Compared to conventional fabrication methods, additive manufacturing (AM) introduces new opportunities in terms of design freedom and part complexity due to the incremental layer‐by‐layer process. For tooling applications, higher cutting speeds can be realized by implementing of internal cooling channels in tools that could not be fabricated otherwise. However, processability of high‐alloyed tool steels with laser powder bed fusion (LPBF) faces certain restrictions. In addition to pore formation, severe cracking caused by a combination of process‐related stresses due to the high thermal gradient and susceptible materials may occur. This work aims to clarify the occurrence of process‐related defects in dependence of the applied energy input of a high‐alloyed cold‐work tool steel and to correlate it to the evolution of microstructure respectively solidification structure. Defect surfaces and structural evolution are investigated. The results exhibit that with increasing energy input porosity changes from lack‐of‐fusion to keyhole porosity. Most recently published investigations suggest cold cracking as predominant failure mechanism during LPBF of tool steels. However, for the investigated material, the present study clearly reveals that, irrespective of the chosen energy input, hot cracks are formed. Crack propagation can be connected to the solidification structure and possible thermal stress accumulations caused by the process.
High‐speed steels (HSS) exhibit a microstructure that comprises a martensitic matrix with carbides. Due to the generally high thermal stability of carbides, it is necessary to perform hardening at very high austenitizing temperatures. Nevertheless, there are certain carbides in HSS that are not dissolved. Therefore, the composition of the austenite, which can be transformed into martensite, is significantly different from the alloys’ nominal composition. Consequently, commonly applied formulae for the calculation of martensite start temperature cannot be used for HSS. The current study demonstrates how empirical formulae, which are basically applied for low‐alloyed steels, where no carbides are present at austenitizing temperature, can be modified for high‐alloyed HSS by applying thermodynamic calculations. Thermo‐Calc software is utilized to calculate the composition of the austenite at two different austenitizing temperatures, and with these compositions are calculated subsequently. For experimental verification, of four alloys, which are quenched from these austenitizing temperatures, are determined using dilatometry. The experimental results show good agreement with the corresponding thermodynamic equilibrium calculations. Furthermore, the results reveal that Co does not raise as predicted by the commonly applied empirical formulae. Therefore, adapted formulae for HSS in a wide composition range are proposed.
Additive manufacturing of steel powders has gained a lot of attention in recent years. In the early stages of Laser Powder Bed Fusion (L-PBF) of steel powders, the well-known materials 1.2709, 316L, and 17-4 PH were used due to their very low carbon content. Yet, since these materials are, on the one hand, quite soft (316L) but, in some cases, too highly alloyed for specific engineering applications (1.2709) on the other hand, also carbon steels are increasingly considered for a use in L-PBF processes. In general, it is well known that carbon limits the weldability of the steel materials. As a rule of thumb, steels with a carbon content below 0.22 wt.-% are suitable for L-PBF processes without powder bed preheating. This contribution presents a new carbon-steel alloy concept which can be processed by L-PBF without powder bed preheating. Due to the special alloy design, it will be shown that the printed parts are ready-to-use in the as-built state with a well-balanced property relationship of strength, ductility, and impact toughness. Apart from the usability in the asprinted condition, it will also be shown that an additional heat treatment or even a surface hardening process can be used to gain even better material and part properties compared to the as-built condition.
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