Influence of steel composition and plastic deformation on the surface properties induced by low temperature thermochemical processing

Low-temperature thermochemical surface hardening by nitriding, carburizing and nitrocarburizing is used to improve the performance of stainless steels with respect to wear, fatigue and corrosion resistance.The dissolution of nitrogen and/or carbon atoms in the materials surface leads to the formation of a supersaturated solid solution known as expanded austenite, or S-Phase.

Expanded austenite is characterized by high hardness, up to 1400 Vickers, and high compressive stresses in the surface region, which result in improved wear and fatigue resistance of the components. Along with the improvement of these properties, the corrosion resistance of the stainless steel is fully maintained or even enhanced.

Despite low-temperature thermochemical processing of austenitic stainless steels has been widely studied in literature, other stainless steel classes and the influence of steel´s initial condition, such as phase composition and/or plastic deformation, have not received the same attention. The topic of this Ph.D. thesis is the study of the influence of the material’s initial phase composition and the presence of plastic deformation on the properties of the surface layers obtained through low temperature thermochemical processes of austenitic, precipitation hardening and martensitic stainless steels.

Parts of the work presented in this Ph.D. project were implemented in the European project “”PressPerfect”, wherein several industrial and university partners were involved. The main goal of the“PressPerfect” Project was to create a methodology to predict the performance of high quality stainless steels after forming and finishing treatments.

The Ph.D. Project focused on the optimization of low-temperature thermochemical processes on severalstainless steel classes used for the surface treatment of industrial products. The activities carried out encompass the study and the characterization of the following aspects: ‐ Influence of plastic deformation prior to the low-temperature thermochemical process ‐ Influence of initial phase composition on the properties and morphology of thenitrided/nitrocarburized surface layer ‐ Influence of interstitial alloying on the mechanical properties of stainless steels ‐ Study of the corrosion performance of expanded austenite layer. ‐ Study of compressive residual stresses in expanded austenite and the influence of the material condition prior to nitriding/nitrocarburizing on the stress distribution.

The experimental techniques applied entail X-ray diffraction (XRD), Glow Discharge Optical EmissionSpectroscopy (GD-OES), Scanning electron microscopy (SEM), light optical microscopy (LOM) and hardness measurement.