The nitriding behaviour of iron-chromium-carbon alloys

Abstract

The nitriding behaviour of an FeCrC (3.16 wt% Cr, 0.20 wt% C) alloy, as a model system for En40B (a classical gas-nitriding steel), was investigated. By comparing the response of a hardened structure, a quenched and tempered structure and an annealed structure, the effect on nitriding of different initial chromium and carbon disbributions over the phases present could be studied. The structure after nitriding was described employing metallographic methods (optical and scanning electron microscopy; hardness measurements), electron microprobe analysis (concentrationdepth profiles) and X-ray diffractometry (residual macrostress determination; phase identification). Further the weight and thickness increases of the specimens during nitriding were determined. During initial stages of nitriding, excess-nitrogen uptake and the development of grain-boundary carbides and a compressive residual surface macrostress took place. At later stages discontinuous precipitation, void formation, decarburization and relaxation of the compressive residual macrostress occurred. The observations lead to a model description for the nitriding of an FeCrC alloy.