Microstructure and corrosion behaviour of DC-pulsed plasma nitrided AISI 410 martensitic stainless steel

AISI 410 martensitic stainless steel samples with different metallurgical structures were DC-pulsed plasma nitrided at 623, 723 and 773 K. The samples were ion nitrided in an industrial equipment using a gas mixture consisting of 25% N₂+75% H₂ under a pulsed DC glow discharge. Optical and scanning electron microscopy, as well as glancing angle X-ray diffraction and microhardness measurements have been used to study the ion nitrided surfaces. All plasma nitrided samples showed surface hardness values higher than 1000 HV; the highest value was obtained at 673 K. The case depth was approximately 30 μm for samples nitrided at 673 and 773 K, while the sample nitrided at 623 K showed an 'expanded ferrite' phase (α_N) and an incipient precipitation of Fe₄N. Depending on the treatment temperature and time, two fronts were formed, termed diffusion front and transformation front; the latter produces a softening of the nitrided case. Electrochemical measurements showed a decrease of corrosion resistance in the samples nitrided during 20 h at 673 and 773 K. Conversely, the sample nitrided at 623 K presented a low corrosion current and more noble corrosion potential.