Phase transformation behavior of oxide scale on plain carbon steel containing 0.4 wt.% Cr during continuous cooling

Abstract

Pre-oxidation in air at 750°C for 10 min, the plain carbon steels without Cr and containing 0.4 wt.% Cr tended to rapidly form oxide scale composed of outer Fe3O4 layer and inner FeO layer. Moreover, a FeCr2O4 layer was observed at the FeO/substrate interface on the steel containing Cr. A comparative study was carried out between the scales on the two steels in inert gas cooled from 350–600°C to room temperature at the cooling rate range of 1–40°C/min, to determine the effect of low concentration Cr addition on the phase transformation of FeO. Based on the cross-sectional morphologies of oxide scale during various cooling conditions, the relationship between cooling rate and start cooling temperature were constructed, and the area fraction of eutectoid structure was analyzed. The result shows that the transformation rate and area fraction of eutectoid structure in oxide scale on steel containing Cr were greater than that on steel without Cr, and then the nucleation, growth and 100% transformation region of eutectoid structure in oxide scale on steel without Cr were delayed. This study proposed two mechanisms to discuss the experimental results. Firstly, the formation of FeCr2O4 layer reduced the consumption of O, and then prevented short-range uphill diffusion of Fe from FeO to substrate. Secondly, combined with the lamellar spacing in eutectoid structure and the Fe–Cr–O equilibrium phase diagram, the Cr addition increased the formation temperature of FeO in oxide scale, which provided sufficient driving force for eutectoid transformation during continuous cooling.