Change in the surface oxide layer of piercing-mill mandrels over time

Satisfactory development of the internal and external oxide layers on piercing-mill mandrels may be ensured by annealing. In addition, strong adhesion of the layers with the metal base is observed. In electron- microscope images, the structure of the boundary layer at the metal surface is seen. It resembles stalagmites. This layer consists of metal oxides, with chaotically distributed particles of the oxides of iron, nickel, and other alloying elements. With increase in the number of piercing cycles, the oxide layer becomes thicker, and its phase composition changes. With high temperature and cooling conditions, the content of wustite FeO in the surface oxide layer increases to 80%.