What are the purposes and requirements for stainless steel bright annealing?

The bright annealing furnace is mainly used for heat treatment of finished products of stainless steel under a protective atmosphere. When the performance requirements are different, the requirements for the metallographic structure after bright annealing are different, and the process of bright heat treatment is also different. The typical heat treatment process for 300 series austenite stainless steels is solution treatment. During the heating process, the carbide is dissolved in austenite, heated to 1050~1150 °C, and properly kept for a short time, so that the carbide is completely dissolved in austenite, and then rapidly cooled to below 350 °C to obtain a supersaturated solid solution. One-way austenitic structure. The key to this heat treatment process is rapid cooling, which requires a cooling rate of 55 ° C / s, and quickly passes through the re-precipitation temperature zone (550 ~ 850 ° C) after solid solution of carbide. Keep the holding time as short as possible, otherwise the crystal grains will be coarse and affect the surface finish.

The main alloy components of the stainless steel strip are Fe, Cr, Ni, Mn, Ti, Si, and the like. Oxidation of Fe and Ni is not a major problem in the annealing temperature range. However, the oxidation interval of Cr, Mn, Si, and Ti is just within the heating temperature range. It is the oxidation of these alloying elements that affects the surface brightness of the strip. In particular, the oxidation of chromium dechromes the surface of the strip, which reduces the corrosion resistance of the stainless steel. When the Cr content is between 17% and 18% and the Ti content is 0.5%, the H2 dew point must be lower than -60 °C to avoid oxidation of Cr and Ti in the heating range of 800C to 1150 °C.

At the same time, how to maintain the purity of the protective gas in the furnace is also important. In this regard, the muffle cover is well sealed, and does not cause pollution of the protective atmosphere such as the electric heating furnace material, providing a relatively pure environmental space. In addition, the strip inlet and outlet of the vertical furnace are located at the lowermost part of the furnace, and the furnace pressure is stable, so the risk of air entering is much less than that of the horizontal furnace. However, the inlet and outlet sealing box, the strip conveying section, the tension adjusting roller and the top guiding roller box of the bright annealing furnace should ensure the sealing requirements of parts per million (ppM level), so that oxygen and water vapor cannot enter, and the shielding gas can’t vent.