Austenitic stainless steels are carburized or nitrided (i.e., surface hardened) at low temperatures in order to maintain their superior corrosion resistance. Treatment temperature must be low enough to prevent precipitation in the diffusion zone, yet high enough to allow sufficient diffusion depths to meet design specifications. At these temperatures, prior machining processes can have a significant effect not only on diffusion, but also the surface hardness and corrosion resistance achieved. This paper presents practical examples showing how cutting, grinding, honing, and polishing processes influence the results of low temperature surface hardening treatments for stainless steel parts. It also discusses the influence of surface deformation and finish.

Gas nitriding is proving to be a viable low temperature case hardening process for stainless steels used in numerous applications. In this study, a comparison between austenitic (grade 304) and martensitic (grade 401) stainless steels shows how pre-oxidation temperature affects the thickness and porosity of the compound layer produced as well as hardness and nitriding diffusion depth. The results indicate that austenitic stainless steel would be the best choice for a part requiring wear resistance and strength, and that a standard rolled martensitic stainless steel would suffice if only a wear resistant surface is needed.

In this study, the creep properties of three titanium alloys were experimentally obtained at different applied stresses and at 683 K. X-ray diffraction and optical and electron microscopy were used to help characterize the microstructure before and after creep deformation and to show how changes in hardness correlate with the precipitation of α and ω phases in the β titanium matrix. The results of the study show that Ti-12Cr-1Fe-3Al is the most creep resistant followed by Ti-12Cr-3Al and Ti-12Cr.

This paper presents the preliminary results of experiments designed to mimic typical machining and thermal processing practices for aerospace titanium alloys. The most significant finding is that multiple side mill passes result in lower magnitude compressive stresses than a single side pass, which suggests that successive interactions with the milling tool serves to relieve residual stresses at the surface. The most likely mechanism for this is that Ti exhibits significant springback during machining, and multiple tool passes essentially remove the “springback” layer. Each successive removal of material allows stress relaxation in the remaining surface layer. By contrast, with only a single pass, surface residual stresses did not sufficiently relax.