Determination of residual stress distribution in ion-nutrided steel

Abstract

Flat narrow plates ion-nitrided on one side under various nitriding conditions are studied for the determination of mechanical properties of nitrided layers. The change of elastic modulus is calculated from deflection test of a cantilever set-up and the change of thermal expansion coefficient is obtained from curvature-temperature relation on the assumption that these changes are in proportion to the amount of nitrogen concentration, which is again to follow one-dimensional diffusion equation. It is found that the thermal expansion coefficient at the nitrided surface has been increased by 2 to 12% over the value of non-nitrided core, and elastic modulus at the nitrided surface has been increased by a maximum of 700%, whose value is dependent on the nitriding conditions. Also, the axial extension due to the diffused nitrogen into Fe lattice is found to be about $0.1×10^{-3}$ to $0.3×10^{-3}$ in terms of strain. Using the above results, the distribution of residual stress is obtained. At room temperature, the core has a low level of tensile stress having about 2 to 25kg/㎟, while the nitrided surface is in compression with a peak of about 50 to 200kg/㎟.