Modeling of distortions induced by the nitriding process

Nitriding is a thermochemical process that is used to improve the fatigue life of structural parts. It also involves residual stresses and non-negligible distortions of treated parts. This work deals with a pre-dimensioning tool able to simulate distortions of complex parts, generated during treatment. The presented model uses the analogy between diffusion of species and heat conduction to describe development of apparent volumic eigenstrains of the treated material, which is the source of global part distortions. The first step of the model consists in identifying these apparent volumic eigenstrains using experimental distortion measurements of a nitrided sample. Then, the unstationnary equivalent boundary condition of an equivalent diffusion problem is developed in order to numerically generate the volumic strains previously identified. Finally, this equivalent boundary condition is integrated in a heat conduction finite element code to simulate distortions of parts with complex geometries. In case of an industrial sample, the predicted distortions are compared to experimental measurements performed on a 3D coordinate measuring machine. The presented tool allows evaluating the consequences of a design choice on the final part distortions induced by the treatment.