Selective laser melting manufacturing of stainless steels: heat treatment effect on microstructure and hardness of maraging steels

Laser Powder Bed Fusion (L-PBF) Selective Laser Melting (SLM) is a widespread additive manufacturing technology in industrial applications, for metal components manufacturing. Maraging steel is a special class of Fe-Ni alloys, typically used in the aerospace and tooling sectors due to their good combination of mechanical strength and toughness. This work reports about the heat treatment effect on the microstructure and hardness value of 300-grade maraging steel manufactured by the L-PBF SLM process. The considered heat treatment consists of a solution annealing treatment followed by quenching and ageing hardening treatment. The effect of ageing temperature is reported, in a wide temperature range. Results show that solution annealing treatment fully dissolves the solidification structure caused by the SLM process. Moreover, the ageing hardening treatment has a significant impact on the hardness, hence on strength, of SLM maraging steel. The optimal ageing conditions for the SLM maraging steel are identified and reported: in particular, results show that the hardness of 583 HV is achieved following ageing treatment at 490 degrees C for 6 hours. A higher treatment temperature leads to over-ageing resulting in a decrease of hardness. Conversely, an excessive ageing time does not seem to affect the hardness value, for the ageing temperature of 490 degrees C.