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Why Does Stainless Steel Tube Turn Yellow Or Black After Bright Annealing?

Views: 156     Author: Iris     Publish Time: 2024-05-23      Origin: Hangao(SEKO)


At present, stainless steel pipes are widely used in the market, and they play a very important role in many industries.

The principle of annealing stainless steel pipes:

A satisfactory metallographic structure can be obtained by eliminating the work hardening of stainless steel pipes. The annealing equipment is a stainless steel continuous bright annealing furnace, which is mainly used for heat treatment of finished stainless steel products under a protective atmosphere. When the application performance requirements of stainless steel pipes are different, the requirements for the metallographic structure after bright annealing are different, so the bright heat treatment process will also be 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 austenitic stainless steel, heated to 1050~1150℃, and then kept warm for a short period of time. All the carbides can be dissolved in the austenite structure, and then quickly cooled to below 350℃. A supersaturated solid solution is obtained, which is a uniform unidirectional austenite structure. The focus of this heat treatment process is rapid cooling, which requires the cooling rate to reach 55°C, and then quickly pass through the re-precipitation temperature zone (550~850°C) after carbide solid solution. In addition, the heat preservation time of stainless steel pipes should be as short as possible, otherwise the particles will become coarse and affect the surface finish.

固溶 保温细节

The heating temperature of 400 series ferritic stainless steel is relatively low (around 900°C), and to a large extent, slow cooling is used to obtain an annealed and softened structure. The annealing method used for martensitic stainless steel can also be treated by segmented fire and then tempering. From the above introduction, we can know that there is a big difference in heat treatment processing technology between 300 series and 400 series. In order to obtain a qualified metallographic structure, the cooling section equipment of the bright annealing furnace requires a large adjustment room. Therefore, current advanced bright annealing furnaces often use strong convection cooling in their cooling sections, and are equipped with three cooling sections.

The purpose of annealing stainless steel pipes:

1. Reduce hardness and improve plasticity to facilitate cutting and cold deformation processing.

2. Refine the grains, uniform the structural composition of the steel, improve the performance of the steel or prepare for subsequent heat treatment.

3. Eliminate residual internal stress in steel to prevent deformation and cracking.

However, many users report that the annealed stainless steel pipe turns yellow or blue and always fails to achieve the expected bright effect. So how to solve this problem?

Now, engineers from Hangao Machinery will discuss with you:

1. It may be caused by unstable heating temperature. When the pipe is heated, the surface temperature is high but the inside temperature is low. It may be caused by problems with the annealing temperature control or the design of the temperature zone division of the annealing furnace. Nowadays, there are many kinds of annealing furnaces in the market, and the prices vary greatly, making it difficult for users to distinguish good from bad.

2. Find the cause from the process flow and technology, which is related to the user's temperature setting, the surface cleanliness of the stainless steel pipe, and the material of the stainless steel pipe.


In order to solve the above problems and make the stainless steel pipe bright after annealing, the following are the main things to do:

1. The air tightness of the heating furnace body and cooling section is a key factor in whether the stainless steel pipe is bright.

2. Whether the structure of the annealing furnace, the distribution of the temperature zone, and the thermal field of the annealing furnace are reasonable. These factors directly affect the heating of stainless steel pipes.

Uniformity, the stainless steel pipe must be heated to an incandescent state, but the heated pipe body must not soften and sag. Heating temperature that is too high is not conducive to restoring the internal structure of the pipe.


3. If the stainless steel pipe itself has too much oil or water stains, the protective atmosphere in the furnace will be destroyed and the purity of the protective gas will not be reached. Stains or water vapor will adhere to the surface of the pipe body and affect the smoothness. At this time, we can check the cleanliness of important accessories and whether there is leakage in the cooling water pipes, and investigate them one by one.

4. Ensure a slight positive pressure in the atmosphere in the furnace so that air will not be sucked back into the furnace. If it is ammonia decomposition mixed gas, it usually requires more than 20kBar.

Hope this knowledge and analysis can help you. If there are any questions or needs about steel pipe induction annealing furnace, please feel free to contact us.

Related Products

Every time the finishing tube is rolled, it must go through the process of solution treatment. Ta ensure that the performance of the steel pipe meets the technical requirements. and to provide guarantee for the post-process processing or use. Bright solution treatment process of ultra-long seamless steel pipe has always been a difficulty in the industry.

Traditional electric furnace equipment is large,covers a large area, has high energy consumption and large gas consumption, so it is difficult for realize bright solution process. After years of hard work and innovative development, the use of current advanced induction heating technology and DSP power supply. Precision control of heating temperature to ensure that the temperature is controlled within t2C,to solve the technical problem of inaccurate induction heating temperature control. The heated steel pipe is cooled by "heat conduction"in a special closed cooling tunnel, which greatly reduces the gas consumption and is more environmentally friendly.
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