Microstructure and residual stresses of a plasma-nitrided M2 tool steel

Abstract

Abstract Nitriding leads to improved tribological and corrosive properties of iron-alloy components. In order to study the effect of plasma-nitriding parameters on the structure of the compound layer and diffusion zone, a systematic variation of process parameters, temperature and process gas atmosphere has been carried out. Metallographic inspection, X-ray diffraction and glow discharge optical spectroscopy analysis were used in the investigation. The results revealed that, depending on the amount of nitrogen in the gas atmosphere, nitrided layers with and without a compound layer can be generated in the surface of M2 tool steel for temperatures ranging from 350 to 500°C. For plasma nitriding in 5 vol.% nitrogen and 95 vol.% hydrogen, no compact compound layer was formed. The gas mixture of 76 vol.% nitrogen resulted in compound layer formation for all temperatures from 350 to 500°C. X-ray phase analysis indicated an almost 100% ϵ-(carbo)nitride phase, but the existence of the γ′-(carbo)nitride could not be excluded completely from the X-ray phase diagrams. After corrections had been made according to the nitrogen gradient, high compressive surface residual stresses were measured in the diffusion zone and were found to increase with temperature. After a qualitative correction for chemical composition gradients, high tensile residual stresses were found, probably existing in the ϵ-(carbo)nitride phase for the investigated plasma-nitrided tool-steel samples.