms point,the microstructure of the specimens was a mixture of lath martensite,lower bainite as well as a small amount of ferrite,and the content of lower bainite increased with the increase of isothermal temperature.When the isothermal temperature was 250 °C,due to the strengthening of the lower bainite,the tensile strength and ductility of the specimen were significantly improved compared to the melting-solidified specimen,which were respectively 1714 MPa and14.28%.The fracture mechanism of this specimen was ductile fracture.Therefore,800 °C-250 °C was identified as the optimal isothermal quenching process for SLM-formed 24 CrNiMo low alloy steel.