Study On 18Ni300 Maraging Steel Manufactured By Direct Laser Deposition

18Ni300 maraging steels have exhibited high strength,excellent thermal stability,good machinability and exceptional resistance to crack propagation,and thus widely used in the mold and die making industrial fields.In recent decades,with the growing requirements on the morphologic complexity of mold parts,additive manufacturing has been proved to provide extended freedom in mold design,in comparison to conventional casting.In order to obtain high-performance 18Ni300 steel,this work studied systematically the effect of the T6 heat treatment and the TiC addition.The microstructure and properties of the direct laser deposited 18Ni300 steels were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),vickers hardness tester,universal material testing machine and friction and wear tester.The main work and conclusions are as follows:(1)The effect of different aging time(1h,3h,6h,and 9h)at 500? on the microstructure and mechanical properties of the direct laser deposited 18Ni300 steels were studied carefully.The results show that the microhardness and tensile strength of the as-deposited steels increase with increasing aging time up to 6h,and subsequently decrease with the time extending to 9h.On the contrary,the elongation is shown a reverse trend.The original 18Ni300 maraging steel exhibits cellular microstructure with an average grain size of 3.7?m,composed of martensite and nano-Ti₂N particles.After aging treatment,Ni-rich nano-precipitates(Ni₃(Mo,Ti)and Ni(Mo,Ti))together with reverted austenite were formed,and promoted with extension of aging time.The optimal comprehensive performance of the 18Ni300 maraging steel can be obtained by aging 3h at 500?,with microhardness of 509 HV_0.2,ultimate tensile strength of 1686 MPa,and elongation of 11.5%,respectively.(2)The effect of TiC addition(0?10 wt.%)on microstructure,microhardness and wear resistance of the direct laser deposited 18Ni300steels were studied carefully.The results reveal that microstructure of the TiC-reinforced composites is significantly refined,in which the volume fraction of TiC formed with different morphologies varies with the addition content of TiC particles.However,some cracks exist in the composite sample with the TiC addition up to 10 wt.%.The average microhardness of the as-deposited samples increases initially and then decreases with increasing TiC contents,and the maximum microhardness(444 HV_0.2)is obtained after adding 5 wt.%TiC.In addition,the sample with 5 wt.%TiC has been experienced a solution treatment at 850? for1h and then a subsequent aging treatment at 500? for 3h.Massive nano-sized precipitates(Ni₃(Mo,Ti))and a few patches of austenite are formed,which are dispersively distributed in the martensite matrix after the heat treatment.This results in a high microhardness of 576 HV_0.2and a low wear rate of 1.37×10^-6mm³/(N·m),compared with those of the TiC-free sample(368 HV_0.2and 5.61×10^-5mm³/(N·m)),respectively.