1997 Volume 37 Issue 2 Pages 169-175
Stainless steel processing often involves an annealing step, which is most economically performed in atmospheres formed by the combustion of natural gas and air. During the annealing process the surface of the steel reacts with oxidants such as O2, CO2 and H2O in the furnace atmosphere to form a scale layer. The scale formation is undesirable due to its effect on yield and surface quality. In the present work, an investigation was carried out to determine the rate of scale formation on type 304 stainless steel under short-time annealing conditions. A thermogravimetric apparatus was used to study the rate of scale formation. Gas compositions were chosen to simulate different ratios of air/CH4.
A protective layer forms on the samples that are oxidized in simulated gas mixtures corresponding to air/CH4≥11 at 1 373 K. The layer breaks down after about 12 min of reaction time due to formation of an iron oxide layer on the surface. It was found that the protective layer is composed of small crystals of spinel, Cr2O3FeO.
A non-protective layer forms on 304 type stainless steel when it is annealed in atmospheres corresponding to air/CH4≤9.53 at 1 373 K. The initial rate of scale formation is much faster when type 304 stainless steels are annealed in atmospheres corresponding to air/CH4≤9.53 at 1 373 K. The rate of scale formation is much faster when an iron oxide layer forms on the samples. It was found that an iron oxide layer forms at the initial stage of oxidation when air/CH4≤9.53. The scale is composed of two layers. The outer layer is almost pure wustite when air/CH4≤9.53. The inner layer is spinel in the form of small nodules like crystals. The iron oxide layer detaches from the samples during cooling. The outer layer may contain small amounts of Fe2O3 and Fe3O4 when air/CHCH4≥11 for longer periods (∼20 min). The oxidation occurs at the gas-scale and scale-metal interfaces. The over-all rate of scale formation is controlled by both diffusion of iron upwards and diffusion of oxidizing agents through cracks and microchannels.