Local Equilibrium and Nodule Formation on Al‐TRIP‐Aided Steel Surfaces During Intercritical Annealing

Abstract

Several advanced high strength steels were intercritically annealed at a dew point of À58C. Afterwards, surface morphology was investigated by scanning and transmission electron microscopy (TEM). In Al alloyed TRIP-aided steels, nodules of metallic Fe containing traces of Mn could be proven by TEM analysis. To clarify the mechanism of origin, several further annealing trails were done and finally a new component of the occurring heterogeneous gas/metal reaction during intercritical annealing was postulated. It is concluded that neither the diffusion mechanism of the alloying elements nor the stress gradient between stress-free surface region of the internal oxidation front nor the oxygen partial pressure dominates alone the selective oxidation process. It is suggested that the reactivity of the considered surface, its local surface chemistry and the local thermodynamic equilibrium should be taken into account in greater detail. Preferred dissociation points of absorbed water vapor could lead to a local increased oxygen partial pressure. With this, a nanoscaled oxidation/reduction process could be initiated.