Production of high-nitrogen stainless steels: A novel powder metallurgical approach using hot isostatic pressing with integrated gas quenching

Summary

The incorporation of nitrogen as an alloying element in stainless steels can significantly improve both mechanical properties and corrosion resistance. The conventional production of high nitrogen-alloyed steels (HNS) is hampered by the limited solubility of nitrogen in the steel melt. This study investigates a new powder metallurgical process to produce HNS by mixing stainless steel powder with Si₃N₄ powder, followed by hot isostatic pressing (HIP) of the mixture, in which the Si₃N₄ particles dissolve in the solid austenitic phase, enriching the matrix with nitrogen and exploiting the higher solubility of nitrogen in the solid phase compared to the liquid phase. The innovative aspect of this approach is the use of HIP with integrated gas quenching, which allows simultaneous powder consolidation and heat treatment. This method exploits the austenite-stabilising effect of isostatic pressure, which increases the solubility of nitrogen in the austenitic phase, to achieve previously unattainable nitrogen-supersaturated matrices. The amount of Si₃N₄ to be added is determined based on CALPHAD calculations considering both pressure and temperature during HIP. The study evaluates the potential for producing HNS using two different base alloys: X2CrNi18-9 and X1Cr20. The X2CrNi18-9 steel produces a highly nitrogen-supersaturated austenitic state. The X1Cr20 alloy produces an almost fully austenitic condition in which only nitrogen is the austenite stabilising element. Micro-structural analysis is performed by secondary electron microscopy, electron backscatter diffraction and energy dispersive spectroscopy.