Surface engineering by thermochemical processing is the intentional change of the composition of a material at elevated temperature with the purpose to improve materials performance. As compared to other surface engineering processes, as for example plating or vapour deposition processes where a coating develops onto the surface, in thermochemical processing components from the starting material are essential in the development of the phases at the surface. Perhaps the most well-known and most widely applied example of thermochemical surface engineering is case hardening of steel by carburizing.
In the present lecture current research and innovation activities in our group are used to exemplify the myriad possibilities with thermochemical surface engineering and the interplay of science and innovation. Distinction is made between processing where interstitial elements are dissolved in the metal, and processing where substitutional elements are provided to change the composition. Aspects of thermodynamics, diffusion kinetics, microstructure development, internal stress and improved materials performance are treated. The examples given encompass the following: - Low temperature surface hardening of stainless steel by dissolution of nitrogen/carbon; - Activity-controlled boriding of tool steels; - Surface hardening of titanium alloys with interstitials; - Thermo-reactive diffusion with chromium, vanadium or titanium for extreme wear resistance; - Aluminizing for enhanced high-temperature corrosion protection in biomass-fired thermal power plants.
Residual stresses exist in material independently of the presence of any external loads. Their presence may not be readily apparent and so they may be overlooked or ignored during a process of engineering design. This however can cause great design risk because they can have profound impact on material strength, dimensional stability and fatigue life. Almost all manufacturing processes create residual stresses that can further develop during service life of the manufactured component. Several comparative, qualitative and quantitative methods for stress analysis are nowadays applied in engineering praxis. Among them X-ray diffraction is one of the most used and developed over the past 90 years. In this article we describe novel concept of two-dimensional X-ray diffraction (XRD2) and we demonstrate its applicability on determination of residual strains and stresses in a bimetallic austenite/ferrite steel system. Our analysis is based on X-ray micro-diffraction experimental utilizing hard monochromatic X-rays focused down to micrometer size. In this way bimetal in bulk form was analyzed and microstructural differences between the joined materials and their interface were determined.