What factors affect the bright heat treatment of stainless steel welded pipes？

What factors affect the bright heat treatment of stainless steel welded pipes?
The purpose of online bright heat treatment of stainless steel welded pipes is: first, to eliminate the residual stress generated during the cold processing of rolling stainless steel strips into tubes and the welding process; second, to carry out solid solution treatment so that all carbides generated during welding steel pipes are dissolved in austenite, and then quickly cooled down, so that austenite does not precipitate or undergo phase change during the cooling process. It is an important process to ensure the performance of stainless steel welded pipes. Factors affecting online bright heat treatment

1. Influence of heat treatment temperature

Solution treatment is the most effective softening treatment process for austenitic stainless steel. After solution treatment, the welded pipe can obtain the best corrosion resistance, lower strength, and better plasticity. Only in this way can it meet the requirements of industrial pipes such as condenser pipes and chemical pipes.

According to the standard requirements of condenser stainless steel pipes, the heat treatment temperature of austenitic stainless steel welded pipes should reach 1050 ~ 1150 ℃. At the same time, the inner and outer surfaces of the welded pipes after heat treatment are required to be white, bright and smooth, and no oxidation color should be present. Therefore, the temperature change range (in the furnace body) should be strictly controlled during the heating and cooling of the welded pipes. The steel pipes should be in a good protective atmosphere, and the traditional water quenching method should not be used to avoid the high-temperature steel pipes from decomposing oxygen and oxidizing the pipe surface when they come into contact with water. Usually, the solution treatment temperature of austenitic stainless steel is 1050 ~ 1150 ℃. If this temperature is not reached, its internal structure is unstable, and carbides will precipitate, resulting in the steel pipe surface not being bright and black.

2. Influence of protective gas

The heat treatment of stainless steel welded pipes uses a non-oxidizing continuous heat treatment furnace with protective gas, which can obtain a non-oxidizing bright surface, thereby eliminating the traditional pickling process. The protective gas that can be used is high-purity hydrogen, decomposed ammonia and other protective gases. Since stainless steel welded pipes contain chromium, it is impossible to perform bright heat treatment in common protective gases (such as hydrocarbon decomposition gases, etc.), and it is best to perform it in a vacuum environment. However, for the heat treatment of online stainless steel welded pipes, a vacuum environment is not possible, and inert gas (such as argon) can be used. Although the use of inert gas as a protective gas for heat treatment of austenitic stainless steel welded pipes has the characteristics of not participating in chemical reactions, simple operation, safety and reliability, it does not have a reducing property, so that the heat treatment effect does not meet the ideal bright heat treatment quality requirements, and the pipe surface is silver-gray. Furthermore, the cost of inert gas is high and is not suitable for large-scale production. Based on the research on the heat treatment process and the analysis and repeated tests of the quality of stainless steel welded pipes after bright heat treatment, the method of first exhausting the air in the heat treatment furnace with inert gas and then replacing the inert gas with hydrogen is adopted. Practice has proved that the bright heat treatment quality requirements are met.

3. Influence of cooling temperature

After heating the stainless steel welded pipe to 1050 ~ 1150 ℃, the welded pipe should be cooled down quickly. On the one hand, it is ensured that austenite does not precipitate during the cooling process. On the other hand, after the pipe leaves the furnace (leaves the gas protective atmosphere), its temperature should drop to a non-oxidizing temperature. Therefore, the cooling temperature is very important and the temperature range should be strictly controlled.

4. Influence of welded pipe surface

The surface state of the stainless steel welded pipe before entering the furnace has a great influence on the bright heat treatment. If the surface of the welded pipe is attached with moisture, grease and other dirt when entering the furnace, a light green oxidation color will appear on the surface of the welded pipe after bright heat treatment. Therefore, before entering the heat treatment furnace, the surface of the stainless steel welded pipe should be very clean, and moisture is not allowed on the surface of the welded pipe. If necessary, it can be dried in a dryer before entering the furnace.

5. Influence of heat treatment furnace sealing

The heat treatment furnace should be closed and isolated from the outside air. Especially at the place where the welded pipe enters the furnace body and the place where the welded pipe exits the furnace body, the sealing rings in these places are particularly easy to wear and tear, so they should be checked frequently and replaced in time. 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 is generally required to be greater than the standard atmospheric pressure.

6. The influence of other factors on bright heat treatment

During the working process, it is necessary to ensure that the welding is continuous and stable. When there are holes or cracks on the welded pipe, the heat treatment furnace must be stopped, otherwise the welded pipe may be burned in the furnace. In addition, if the welding effect is not good, the air or moisture ejected from the welding hole will destroy the protective atmosphere in the furnace and affect the bright heat treatment effect.

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