Bright annealing furnace is mainly used for heat treatment of finished stainless steel products in protective atmosphere. When the performance requirements are different, the metallographic structure after bright annealing is different, and the process of bright heat treatment is also different.
The typical heat treatment process of 300 series austenitic stainless steel is solution treatment. In the process of heating, carbides are dissolved into austenite, heated to 1050 ~ 1150℃, properly held for a short period of time, so that all carbides are dissolved in austenite, and then rapidly cooled to below 350℃, susaturation solid solution is uniform uni-symmetric austenite structure. The key of this heat treatment process is rapid cooling, which requires a cooling rate of 55℃/s and a rapid precipitation through the temperature zone (550 ~ 850℃) after solid solution of carbide. The holding time should be as short as possible, otherwise the coarse grain will affect the surface finish.
400 series ferritic stainless steel heating temperature is relatively low (900℃ or so), and more use of slow cooling to obtain annealing softening microstructure. Martensitic stainless steel can be treated by annealing or piecewise quenching and tempering.
It can be seen from the above that there is a great difference between 300 series and 400 series stainless steel in heat treatment system. In order to obtain qualified metallography structure, it is required that the cooling equipment of bright annealing furnace has a great room for adjustment. Therefore, the modern advanced bright annealing furnace often adopts strong convection cooling in its cooling section, with three cooling sections, which can adjust the air volume separately. It is divided into three sections along the width direction of the strip, and the cooling speed in the width direction of the strip is adjusted by air flow diversion, and the plate type is controlled.