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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 stee] reacts with oxidants such as O., C02and H.O 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. Athermogravimetric apparatus wasused to study the rate of scale formation. Gascompositions 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;ZI I at 1373 K. The iayer breaks downafter about 12min of reaction time due to formation of an iron oxide layer on the surface. It wasfound that the protective layer is composedof small crystals of spinel, Cr.O.FeO. Anon-protective layer forms on 304 type stainless steel when is annealed in atmospherescorresponding to air/CH4~9.53 at 1373 K. The initial rate of scale formation is muchfaster whentype 304 stainless steels are annealed in atmospherescorresponding to air/CH4~ 9.53 at 1373 K, The rate of scale formation is much faster whenan iron oxide layer forms on the samples. It was found that an iron oxide layer forms at the initial stage of oxidation whenair/CH4~9.53. Thescale is composedof two layers. Theouter layer is almost pure wustite whenair/CH*~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 maycontain small amountsof Fe.03 and Fe.04 Whenair/CH4~~l I for longer periods (-20min). The oxidation occurs at the gas-scale and scale-metal interfaces. Theover-all rate of scale formation is controlled by both diffusion of iron upwards and diffusion of oxidizing agents through cracks and microchannels.

Oxidation of Type 304 Stainless Steels under Simulated Annealing Conditions.