Low temperature nitriding, nitrocarburising and carburising of AISI 316L austenitic stainless steel

Abstract

AISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (γ_N). After LTPN, a second carbon expanded austenite (γ_C) layer was formed beneath the nitrogen expanded austenite layer (γ_N). LTPC led to formation of a carbon supersaturated expanded austenite (γ_C). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (γ_N) is higher than that calculated for carbon expanded austenite γ_C. As a consequence, the lattice expansion Δa/a for the nitrogen rich (γ_N) phase is higher than the one observed for the (γ_C) phase and the nitrogen rich expanded austenite layer displays higher hardness than the carbon rich expanded austenite layer. The LTPNC bilayer displays a less steep hardness gradient, indicating that the carbon rich expanded austenite layer can grant mechanical support to the harder nitrogen rich layer.

Keywords:

• Nitriding
• Nitrocarburising
• Carburising
• Expanded austenite
• Plasma processes

The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São Paulo for financial support to Project PRONEX-Temático (No. 03/10157-2).