Low Temperature Surface Hardening of Stainless Steels in Automotive Applications

Handling of more corrosive (bio) fuels, corrosion loaded outside kinematics of turbochargers, high pressure injection systems as well as additional exhaust gas treatments lead to more and more automotive applications where stainless steels are needed. While the excellent corrosion resistance and (para-)magnetic properties make stainless steels the material of choice for such applications, poor tribological behaviour, especially low abrasive / adhesive wear resistance and a tendency for fretting, has prevented the use of these materials in applications where both corrosion and wear resistance are required. With low temperature (<500°C) surface hardening methods – carburizing and nitriding – it is possible to enhanced wear properties while maintaining corrosion properties. Suitable not only for austenitic, but also duplex and martensitic stainless steels as well as nickel base alloys, this thermo-chemical diffusion process enriches the surface of the treated alloys with carbon and/or nitrogen, forming a carbon or nitrogen supersaturated layer or S-phase. At the same time the formation of precipitations, which cause sensitization and a loss of superior corrosion properties, is avoided. This paper provides a brief introduction into these technologies. A more detailed review follows on Bodycote’s commercially available low temperature treatment known as Kolsterising® and S³P. The mechanical benefits on galling behavior, wear resistance, cavitation erosion, and fatigue strength are discussed. Additionally, environmental resistance is reviewed and typical automotive applications are presented, including possible future trends for automotive applications.