Investigation of the Fundamental Processes in the Internal Oxidation of Binary and Ternary Iron Based Alloys at Elevated Temperatures

In different production steps steels are exposed to high temperature either in reducing condition (for e.g. N₂-H₂during recrystallization annealing) or in oxidizing environments (e.g. during hot rolling and cooling after hot rolling).  These processes are accompanied by external and internal oxidation, which can affect further processing steps. For example, during cooling after hot rolling of high strength steels especially grain boundary oxidation can lead to significant problems during pickling and cold rolling. In this paper a fundamental investigation of grain boundary oxidation during annealing under model atmospheres simulating the oxygen partial pressure beneath the scale will be addressed, based on model alloys which are relevant to industrial steel grades. The focus will be on questions such as how the different alloying elements and the various annealing conditions influence the internal selective oxidation.

In order to develop a fundamental understanding, binary and ternary model alloys (for instance, Fe‑Mn, Fe-Cr etc.) are considered. Significant focus will be given to the oxidation kinetics i.e., monitoring of the mass gain during isothermal oxidation using a thermo‑gravimetric balance (TG). The partial pressure of oxygen during the experiments is kept just below the equilibrium pressure for FeO/Fe. As the mass changes during internal oxidation are very low, errors from buoyancy effects in a standard set-up are in the same range as the mass change to be investigated. Hence, a novel under-pressure TG set-up was developed and applied here. In this under-pressure TG the same gas composition of water vapor and hydrogen are used, but the inert carrier gas N₂is omitted, significantly decreasing buoyancy effects. Thus the mass gain measurement is carried out with so far unequaled accuracy.

Furthermore, the role of elemental segregation to the grain boundaries on grain boundary oxidation was investigated. The results will be discussed also on the basis of earlier simulation work [1].

[1] M. Auinger, S. Borodin, S. Swaminathan and M. Rohwerder, Materials Science Forum 696 (2011) 76-81