Influence of increased carbon content on the processability of high-speed steel HS6-5-3-8 by laser powder bed fusion

By using laser powder bed fusion (LPBF) in the tooling industry, tools with complex geometries and integrated functions, such as adapted cooling channels, can be manufactured with high potential in terms of product lifetimes or cycle times. When processing tool steels with a carbon content > 0.6 wt.-% by means of LPBF, hard and brittle phases are formed as well as soft retained austenite and carbides. Combined with high internal stresses in the part this often leads to crack formation. Using base plate preheating up to 500 degrees C can enable the production of crack free components, however, to increase the processability of high-speed steel, varying the alloy composition by stabilizing austenite could lead to reduced residual stresses. In this study, we investigate the processability and microstructure of high-speed steel HS6-5-3-8 reference and HS6-5-3-8 with increased carbon content by rapid alloy development (RAD) in the LPBF process. The manufactured specimens are analysed based on part density, hardness, microstructure, and homogeneity of the microstructure. Light-optical and electron microscopic methods (i.e. SEM, EDS, EBSD) are used for microstructure investigations. We found that an increased carbon content in the alloy leads to an increased carbide volume fraction and a fully austenitic matrix. However, contrary to the hypothesized beneficial impact of austenite stabilization on crack formation, we observe a worsening of cracking with C content. Based on these results, we discuss which microstructural features are the deciding factors for cold cracking in LPBF of high-carbon steels. Based on the results, a relationship between alloy composition of high-speed steel and their processability is discussed and the application of RAD by means of LPBF is assessed.