PHASE TRANSFORMATION IN Fe-Mo-C AND Fe-W-C STEELS—II. EUTECTOID REACTION OF M23C6 CARBIDE DECOMPOSITION DURING AUSTENITIZATION

Abstract

The dissolution of globular M23C6 carbide in Fe-W-C and Fe-Mo-C steels during austenitization has been studied by means of transmission electron microscopy (TEM). It was observed that the M23C6 carbide formed at 700°C decomposes by two different eutectoid reactions. In the early stage of dissolution, before the α → γ transformation is completed at austenitization temperature, the M23C6 carbide transforms into M6C + α with the formation of spheroidal M6C precipitates. This first reaction takes place while the carbide is still surrounded by ferrite. After the α → γ transformation, the M23C6 carbide decomposes into M6C + γ in Fe-W-C steels and into Fe2MoC + γ or M6C + γ in Fe-Mo-C steels. In the latter cases a rod-like or lamellar structure was observed. It was established that phase boundary migration during M23C6 carbide dissolution initiates the precipitation reaction at the interface. The precipitates were characterised by X-ray diffraction and by TEM-EDS. The orientation relationship between M6C and austenite and M6C and martensite after M23C6 decomposition has been determined. The results obtained indicate that in both Fe-Mo-C and Fe-W-C steels, M23C6 carbide is not stable in the temperature range of 800–1000°C. The observed results will be discussed in terms of local equilibrium at the phase interfaces during the reactions. © 1997 Acta Metallurgica Inc.