Study On Selective Laser Melting And Post-heat Treatment Of 24CrNiMo Alloy Steel

Selective laser melting(SLM)is currently one of the most attention and rapid developed laser additive manufacturing,which is widely used in the preparation of personalized metal parts with high precision and complex structure.At present,low-alloy steel parts which are widely used in industry processed by SLM,have not yet been applied to applications with the lack of systematic research on the structural characteristics and heat treatment processes.Based on the above status quo,this subject focuses on the basic technology,microstructure and properties control and post-heat treatment processing optimization of SLM-processed 24CrNiMo alloy steel.The main conclusions are as follows:The cladding layer of SLM-processed 24CrNiMo alloy steel is mainly divided into two areas:the molten pool zone and the heat affected zone.The microstructure is mainly tempered martensite with a small amount of retained austenite.Twin martensite and a large amount of ?-Fe3C were observed in the molten pool zone.In the heat affected zone,the martensite lath is significantly broadened with some massive retained austenite at lath interface.With the increase of laser power and the decrease of laser scanning speed,the volume of molten pool increases,the heat effect is intensified and the microhardness of the molten pool area and the heat affected area decreases simultaneously.At the same time,the relative density of SLM-processed alloy steel increases.The highest relative density of 99.93%was obtained at 320 W and 750 mm/s.When the laser power is 320 W and the scanning speed is 950 mm/s,the SLM-processed alloy steel has the best mechanical properties,its yield strength and tensile strength are 1252 MPa and 1362 MPa,respectively,and its elongation is 16.2%.Under reasonable SLM parameters,the comprehensive mechanical properties of 24CrNiMo alloy steel formed by SLM are significantly better than that of the as-cast one.In this paper,quenching and isothermal quenching are used to optimize the structure and properties of the SLM-processed alloy steel(laser power is 320 W,scanning speed is 950 mm/s).The microstructure of the quenched sample is twin martensite.With the austenitizing temperature increases from 800? to 1000?,the average prior austenite grain size increases from 5.6 ?m to 18.9 ?m,and the uniformity of grain size is significantly improved.The sample with austenitizing temperature of 900? has the highest yield strength and tensile strength,1390 MPa and 1987 MPa,respectively,but the elongation is only 6.0%,showing brittle fracture behavior.The strength of the sample with austenitizing temperature of 1000? decreases,but the elongation increases to 11.9%,showing quasi-cleavage fracture behavior.The microstructure of the isothermally quenched samples with isothermal temperature higher than Ms point(297?)is upper bainite and fresh martensite,and their mechanical properties are comparable to the quenched sample,but their plastic deformation ability is poor,mostly showing brittle fracture behavior.The microstructure of the isothermally quenched samples with isothermal temperature lower than the Ms point are consisted of initial martensite,lower bainite,and fresh martensite.Comparing the microstructure and mechanical properties of the samples with different isothermal times which austenitized at 800? and isothermal at 200?,it is found that a large number of nano-twins and nano-carbides are in the samples,which play the role of interfacial strengthening and precipitation strengthening,respectively.The nano-carbides mainly are ?-Fe2C and ?-Fe3C,which staggered at 90°and 60° in the lath,respectively.With the extension of the isothermal time,the tempering effect of initial martensite increases and the amount of lower bainite increases,which both cause a large amount of carbide precipitated.At the same time,the amount of fresh martensite decreases.The samples with isothermal time of 10?120 s all have excellent comprehensive mechanical properties,the tensile strength is close to 1800 MPa,but the yield strength is reduced from 1279 MPa to 715 MPa with the increases of isothermal time,the yield ratio is reduced from 0.71 to 0.4,indicated the plastic deformation ability is enhanced,and the elongation is maintained at about 15%,showing ductile fracture behavior.