High strength and damage tolerant steel for additive manufacturing

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The project focuses on research into the high-strength austenitic steel alloy X15CrMnNiN 15-3-3-0.15 for additive manufacturing using both laser beam and electron beam powder bed processes (LB-PBF, EB-PBF). In contrast to previously available materials such as AISI 316L or 17-4PH, this steel is expected to exhibit extensive isotropy of mechanical properties, possibilities to selectively influence process-induced residual stresses, and good potential for high strength through downstream Q&P treatment (Rm > 1600 MPa) for a wide range of applications, and to have high damage tolerance to structural defects as well as cyclically stable fatigue properties. The necessary boundary conditions for an isotropic, fine-grained microstructure are being studied in parallel in both processes. A major contribution is made by primary ferritic solidification, which leads to an additional phase transformation (ferrite to austenite). Even though the EB-PBF and LB-PBF processes are very similar in principle, the components are manufactured under significantly different process conditions (build platform temperatures, powder particle size, layer thicknesses). This in turn leads to different cooling conditions and changes in microstructure and residual stress states, which will ultimately be reflected in the mechanical properties of LB-PBF and EB-PBF manufactured components. Therefore, the entire process chain of LB-PBF and EB-PBF processes for additive manufacturing will be investigated on a fffor both processes adjusted alloy design of steel X15CrMnNiN 15-3-3-0.15. This includes not only processability in the actual build process, but also prior atomization and downstream heat treatment. Additive manufacturing is followed by Q&P heat treatment, the adaptation of which to the different alloy initial states (i.e., after EB-PBF and LB-PBF) needs to be explored in order to set targeted mechanical properties of the material. Since the LB-PBF fabricated material is expected to have significant residual stresses, these must be considered comparatively when evaluating the properties before and after Q&P heat treatment. In addition, due to the wide variance of possible build platform temperatures (from RT up to 800°C), the intermediate step of solution annealing may not be necessary for the LB-PBF process. In principle, the effect on the formation of residual stresses and possibly carbides is of interest here. In addition to the static material properties, the cyclic properties in the low- and high-cycle range will be investigated in particular, since this is of central importance for a large proportion of industrially used components. The mechanical characterization of the sample material is accompanied by microstructural investigations.