Surface Hardening by Gas Nitriding

Abstract

Gas nitriding is one of the main surface hardening treatments for wear and corrosion improvements in ferrous and non-ferrous alloys. Nitriding of steels is usually conducted within the temperature range of 495–565°C in NH3 or NH3–H2 atmosphere and the nitrided microstructure mainly consists of ɛ-Fe2N1-z and γ′-Fe4N1-x compound layer (i.e., white layer) on top of a nitrogen diffusion zone (i.e., case). In the presence of nitride-forming alloying elements such as Cr, Al, V, precipitation of fine nitrides in the diffusion zone leads to enhanced hardenability, compressive residual stresses and fatigue improvement. Nitriding of titanium alloys is usually conducted at 700–1100°C under N2/H2 atmosphere, resulting in the formation of TiN and Ti2N compound layer and a nitrogen-strengthened diffusion zone. Nitriding improves wear and friction behavior of titanium alloys, but it has an adverse effect on their fatigue strength and ductility by promoting detrimental microstructural changes such as grain growth. Nitriding of aluminum alloys forms an aluminum nitride (AlN) surface layer with high hardness and wear resistance. Nonetheless, several challenges are faced due to the insolubility of nitrogen in aluminum, retardation of Al–N reactions due to native surface oxides and high thermal expansion mismatch between Al and AlN.