Effect of Post-Heat Treatment on the AISI M4 Layer Deposited by Directed Energy Deposition

Abstract

Currently, high-speed steel (HSS) powders are deposited locally on a metal surface through direct energy deposition (DED) onto hardface tool steel. Although the HSS powder enhances the hardness and the abrasion resistance of a metal surface, it makes the tool steel brittle because of its high carbon content. In addition, the steel is likely to break when subjected to a high load over time. This study focused on improving the steel toughness by applying a post-heat treatment. To fabricate a uniformly deposited layer through DED, M4 powder was deposited onto a pre-heated substrate (AISI D2). In addition, four post-heat-treated specimens were prepared, and their mechanical properties were compared. The Charpy impact and hardness tests were conducted to evaluate the durability required for the D2 die. The deposited M4 powder possessed a high hardness but a relatively low impact toughness. During laser melting, a stable bond formed between M4 and D2 without any cracks or delamination. The hardness of the initial M4 deposited layer was 63 HRC, which changed to 54–63 HRC depending on the effect of the post-heat treatment. Moreover, the post-heat-treatment process improves the impact toughness of the M4 deposited layer by changing its microstructure.