Heat treatment and properties of a hot work tool steel fabricated by additive manufacturing

Additive manufacturing (AM) is increasingly used for the manufacturing of tools and dies; in this respect, apart from the optimization of processing parameters, it is important to establish the most proper heat treatment conditions for the fabricated parts. In this paper, the microstructure, and some properties of H13 hot work tool steel fabricated by selective laser melting (SLM) have been evaluated after direct tempering and in quenched and tempered condition. The as-built microstructure consists of a partially tempered martensite and a much higher amount (up to 19%vol) of retained austenite (RA) compared to the quenched steel (RA<2%vol), leading to a much stronger secondary hardening during tempering. Quenching further produces a partial recovery of the solidification structure, removing the cellular/dendritic SLM structure as well as the uneven local hardness. Dilatometry highlights a much different tempering behaviour for as-built and quenched steel. Very promising fracture toughness values were measured particularly when the samples were tested with a notch plane perpendicular to the build plane (XY plane). In spite of higher hardness, the fracture toughness of tempered samples was comparable to that of quenched and tempered ones.