Influence of ferrite-austenite distribution in 2205 duplex stainless steel on high-temperature solution nitriding behaviour

Duplex stainless steel 2205 was subjected to annealing treatments in the range of 1000–1200 °C to adjust the phase fractions of ferrite and austenite. Annealed steels were subsequently subjected to high-temperature solution nitriding, which transformed the surface-adjacent region into austenite under the influence of nitrogen ingress. Microstructural characterization of the pre-annealed state and the solution-nitrided cases were performed with X-ray diffractometry, light-optical microscopy, electron back-scatter diffraction and hardness indentation. The phase fractions of austenite and ferrite have a significant influence on the nitriding kinetics. A relatively high ferrite phase fraction results in finer grains in the nitrogen-stabilized austenite case. The developing austenite case and the evolution of the microstructure during solution nitriding are discussed in a computational thermodynamics and kinetics context. Slower grain growth in the austenite cases for higher ferrite phase fractions can be understood in terms of phase distribution, alloying element partitioning, and possibly, pinning effect by M₂N nitrides.