Pulsed-plasma nitriding of a niobium-alloyed PM tool steel

The aim of this work is to describe the processes during plasma nitriding of Nb-containing, powder metallurgy prepared (PM) tool steel. PM steel was studied containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W, produced by melt atomization and hot isostatic pressing. The hardened and tempered steel was plasma nitrided at temperatures ranging from 470 to 530 °C. The nitriding time was 30-180 min. Optical microscopy, electron microscopy, XRD, EDS, WDS and hardness measurements were used to study the steel's properties, microstructure, phases and chemical composition. The nitriding temperature of 470 °C was found to be unsuitable for practical use because the layers prepared at this temperature were non-uniform. The compound layer formed by M2,3(C,N) and M4N was found after nitriding at temperatures of 500 and 530 °C for at least 60 min. The effect of niobium on the formation of this layer was studied.