INVESTIGATION OF THE MICROSTRUCTURE AND SURFACE HARDENING EFFECT OF GAS AND PLASMA NITRIDING ON DUPLEX STAINLESS STEELS AND WELD OVERLAYS

Abstract

This study examines the effects of gas and ion (plasma) nitriding on the microstructural transformations and surface hardening of duplex stainless steels UNS S32205 (2205) and UNS S32750/S32760 (1.4410), including welded overlays produced by SMAW surfacing. Nitriding was performed under two regimes - 420°C/20 h and 560°C/6 h. Microstructural characterization was carried out using optical metallography, accompanied by microhardness measurements at 0.025 kgf.

The results show that plasma nitriding produces a finer and more uniform nitrided layer with controlled thickness and less pronounced nitride precipitation. Gas nitriding, on the other hand, forms thicker layers with coarse lamellar morphology and steeper hardness gradients. Welded overlays exhibit significantly higher nitriding efficiency due to the increased nitrogen solubility in the austenitic–ferritic regions and the presence of chromium-rich secondary phases, which promote nitride network formation. A clear relationship between nitriding temperature, processing method, and resulting hardness is established, providing a basis for optimizing surface treatments of duplex alloys intended for service in corrosive or abrasive environments.