Effect of Compressive Stresses on Microstructure of Scales Formed on Pure Iron During Continuous Air Cooling

The microstructure development of oxide scale on pure iron under the mutual action of compressive stress and cooling conditions was investigated. Oxide scale structure was examined by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that oxide scale formed under normal cooling conditions had a structure mainly consisting of an outer magnetite and an inner wustite layer. When a compressive stress was applied, numerous magnetite precipitates formed within wustite layer homogeneously at starting cooling temperature of 900 °C, and the wustite layer in the scale was transformed into a mixture of mostly magnetite/iron eutectoid and magnetite layer at starting cooling temperature of 700 °C. The wustite decomposition and precipitation of magnetite in wustite under compressive stress were discussed.