Active-Screen Plasma Nitriding of an Austenitic Stainless Steel Small Thin Rolled Plate

Austenitic stainless steels (ASS) are used in a wide range of items such as household products, construction materials and automobile parts as well as in applications related to power generation and chemical and food industries owing to its high functionality, excellent corrosion resistance, ductility, and toughness. However, ASS is limited by a low-hardness, poor bending rigidity, and wear resistance. Surface hardening treatments such as nitriding can improve the mechanical properties to extend the applicability of ASS to fine and precise machining.

Plasma nitriding and carburizing methods are considered suitable as surface modification treatments for ASS with a strong passivation film because the sputtering effect can remove the oxide film on the surface of the object to be treated. However, high temperatures may significantly reduce corrosion resistance. The deterioration of corrosion resistance can be prevented by lowering the processing temperature below 698 K. Nitrided and carburized layers with hardness and corrosion resistance generated by low-temperature processing are called “expanded austenite” or “S phase.” Active screen plasma nitriding (ASPN) and carburizing (ASPC) are superior methods for producing active species and generating uniform heating regions.

In this study, low-temperature ASPN are applied to small thin ASS rolled plate to investigate formation and mechanical property of S phase. A solution-treated ASS plate was rolled at a rolling reduction rate of approximately 30%. By rolling, the hardness increased from 160 HV to approximately 400 HV. XRD pattern showed that strain-induced martensite was generated by rolling process. The rolled sample was treated by ASPN to generate S phase and the nitrided sample properties of layer thickness, layer structure, surface hardness, and corrosion resistance are evaluated by varying the processing temperature and time.