Materials characterization of M789 steel manufactured by laser powder bed fusion
University of New Brunswick
The recent advances in the design and development of new alloys compatible with additive manufacturing (AM) are underway to address the manufacturing industry's evolving needs. In this work, a new commercial alloy M789 steel, with printability similar to that of maraging steels combined with the corrosion resistance of stainless steels designed for the laser powder bed fusion (LBDF) process, has been characterized. The microstructure of the as-printed condition exhibited melt pool boundaries; elongated martensitic cellular grains with a high density of dislocations were detected using electron microscopy. The optimum aging heat treatment was then determined by thermodynamic simulation backed up with experimentation; the maximum hardness and tensile strength were achieved at the temperature range of 450°C to 500°C for 2 hours. Aging at temperatures above 500°C allows the formation of reverted austenite, reducing the hardness and tensile strength. The aging process using the optimum parameters transformed the grains into martensitic needle-like structures with the presence of spherical ETA-N₁₃(Ti,Al) precipitates that led to a significant increase in the mechanical properties. Specifically, the material's hardness, tensile strength, and yield strength increased from 31.3±1.4 HRC, 1019±13 MPa, and 843±17 MPa in the as-printed condition to 52.4±0.7 HRC, 1798±4 MPa, 1714±13 MPa in the heat-treated condition, respectively. The simulation and experimental results in this work help the manufacturing industries modify the physical and mechanical properties of additively manufactured M789 steel according to the target application.