Influence of stress and strain hardening on gaseous nitriding

Machining is known to introduce plastic straining and residual stress gradients in surfaces of mechanical parts. If not controlled, the mechanical state of a surface can influence adsorption and diffusion of atoms, such as nitrogen, during subsequent surface treatments. In the present work, the influence of stress and 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 32CrMoV12-9 steel samples. Bending set-ups are designed in order to nitride flat samples under different elastic loads as a function of the yield strength. Homogeneously strain hardened samples are obtained by controlled tensile tests, enabling to nitride samples at different levels of strengthening. Nitrided samples are then characterized by carbon and nitrogen in-depth concentration profiles, electron microscopy, microroughness measurements and residual stress analysis. Results are compared to a reference state, obtained on different samples, under the same conditions of pretreatment and nitriding, and characterized by thermogravimetric analysis.