Nitrogen-Induced Microstructure and Property Evolution in Ferritic Stainless Steel during Thermochemical Hot Isostatic Pressing

Thermochemical hot isostatic pressing (TC-HIP) in a nitrogen (N) atmosphere using a HIP system with uniform rapid quenching (URQ ® ) integrates high pressure, elevated temperature, and rapid cooling to introduce significant amounts of N into stainless steels. Interstitially dissolved N increases hardness and corrosion resistance while stabilizing the austenitic phase, making TC-HIP suitable for microstructural design and the improvement of near-surface properties. In this feasibility study, ferritic stainless steel X6Cr17 (DIN EN 1.4016, AISI 430) was TC-HIP heat-treated in an N 2 atmosphere followed by URQ ® . Microstructural analysis by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and glow discharge optical emission spectrometry (GDOES) demonstrated the formation of nitrides, N-enriched austenite, and martensite. GDOES, CALPHAD calculations, and DICTRA simulations were used to investigate N uptake during the nitriding process. Microhardness and corrosion testing revealed that CrN precipitation increases hardness (880 HV0.05) but impairs corrosion resistance, while the N-enriched austenite combines elevated hardness (365 HV0.05) with excellent corrosion performance. Martensitic regions exhibit high hardness (640 HV0.05) together with a corrosion resistance comparable to conventional austenitic grades. The present work thus lays the foundation for TC-HIP as a tool for microstructural design and targeted enhancement of near-surface properties in stainless steels.