Effect of pressurized solution nitriding on phase changes and mechanical properties of ferritic Fe-22.7Cr-2.4Mo stainless steel

Pressurized solution nitriding (PSN) of ferritic Fe-22.7Cr-2.4Mo stainless steel at 1200 °C in nitrogen gas under pressure of 0.25 MPa for different times has been investigated. The structure and mechanical properties were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy and tensile testing. Phase transformation of ferrite to austenite starts from the sample surface and grows further into the core with increasing nitriding time. A fully austenitic structure was achieved after 9 h nitriding for plates of 2 mm thick. Strip-like chromium nitride precipitates with discontinuous morphology are found in the edges of the sample nitrided for 12 h. The average solute nitrogen content of austenite phase was estimated based on lattice parameter calculations to vary from 1.25 to 1.75 wt%. The yield strength is monotonically increased above 900 MPa with increasing the PSN time. The solution nitrided samples for 6 h exhibit high elongation to fracture (above 30%) and high tensile strength (above 1000 MPa). Solution nitriding changes the fracture mode from ductile to a mixed mode inter-granular and trans-granular brittle fracture. Formation of the planar slip bands plays a major role in brittle fracture of the austenite phase. Brittle fracture is favored with precipitation of chromium nitrides. The time of the PSN should be optimized to suppress chromium nitride precipitation.