Influence of the Cooling Rate on the Wüstite Content in Oxide Layers Formed During High-Temperature Oxidation of Hot-Worked Tool Steel with High Thermal Conductivity

The transformation of wüstite (FeO) in the oxide layer formed during high temperature oxidation (600 °C and 700 °C) on hot-worked tool steel was investigated. Wüstite plays an important role in the oxide layer of these steels used for hot working. However, understanding its transformation behavior during cooling is crucial for controlling the final oxide layer structure. Slow cooling rates have a significant influence on the final wüstite content, resulting in inaccurate representations of the composition of the oxide layer at temperatures above 570 °C. The aim of this study was to determine the influence of cooling rate on the wüstite content in the oxide layer after high temperature oxidation. It was found that for hot-worked steel samples oxidized at 700 °C or higher, a cooling rate of more than 1000 °C min⁻¹ is required to suppress the eutectoid transformation and maintain the realistic wüstite content. At lower temperatures (570 °C–600 °C), a cooling rate of more than 100 °C min⁻¹ is required to achieve the wüstite content observed at oxidation temperatures in the oxide layer. Overall, the hematite and magnetite contents also vart with the cooling rate, which is associated with changes in the wüstite content.