Influence of strain hardening on gaseous nitriding of steels

Abstract

Machining is known to introduce plastic straining and residual stress gradients in surface of mechanical parts. If not controlled, the mechanical state of a surface can influence adsorption and diffusion of atoms during subsequent chemical surface treatments, such as nitriding. In the present work, the influence of strain hardening on gaseous nitriding of steels is studied. The nitriding process was carried out using a laboratory set-up enabling the control of nitriding parameters (time, temperature, nitriding potential) on 33CrMoV12-9 steel samples. Homogeneously strain hardened samples are obtained by controlled tensile tests, enabling to nitride samples at different levels of strengthening. Results are compared to a reference state, obtained from tensile specimen not submitted to strain-hardening, as well as an industrially representative sample nitrided using the same conditions. Nitrided samples are characterized by SEM observations, hardness measurements and carbon and nitrogen in-depth concentration profiles.