Low temperature surface hardening of stainless steel; the role of plastic deformation

Thermochemical surface engineering by nitriding of austenitic stainless steel transforms the surface zone into expanded austenite, which improves the wear resistance of the stainless steel while preserving the stainless behavior.  As a consequence of the thermochemical surface engineering, huge residual stresses are introduced in the developing case, arising from the volume expansion that accompanies the dissolution of high interstitial contents in expanded austenite. The presentation will address two aspects of the role of plastic deformation on the case developing during low temperature nitriding.

In practice, plastic deformation of metastable austenitic stainless is the rule rather than the exception. Plastic deformation of metastable austenite leads to the development of strain-induced martensite. Experimental work has demonstrated that the presence of bcc martensite leads to a thicker case, while the precipitation of CrN is promoted. On the other hand, for plastically deformed stable austenite, or austenite stabilized by a high temperature solution nitriding treatment, the case depth is insensitive for the degree of plastic deformation, and high dislocation density does not appear to promote the precipitation of CrN.

Modelling of the composition and stress profiles developing during low temperature surface engineering from the processing parameters temperature, time and gas composition is a prerequisite for targeted process optimization. The evolution of composition- and stress-profiles over the developing case was developed can be simulated starting from the fundamental thermodynamic, kinetic and crystallographic data. It is shown that is necessary to include stress-induced diffusion as well as elastic-plastic accommodation of the composition-induced strains and interstitial strengthening.