Influence of Fast Path Diffusion on the Oxidation of Mn Strengthened IF Steels

Abstract

During the in-line coating process of Mn-strengthened-Interstitial-Free-steels, selective oxidation under low dew-point atmospheres may result in surface oxides that are not wetted by the coating alloy. In the current study, we investigate the formation of external and internal oxides in the metal in order to characterize the influence of fast path diffusion. The short-time oxidation in the metal was carried out in a gold-image furnace and the resulting external and internal oxides were characterized through scanning-electron microscopy. An IF steel sample containing 1.5wt.% Mn was used and the effects of dew point (DP) ranging from –75 to –15oC on oxidation was investigated at 800oC by varying the PH2/PH2O ratio in the oxidizing gas. Simulations were carried out by considering the combined effect of diffusion and mass transfer represented by a system of partial differential equations. The model equations were solved using finite element method in a Multiphysics modeling software COMSOL. The experimental and simulated results were found to be in agreement, and showed that, for DP up to –30oC, gas phase mass transport of oxidant gas controls the oxidation which results in exclusively external oxide nodules distributed uniformly on the surface. At –15oC > DP > –30oC, the oxidation evolves as ridges along grain boundaries and the simulations indicate that this is due to solid state diffusion control and the controlling mechanism is fast path diffusion through the alloy. At this point, internal oxides also start to appear.