01 The purpose of the stainless steel bright annealing process

Bright annealing furnace is mainly used for heat treatment of finished stainless steel products under protective atmosphere. When the performance requirements are different, the requirements for the metallographic structure after bright annealing are different, and the bright heat treatment process 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 into the austenite, heated to 1050~1150°C, and kept for a short period of time to allow all the carbides to dissolve in the austenite, and then rapidly cooled to below 350°C to obtain a supersaturated solid solution that is uniform. Unidirectional austenite structure. The key to this heat treatment process is rapid cooling, which requires the cooling rate to reach 55°C/s and quickly pass through the re-precipitation temperature zone (550~850°C) after carbide solid solution. The holding time should be as short as possible, otherwise the grains will become coarse and affect the surface finish. The heating temperature of 400 series ferritic stainless steel is relatively low (around 900°C), and slow cooling is often used to obtain an annealed and softened structure. Martensitic stainless steel is annealed, and can also be treated by segmented quenching and then tempering.

From the above, it can be seen that the heat treatment systems of 300 series and 400 series stainless steel are very different. In order to obtain a qualified metallographic structure, the cooling section equipment of the bright annealing furnace is required to have a lot of room for adjustment. Therefore, modern advanced bright annealing furnaces often use strong convection cooling in their cooling sections. There are three cooling sections and the air volume can be adjusted individually. It is divided into three sections along the width direction of the strip. The cooling rate in the width direction of the strip is adjusted through air volume diversion. Another key issue in the heat treatment of plate-shaped stainless steel cold-rolled strip is that the entire strip is required to be in width and length. The upper tissue is very uniform.

The muffle type bright annealing furnace uses large-size muffle tubes to evenly organize the heating airflow from the outside of the muffle tubes to spiral around it to evenly heat the strip. To ensure that the strip has a uniform structure along its length, it is necessary to keep the linear speed of the strip in the heating furnace constant. Therefore, precision-adjustable roller tension adjustment devices are installed at the front and rear of modern vertical bright heat treatment furnaces. It not only ensures that the inlet and outlet speed of the strip meets the requirements of the heat treatment speed and is not affected by the empty or full loop size, but also establishes and precisely adjusts the small tension of the strip according to the shape of the strip to meet the shape of the strip. Require.

02  Whether the annealed stainless steel pipe is bright or not mainly depends on the following factors

1. Whether the annealing temperature reaches the specified temperature. The heat treatment of stainless steel generally adopts solid solution heat treatment, which is what people usually call “annealing”. The temperature range is 1040~1120℃ (Japanese standard). You can also observe through the observation hole of the annealing furnace. The stainless steel pipe in the annealing area should be in an incandescent state, but there should be no softening and sagging.

2. Annealing atmosphere. Generally, pure hydrogen is used as the annealing atmosphere. The purity of the atmosphere is preferably above 99.99%. If the other part of the atmosphere is an inert gas, the purity can be lower, but it must not contain too much oxygen or water vapor.

3. Furnace body sealing. The bright annealing furnace should be closed and isolated from the outside air; if hydrogen is used as the protective gas, only one exhaust port is open (used to ignite the discharged hydrogen). The inspection method can be to apply soapy water on the joints of the annealing furnace to see if there is air leakage; the most likely places for air leakage are the places where the tubes enter and exit the annealing furnace. The sealing rings in this place are particularly easy to wear. Check and change frequently.

4. Protective gas pressure. In order to prevent micro-leakage, the protective gas in the furnace should maintain a certain positive pressure. If it is hydrogen protective gas, it generally requires more than 20kBar.

5. Water vapor in the furnace. On the one hand, check whether the furnace material is dry. When installing the furnace for the first time, the furnace material must be dried; secondly, whether there are too many water stains on the stainless steel pipe entering the furnace. Especially if there are holes on the pipe, be sure not to allow water to leak in. Otherwise, the atmosphere of the furnace will be completely destroyed.

Cherry
Website: www.jinyoumetal.com
Email: Cherry@jinyoumetal.com
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