Eliminating Work Hardening to Obtain Satisfactory Metallographic Structure

The bright annealing furnace is mainly used for heat treatment of stainless steel products in 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 austenitic stainless steel is solution treatment. During the heating process, the carbides are dissolved in austenite, heated to 1050~1150 ℃, properly kept for a short time, so that all carbides are dissolved in austenite, and then rapidly cooled to below 350 ℃ to obtain Supersaturation Solid solution, that is, uniform unidirectional austenite structure. The key to this heat treatment process is rapid cooling, which requires a cooling rate of 55 ℃/s to quickly pass through the precipitation temperature range (550-850 ℃) after carbide solid solution. The insulation time should be as short as possible, otherwise the grain size will be coarse and affect the surface smoothness.

The heating temperature of 400 series ferritic stainless steel is relatively low (around 900 ℃), and slow cooling is often used to obtain annealed and softened microstructure. Martensitic stainless steel shall be annealed, and it can also be treated by sectional quenching and tempering.

From the above, it can be seen that there are significant differences in heat treatment systems between the 300 series and 400 series stainless steels. To obtain qualified metallographic structures, it is necessary to have a large margin of adjustment for the cooling section equipment of the bright annealing furnace. Therefore, modern advanced bright annealing furnaces often use strong convection cooling in their cooling section, with three cooling sections that can adjust the air volume separately. Along the width direction of the strip steel, it is divided into three sections, and the cooling speed in the width direction of the strip steel is adjusted through air flow diversion to control the plate shape

Another key issue in the heat treatment of cold rolled stainless steel strips is the requirement for the entire strip to have a uniform structure in both width and length. The muffle type bright annealing furnace adopts large-sized muffle tubes, and the heating airflow is uniformly organized and spirally surrounded from the outside of the muffle tubes to evenly heat the strip steel. To ensure the uniform organization of the strip steel along the length direction, it is necessary to maintain the constant linear velocity of the strip steel in the heating furnace. Therefore, modern vertical bright heat treatment furnaces are equipped with precision adjustable roller tension adjustment devices before and after. It not only needs to ensure that the inlet and outlet speed of the strip steel meets the requirements of heat treatment speed, without being affected by the empty or full loop quantity, but also needs to establish and precisely adjust the small tension of the strip steel according to the plate shape situation to meet the requirements of the plate shape.