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

Nitriding leads to improved tribological and corrosive properties of iron-a lloy components. In order to study the effect of plasma-nitriding parameter s on the structure of the compound layer and diffusion zone. a systematic v ariation of process parameters, temperature and process gas atmosphere has been carried out. Metallographic inspection, X-ray diffraction and glow dis charge optical spectroscopy analysis were used in the investigation. The re sults revealed that, depending on the amount of nitrogen in the gas atmosph ere, 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 degre es C. For plasma nitriding in 5 vol.% nitrogen and 95 vol.% hydrogen. no co mpact compound layer was formed. The gas mixture of 76 vol.% nitrogen resul ted in compound layer formation for all temperatures from 350 to 500 degree s C. X-ray phase analysis indicated an almost 100% epsilon-(carbo)nitride p hase, but the existence of the gamma'-(carbo)nitride could not be excluded completely from the X-ray phase diagrams, After corrections had been made a ccording to the nitrogen gradient, high compressive surface residual stress es were measured in the diffusion zone and were found to increase with temp erature. After a qualitative correction for chemical composition gradients, high tensile residual stresses were found, probably existing in the epsilo n-(carbo)nitride phase for the investigated plasma-nitrided tool-steel samp les. (C) 1999 published by Elsevier Science S.A. All rights reserved.