On the microstructure and phase identification of plasma nitrided 17-4PH precipitation hardening stainless steel

Systematic microstructure characterisation of plasma nitrided (350–500 °C for 10 to 30 h) 17-4PH alloy was carried out using SEM, XRD and TEM. Experimental results have shown that the microstructure and phase constituents of the plasma surface alloyed cases are highly treatment temperature dependent. When treated at low-temperatures (≤ 420 °C), the microstructure is dominated by nitrogen supersaturated martensite (α'N-expanded martensite); Nitrogen S-phase grains can be formed from the pre-existent retained austenite by converting the retained austenite grains in 17-4PH but no continuous S-phase layer was found. When treated at high-temperatures (above 420 °C), a surface γ′–Fe4N compound layer was formed, CrN precipitated and S-phase was decomposed.