Structural Characterization of Dual Layer Formed after Plasma Nitrocarburizing of Aisi 304l

Abstract

Plasma nitrocarburizing (PNC) processes are employed for case hardening of austenitic stainless steels (ASSs) by incorporating nitrogen (N) and carbon (C) in to the surface of the steel for improving the hardness and corrosion resistance properties. Discharge of N and C containing gases during plasma nitrocarburizing enables surface modification at low temperatures compared to gas nitrocarburizing processes. The modified surface of ASSs treated at low temperatures (<450 ºC) are composed of expanded austenite of N and C (γN,C) and at high temperatures(>450 ºC) phases of Fe (C, N) nitrides and CrN are present The incorporation of N and C is largely diffusion controlled and forms a dual layer whose thickness is influenced by the PNC process temperature. In the present work PNC processes were carried out on AISI 304L at process temperatures of 350-500 ºC. The phases in the modified surfaces were identified through X-ray diffractometry (XRD) and the dual layer thickness using scanning electron microscopy (SEM). At low temperatures the observed phases were supersaturated phases of N and C in austenite (γN and γC) along with secondary supersaturated phase γ′N, and ε′N phase. At high temperatures the supersaturated phases decompose to CrN, Fe2-3N, Fe4N and Fe3(C, N), though γ′N can still be seen at 450 ºC. Diffusion activation energies of 99.1 KJmol-1 and 91.7 KJmol-1 for nitrogen and carbon respectively were calculated from Arrhenius equations for the process. Also, surface properties in terms of microhardness, roughness and corrosion resistance were measured after the plasma nitrocarburizing processes. It was found that the sample treated at 400 ºC exhibited better pitting corrosion resistance along with higher surface microhardness due to the supersaturated phases and ε′N.

Keywords: Plasma Nitrocarburizing, AISI 304L, supersaturated, secondary supersaturated phase, microhardness, Corrosion resistance.