Investigation of Compound Layer Structures after Nitriding and Nitrocarburizing of Quenched and Tempered Steels

A compound layer is formed by ingress of nitrogen from an external nitrogen source into the surface layer and the formation of nitrides when the solubility of nitrogen in the bulk material is exceeded. In the surface layer, where the nitrogen concentration is at its maximum level, the nitrides form a closed layer. The compound layer continues to contain alloy nitrides which have formed from the carbides and other precipitates from the bulk material. The properties of the compound layer have a decisive influence on the wear and fatigue behavior of the loaded surfaces. The current investigations deal with the extensive characterization of compound layers that have been produced in heat treatment processes with the aim of producing stress-resistant nitriding layers. The commonly used nitriding and quench and temper (Q&T) steels 31CrMoV9 and 42CrMo4 served as examination material. The structure of the compound layers was varied within the nitriding trials regarding the phase composition, porosity and layer thicknesses. The phase composition of the compound layers was determined by special etching, scanning electron microscopy (SEM), X-ray diffraction and GDOES.