Manufacturing through Electron Beam Melting of hot-work tool steels

The use of hot work tool steels is widespread in the production of tooling dedicated to metal forming processes, including forging and high-pressure die casting. A viable strategy to mitigate thermal gradients and enhance temperature distribution, thereby optimising the metal forming process and the working condition of the tool, is the production of tooling with internal hollow shapes and conformal cooling channels. The application of innovative metal additive manufacturing techniques, such as electron beam melting (EBM) offers a competitive advantage in the production of components with complex shapes that would otherwise be unattainable through subtractive manufacturing. However, due to the rapid melting and solidification process starting from a material in powder form, the materials produced by additive processes are often characterized by anisotropic microstructures, high residual stresses and microstructural inhomogeneity. Such techniques also allow to obtain finer microstructures with great advantage of mechanical properties such as wear resistance and impact toughness. The microstructural properties of hot-work tool steels are particularly suitable to face wear and mechanical stress at high temperature, nevertheless the correct heat treatment has to be identified to achieve the most appropriate microstructural properties. This work covers the study of the heat treatment of an AISI H13 hot work tool steel, obtained from nitrogen atomized powders and manufactured through EBM with a powder bed preheat temperature of 1040°C. Despite showing a hardness of 47 HRC, the EBM material showed clear carbide precipitation phenomena, due to long high temperature permanence (about 5.3 hours) and subsequent slow cooling (about 6.2 hours) in He. Starting from the literature on nitrogen-containing wrought AISI H13. The effect of austenitization temperatures between 1025°C and 1100°C on the as quenched alloy was studied by microstructural studies and hardness assessment; the tempering curves were built in a range between 400°C and 650°C for samples austenitized at 1050°C and 1100°C respectively. The microstructure was studied by scanning electron microscopy (SEM), which revealed the presence of undissolved carbides. The size of these carbides was measured by means of image analysis (Imagej software). Carbide size distribution curves were determined for each heat treatment condition to characterize the effect of solubilization at higher temperatures due to austenitization, as well as the effect of precipitation of carbides after tempering and hardness after double tempering was correlated to the impact released energy starting from the as-built condition. Moreover, the tempering curves were built using macro-hardness measurements as a function of tempering temperature and correlated with impact released energy, obtained from un-notched samples according to SEP 1314, to determine the optimal heat treatment condition.