Relation between oxygen activity gradient in the internal oxidation zone of Mn alloyed steel and the composition of oxide precipitates

Abstract An internal oxidation zone with (Mn 1 −  x ,Fe x )O mixed oxide precipitates occurs after annealing a Fe – 1.7 at.% Mn steel at 950 °C in N 2 plus 5 vol% H 2 gas mixture with dew point of 10 °C. Local thermodynamic equilibrium in the internal oxidation zone is established during annealing of the Mn alloyed steel. As a result, the composition of (Mn 1 −  x ,Fe x )O precipitates depends on the local oxygen activity. The oxygen activity decreases as a function of depth below steel surface, and consequently the concentration of Fe decreases in the (Mn 1 −  x ,Fe x )O precipitates.