The effect of thermal cycling on direct laser-deposited gradient H13 tool steel: Microstructure evolution, nanoprecipitation behaviour, and mechanical properties

Abstract

Gradient-structured (GS) materials are attracting much interest due to their ability to deliver stronger and more ductile steels. However, the fabrication of GS H13 steels via direct laser deposition (DLD) has been rarely studied although they are widely employed as hot-work steels for use at high temperatures. Therefore, in this investigation, three scanning strategies with different thermal cycling protocols were applied to fabricate GS H13 steels using DLD and the effect of these conditions on the microstructural evolution and mechanical properties of the resulting steels was evaluated. Using an optimised scanning strategy with suitable time intervals between individual layers and incident laser energy, it can be obtained gradient microstructures with grain sizes ranging from ∼7.1 μm at the top to ∼4.7 μm at the bottom. The sample surfaces constituted of martensite while the interior was transformed from martensite into tempered martensite and nanoprecipitates (Cr23C6) after thermal cycling. Thus, the sample surfaces exhibited a high ultimate strength (1981 MPa) and hardness (∼660 HV), whereas the interior exhibited better plasticity (∼12 %). Finally, the methodology outlined in this article provides a strategy to control thermal cycling to achieve the necessary gradient in the microstructure and mechanical properties of DLD H13 steels.