Influence of nitriding treatment on the microstructure and mechanical properties of AISI 422 martensitic stainless steel

This study investigates the microstructural evolution and residual stress mechanisms in plasma-nitrided AISI 422 martensitic stainless steel through integrated experiments and phase-field simulations. Plasma nitriding at 560 °C induced sequential phase transitions, initiating from Guinier-Preston (GP) zones that transformed into coherent CrN precipitates, followed by continuous coarsening growth. Advanced characterization techniques, including TEM, XRD, and geometric phase analysis (GPA), revealed depth-dependent structural gradients: CrN precipitates evolved from coherent needle-like morphologies (100 nm length, 5 nm width) near the matrix interface to coarser rod-like structures (50 nm length, 20 nm width) in shallower regions. Surface layers were dominated by γ′-Fe4N phases, exhibiting residual tensile stress, while peak compressive stress (−550 MPa) occurred at 150 μm depth due to coherent strain fields arising from CrN-matrix lattice mismatch. Phase-field simulations successfully replicated CrN nucleation, anisotropic growth, and coarsening, aligning closely with experimental observations. The saddle-shaped residual stress profile was attributed to nitrogen diffusion-induced lattice expansion, strain relaxation during precipitate coarsening, and contraction of the γ′-Fe4N phase. These findings establish a microstructure-property relationship, providing theoretical support for industrial-scale production of nitrided martensitic stainless steels.