PhD Defence at DTU Mechanical Engineering

PhD Defence 13th January: "Interaction of stress and phase transformations during thermochemical surface engineering"

Tuesday 05 Jan 16

Freja Nygaard Jespersen from DTU Mechanical Engineering defends her PhD, "Interaction of stress and phase transformations during thermochemical surface engineering" Wednesday, the 13th of January, at 13:00. The defence takes place in Building 101, Meeting room no. S04, at the Technical University of Denmark. Professor Marcel Somers is principal supervisor, Professor Viggo Tvergaard and Professor Jesper Hattel are co supervisors.

Abstract:
Low temperature nitriding of austenitic stainless steel causes a surface zone of expanded austenite, which improves the wear resistance of the stainless steel while preserving the stainless behavior. During nitriding huge residual stresses are introduced in the treated zone, arising from the volume expansion that accompanies the dissolution of high nitrogen contents in expanded austenite. An intriguing phenomenon during low-temperature nitriding, is that the residual stresses evoked by dissolution of nitrogen in the solid state, affect the thermodynamics and the diffusion kinetics of nitrogen dissolution. The present project is devoted to understanding the mutual interaction of stresses and phase transformations during thermochemical surface engineering by combining numerical modelling with experimental materials science.

The modelling was done by combining solid mechanics with thermodynamics and diffusion kinetics to simulate the evolution of composition-depth and stress-depth profiles resulting from nitriding of austenitic stainless steel. The model takes into account a composition-dependent diffusion coefficient of nitrogen in expanded austenite, short range ordering (trapping) of nitrogen atoms by chromium atoms, and the effect of composition-induced stress on surface concentration and diffusive flux. The effect of plasticity was also included.

Temperature and concentration dependencies of mechanical and diffusion material properties were studied, and the effect of incorporation in the model examined. The effect of pre-stressing the sample was also tested, to investigate the effects of a residual stress-state that might be present from processing of the metal specimen. Controlled thermochemical treatment of austenitic stainless steel was investigated experimentally by in-diffusion of nitrogen from a gaseous environment. Measurements of diffusion coefficient were conducted using thermogravimetry.