Characterization of Transition Metal Carbide Layers Synthesized by Thermo-reactive Diffusion Processes

Hard wear resistant surface layers of transition metal carbides can be produced by thermo-reactive diffusion processes where interstitial elements from a steel substrate together with external sources of transition metals (Ti, V, Cr etc.) form hard carbide and/or nitride layers at the steel surface. In this study halide-activated pack cementation techniques were used on tool steel Vanadis 6 and martensitic stainless steel AISI 420 in order to produce hard layers of titanium carbide (TiC), vanadium carbide (V8C7) and chromium carbides (Cr₂₃C₆ and Cr₇C₃). Surface layers were characterized by scanning electron microscopy, X-ray diffraction and Vickers hardness testing. The study shows that porosityfree, homogenous and very hard surface layers can be produced by thermo-reactive diffusion processes. The carbon availability of the substrate influences thickness of obtained layers, as Vanadis 6 tool steel produces thicker layers than AISI 420. X-ray diffraction analysis validates the formation of TiC and V8C7 layers on titanized and vanadized samples respectively, while chromized samples form both Cr₂₃C₆ and Cr₇C₃. It is shown that the two-phase layer of Cr₂₃Cr₆/Cr₇C₃ produced on Vanadis 6 can be transformed into a layer much richer in Cr₇C₃ and poorer in Cr₂₃C₆ by a subsequent heat treatment. The produced chromium-, vanadium- and titanium carbide layers have hardness values of 2116±37HV, 3022±119HV and 3951±66HV respectively, and subsequent hardening and tempering of treated materials can be done without spallation of the hard layers.