Analysis of Oxide Layer Formation During Oxidation of AISI 4140 Steel at 1000 °C over Exposure Time

The high-temperature shaping of steels is accompanied by the formation of surface scales composed of oxide layers. However, the oxidation kinetics and morphology of these scales remain poorly understood. This study analyses the formation of oxide layers on AISI 4140 steel at varying oxidation times (20, 40 and 60 min) at 1000 °C. The analysis revealed the presence of hematite, magnetite, and transformed wustite in the oxide layers, along with clusters of alloying element oxides, predominantly chromium and iron oxide (FeCr2O4). There was a direct correlation between the duration of the oxidation process and the thickness of the scale and the number of defects observed in the material. The coating layer of alloying element oxides demonstrated insufficient adhesion to the steel substrate. Similarly, the oxides of alloying elements within this layer exhibited low cohesion among themselves. The alloying elements are present in all oxide layers, but in greater quantity in the layer in contact with the steel substrate, where a reduction in their concentrations was observed over time. This indicates that the alloying elements tend to disperse as the thickness of the alloying element oxide layer increases over time.