Oxidation of austenitic and duplex stainless steels during primary processing

2009-04-02T09:57:42Z (GMT) by Mark A.E. Jepson
During manufacturing, stainless steels will experience high temperatures at many steps: continuous casting, reheat, rolling and annealing. After continuous casting, it is usual for the ascast metal to cool to ambient temperatures. In order to achieve successful hot forming, the metal must then be reheated to a suitable temperature. This process is called "reheat", and commonly takes place in large, propane-fired walking beam furnaces at temperatures in excess of 1200°C for times of 2 or more hours. During reheat, a thick oxide scale will form which, if left in place during cold working, may cause damage to the surface of the final cold rolled product. As a result, manufacturers generally apply a mechanical descaling procedure immediately prior to rolling. In order to study the effect of reheat conditions on the oxide scale formation, austenitic grades (Types 316L and 304) and duplex grades (Types S32101 and S32205) were oxidised. The temperatures used ranged from 700°C through to 1300°C for times up to, and including, 5 hours. The results of the heat treatments were investigated using a range of analytical techniques including: scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy and site specific cross-section preparation using focused ion beam for both SEM investigation and transmission electron microscopy analysis. The use of these techniques has provided a thorough description of oxides that are expected to form at a variety of temperatures and conditions. On austenitic grades a number of sub-surface oxide structures form, such as dendritic internal oxide particles and sub-surface oxide layering, which may pose great challenges to the mechanical descaling processes. External oxidation has not been neglected in this study and complex chemical and phase distributions within external oxide scales have been characterised. The presence of an as-cast oxide scale during reheat does not have a substantial effect on the reheat oxide formed, however, the as-cast microstructure may contribute to the formation of subsurface oxide banding. Duplex stainless steels produce oxides which are highly dependant on the phase distribution and surface finish of the substrate. Large scale oxide nodules have been found to form only on the austenite with an apparently protective thinner oxide present on the ferrite. The formation of these nodules has been studied from the initial stages of oxidation by the use of environmental SEM and site-specific preparation of important features to reveal that nodules form where the thin, protective chromium rich layer chemically fails during oxidation enabling local breakaway regions to form.