Development of a new processing route for the additive manufacturing of highnitrogen, corrosion resistant, martensitic steels by combining PBF-LB/M and HIP with process-integrated quenching

• Applicants: Dr.-Ing. Nils Ellendt; Professor Dr.-Ing. Sebastian Weber
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 549277253

N-alloyed martensitic, corrosion-resistant steels with an N content of 0.3 to 0.5 mass-% can be conventionally produced only by pressure-assisted metallurgy. If such a steel is atomized using N2 as melting atmosphere and atomizing medium, a significantly lower N content results after PBF-LB/M due to the low N solubility in the melt. By gas nitriding with N2 as nitrogen donor, N contents of more than 1.0 mass-% can be realized in Cr-containing steel powders. If the gas-nitrided powder is mixed with an N-free starting powder to a total N content of about 0.7 mass-% and processed via PBF-LB/M thereafter using different exposure times, the high solidification and cooling rates result in N contents in the components that are far above the maximum equilibrium solubility of N in the melt. Due to the partial outgassing of N2 and the formation of lack of fusion defects, densities of max. 97.7% by volume can currently be achieved. The quantitative relationships between the N content of the starting powder, the PBF-LB/M parameters used (scan speed, layer thickness, laser power and hatch distance), the resulting N content, and the defect and microstructure formation are not yet understood. Defects in additively manufactured components can be closed thermomechanically by HIP processing, which significantly increases the mechanical and chemical performance. The Ar pressure during HIP increases the thermodynamic austenite stability and thus the interstitially soluble N content in the austenite. In HIP plants with integrated gas quenching, highly N-containing conditions can thus be realized, making it possible for the first time to produce steel matrices strongly supersaturated in N during HIP. The process combination of PBF-LB/M production of steel powders with high N content and subsequent HIP post-processing with internal gas quenching thus enables the production of (C+N)-martensites by additive manufacturing. To date, there have been no systematic studies on the production of N-containing martensitic corrosion-resistant steels using additive manufacturing and HIP post-processing. In particular, the underlying phase formation mechanisms are of scientific interest in the context of the austenite-stabilizing effect of pressure. Within the scope of the proposed project, a new process route for N-alloyed martensitic corrosion-resistant steels is therefore to be developed which specifically exploits the above-mentioned relationships in a multi-stage process.

• DFG Programme: Research Grants