Mechanical-corrosion properties of 17-4 PH stainless steel manufactured using laser powder bed fusion: The influence of post-process heat treatments

Laser powder bed fusion (L-PBF), also commonly referred to selective laser melting, has been attracted a lots of attention in the past few years for the fabrication of complicated mold designs with conformal cooling channels that were previously impossible to make via the conventional manufacturing techniques. The L-PBF process is capable of creating complex and near net shape components with relatively high densities and mechanical properties comparable to the wrought components. 17-4 PH (precipitation hardening) stainless steel is the proper material of choice for the fabrication of high strength, anti-wear molds. In the present investigation, 17-4 PH stainless steel components are fabricated by L-PBF. Different combinations of post process heat treatments (hot isostatic press [HIP], solution annealing [SA] and aging) are utilized to modify the microstructure and achieve the optimized mechanical properties. The corrosion resistance of the as-printed samples before and after post process heat treatments are systematically evaluated using the immersion and Tafel polarization tests in NaCl solution. Despite the improvements of the mechanical properties, the specimens demonstrate poor corrosion resistance after “HIP+aging”, while keep good corrosion resistance after “SA+aging”. Further microstructural analysis revealed that a Nb-rich phase (NbC) is precipitated at the grain boundaries of prior austenite in the “HIP+aging” condition, forming a network of grain boundary precipitates. In contrast, NbC precipitates are homogenously distributed in the martensitic grains after the “SA+aging” treatment. It was also found that ferritic microstructures exhibit superior corrosion resistance compared to martensitic microstructures of the as-printed samples.