Carbide Reactions in a 5%W-Cr Tool-Steel on Isothermal Austenitizing

Abstract

Carbide reactions were studied in a steel containing 1.31%C, 0.65%Cr and 4.59%W, which had been previously annealed into a meta-stable equilibrium state, on isothermal austenitizing at a temperature of 850° to 1000°C by means of an X-ray diffraction method and chemical analysis of the residues after electrolytic separation.
The carbide change from a meta-stable to a stable equilibrium state can be expressed by the reactions, M₂₃C₆→M₂₃C₆+M₆C→M₂₃C₆+M₆C+WC→Fe₃C+WC (below 900°C) or WC (above 950°C). The M₆C appears as soon as the specimen is austenitized and the concentration of tungsten in the M₆C increases with the holding time. Nucleation of tungsten carbide crystals takes place after a incubation period which becomes shorter with increasing austenitizing temperature, and the crystals grow larger due to both the precipitation of carbon and tungsten on them from the austenite matrix and the dissolution of the M₂₃C₆ and M₆C, accompanied by the decrease in concentration of tungsten in the M₂₃C₆.