Influence of thermally grown oxide layers thickness on temperature evolution during the forging of large size steel ingots

Medium carbon low alloy specialty steels are preheated in gas-fired furnaces before undergoing open die forging operation. Oxide layers with different thicknesses and compositions are formed on the surface and significantly influence the temperature evolution during the forging process. In the present work, the kinetics of oxide growth of two high-strength medium carbon steels with different Ni concentrations, were determined. Four oxidation temperatures and five oxidation times were used to simulate various forging and intermediate heat treatment conditions. Three distinct oxide layers with different thicknesses were identified and correlated with the applied thermal cycles. The results indicated that the kinetics of oxidation varied with the Ni concentration of the alloys, but interestingly the fractions of different layers were the same at different oxidation temperatures. A 3D finite element model (3D-FEM) was developed, and the effect of the oxide layer investigated on the temperature evolution of the ingots. Correlations are made between the thickness of the oxide layers and the cooling pattern of the ingots, as well as their impact on the forging die temperature. The 3D-FEM predicted critical temperatures were validated by experimental temperature measurements on the ingot during the forging process.