Nitriding of Interstitial Free Steel in Potassium–Nitrate Salt Bath

Abstract

A study has been made of nitriding of interstitial free (IF) steel in the potassium–nitrate salt bath at temperatures ranging from 400 to 650°C. The salt is decomposed to generate nitrogen and oxygen. Nitrogen diffuses into steel, or steel is nitrided, while oxygen reacts on steel surface to form the oxide scale. The oxide scale thickness is much smaller than the nitriding thickness. Most of nitrogen resides in steel as a form of interstitial solid-solution. For nitriding at higher temperatures, nitride precipitates (γ′-Fe₄N and ζ′-Fe₂N) exist mostly in grain boundaries and partly in grains of the steel. The nitrate nitriding gives rise to much larger nitriding depth than other nitriding methods at similar nitriding temperature and time. The nitrate nitriding of steel substantially increase its tensile strength as well as hardness, e.g., an IF steel specimen nitrided at 650°C for 1.5 h shows a tensile strength of 916 MPa, which is 2.2 times higher than that of non-nitrided IF steel specimen, and an elongation of 20% at 70°C. Severe serrations are observed in flow curves of nitrided steel specimens, mainly due to dynamic strain aging that occurs because of interaction between dissolved nitrogen and moving dislocations. The effective diffusion coefficient of nitrogen D_N obtained from the nitriding data, D_N=D₀exp(−Q/RT) with D₀=3.789×10⁻⁷ m²·s⁻¹ and Q=76.62 kJ mol⁻¹, is approximately the same as that for diffusion of nitrogen in α-iron.