Characterisation of Scale Layer Structure and Resulting Mechanical Properties Under Varying Carbon and Chromium Contents

In hot forming processes and during heat treatment, scale layer formation occurs on the surface by temperatures over 570 °C. The structure of the scale layer depends on many different parameters, such as temperature, time, atmosphere and the alloying elements of the material. The aim of this work is to characterise the scale layer forming on steels with different carbon and chromium contents. Focus is on phase analysis using EDX and correlation of the iron oxide phases with hardness using nanoindentation. In addition to the well-known iron oxide phases and the three-layer-structure a fourth layer, called subscale, was identified, which primarily consists of chromium and silicon oxides. As this layer is the interface with the base material, it could influence the adhesion of the scale layer. The hardness measurements show high values in the area of the subscale, which indicates brittle fracture behaviour. The thickness of the subscale and the primary elements it contains depend on the alloying elements of the base material. For an unalloyed steel, the subscale thickness was reduced, compared to the low-alloy steels.