Materials for surface layer treatments

Abstract

Surface engineering employs a number of processes to coat or treat the surface (Chapt. A.3, p. 71), and optimised materials are available for some of them. We will now discuss one thermal and two thermochemical heat treatments that affect the surface layer and also present the appropriate materials. All three processes have a common objective: above a ductile core, the hardness is to be increased and residual compressive stresses introduced into the surface layer in order to improve the wear resistance and the fatigue strength.

The hardness gradually decreases from the near-surface region to the core, which means that a minimum hardness has to be specified for a certain depth below the surface. For surface-hardening, EN 10328 defines this specific minimum hardness as follows: hardness limit=0.8 · minimum required surface hardness, measured in HV1. The effective depth of hardening DS is given in mm. DIN 50190-3 refers to the effective depth of hardening after nitriding Nht in mm and the hardness limit=measured core hardness+50 (HV0.5). For case hardening, the hardness limit is 550HV1 and the effective depth of case hardening, abbreviated to CHD in EN ISO 2639, is given in mm (previously DC in EN 105). Special cases must be agreed upon in accordance with the appropriate standard. In reality, the effect of heating or chemical modification penetrates more deeply than the surface layer thickness specified by the hardness limit.