Investigation on the moderately high-temperature tensile behavior of the laser powder bed fusion-fabricated 17-4 PH steel

The as-built 17-4 PH steel manufactured by powder bed fusion (PBF) generally requires solution and aging post heat treatments to improve performance and meet service requirements. This is because the Cu-rich phase in the microstructure has no time to precipitate and grow due to the high cooling rate of the PBF process. In this study, high-temperature tensile tests were conducted on as-built specimens along horizontal and vertical directions and the specimens with direct-aging and solution-aging heat treatments at 450-550 degrees C and different strain rates. The results show that when the temperature is between 450 and 500 degrees C, the high-temperature tensile properties of the as-built PBF 17-4 PH stainless steel specimens are comparable to or even better than those of the heat-treated specimens. The growth of the Cu-rich phase during the high-temperature tensile process is key to the results. Molecular dynamics simulation results show that crystal defects generated during the high-temperature tensile process significantly promote the formation and growth of Cu clusters. In addition, the grain morphology, the sizes and distributions of the Cu-rich phase and Si-Mn inclusions, and the grain boundary (GB) strength affect together the fracture modes of the four specimens under high temperatures. When the temperature was 550 degrees C, the tensile strengths of all specimens are limited by the coarser Cu-rich phase, the larger Si-Mn inclusion and the more severe migration of GB. The study shows the potential for the direct use of the as-built 17-4 PH steel in service environment characterized by short duration and high temperature such as missile launchers. The idea also improves significantly their application capacity of other similar PBF-ed alloys by avoiding the cost of post heat treatment.