Processability and Microstructural Evolution of W360 Hot Work Tool Steel by Directed Energy Deposition

Abstract

Laser directed energy deposition (L-DED) was used to produce samples of the newly patented W360 hot work tool steel by Böhler. The process parameters were optimized to obtain nearly fully dense samples through the production and analysis of single deposited tracks and single layers. Subsequently, bulk samples underwent a hardening heat treatment, consisting of austenitizing, air quenching, and tempering. The samples were analysed in the as-built condition (AB), after quenching (Q) and following tempering cycles (HT) to observe the microstructural evolution. The microstructure was investigated using optical and scanning electron microscopes, energy dispersive X-ray analysis, and X-ray diffraction analysis. Furthermore, the microstructural evolution was analysed with differential scanning calorimetry, while the mechanical response was evaluated through microhardness test. It was found that the AB samples exhibited a dendritic-cellular microstructure with tempered martensite laths. The thermal history of the AB samples was completely modified by the austenitizing treatment followed by quenching, resulting in a fully martensitic Q sample that did not display the typical dendritic-cellular microstructure of the L-DED process. The completion of the heat treatment with tempering cycles revealed the presence of Mo-rich carbides dispersed in a martensitic matrix. The HT samples exhibited a mean microhardness of 634 HV, remaining constant along the entire building direction from the substrate to the last deposited layer, indicating a homogeneous microstructure. This high value, similar to other hot work tool steels such as H13, makes W360 a very promising candidate for tool build and repair purposes.

Graphical Abstract