Study on microstructure and mechanical properties of laser solid forming 17-4 PH stainless steel

The microstructure and mechanical properties of as-deposited and heat-treated laser solid forming (LSF) 17-4 PH (0Cr17Ni4Cu4Nb) percipitation-hardening stainless steel are investigated. The results of optic microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD) show that the microstructure of as-deposited LSF 17-4 PH steel consists of lath martensite and a small quatity of the second phase strengthening particles which distribute in and between the laths. According to the characteristics of alloy, the precipitating second phase strengthening particles should be M₇C₃ and NbC carbides, etc. The microstructure at the bottom of the deposited LSF 17-4 PH is mainly thin lath hardened martensite, however, there are thick lath martensite at the top of the deposited one. The microstructure of heat-treated LSF 17-4 PH is thin homogeneous lath martensite and there are more second phase strengthening particles percipitated in the subtrate as well. These second phase strengthening particles should be NbC and M₇C₃, M₂₃C₆ carbides. After heat-treatment, the strength and hardness of samples are slightly improved, while the plasticity is highly improved. In addition, the tensile strength and plastcity of the heat-treated samples are higher than forging bar standard, meanwhile, the yield strength is slightly lower than forging bar standard. The analysis on fractography shows that both the as-deposited and heat-treated LSF 17-4 PH steel exhibit toughness fracture and the second phase particles which consist of M₇C₃ carbides are the resource of microcavities and dimples.