Directly forming to square pipe mill is widely used in the production of stainless steel welded pipes for large-scale power station generator sets. Most of the production of stainless steel welded pipes in my country is the introduction or use of advanced foreign equipment and technology.

Heat treatment is an important process to ensure the performance of stainless steel welded pipes. One of its purposes is to eliminate the residual stress generated during the cold working process of coiling the stainless steel strip into a tube and the welding process. The second purpose is to perform solution treatment to make the welded steel pipe The carbides produced at this time are all solid-dissolved into austenite, and then quickly cooled down to prevent the austenite from precipitation or phase change during the cooling process. At present, continuous roller hearth furnaces are generally used in the heat treatment of stainless steel pipes using directly forming to square pipe mill in my country. Since no protective atmosphere is used, and producer gas is used as fuel, then pickling is used to remove the heat generated on the surface of the steel pipe after heat treatment. Oxide skin corrodes the steel pipe surface and causes environmental pollution. Some developed countries have now widely used online bright heat treatment technology. Since it can eliminate the traditional off-line heat treatment and pickling process, it has the advantages of increasing productivity and reducing labor intensity. Therefore, my country should vigorously promote the production of stainless steel welded pipes with protective atmosphere control. System development of bright heat treatment technology. The author analyzed some of the problems that occurred during the on-line bright heat treatment of the welded pipe production line introduced in my country, in order to improve the product quality and yield rate.

The factors that affect online bright heat treatment can be divided into six kinds. We first introduce the influence of heat treatment temperature and the influence of protective gas.

①The influence of heat treatment temperature

Solution treatment is the most effective softening treatment process for austenitic stainless steel. The welded pipe after solution treatment can get 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 requirements of the stainless steel pipe standard for condensers, 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 heat-treated welded pipes should be white and smooth without oxidation color. Therefore, it is required to be strict during the heating and cooling of the welded pipe. To control the temperature change range (in the furnace body), the steel pipe should be in a good protective atmosphere, and the traditional water quenching method cannot be used to prevent the high temperature steel pipe from decomposing oxygen and oxidizing the surface of the pipe when it meets water. Generally, the solution treatment temperature of austenitic stainless steel is 1050 ～ 1150 ℃. If this temperature is not reached, the internal structure of the austenitic stainless steel is unstable, and carbides will precipitate, causing the surface of the steel pipe to not reach the bright color, and the surface of the pipe appears black.

②The influence of shielding gas

The heat treatment of the stainless steel welded pipe adopts a non-oxidation continuous heat treatment furnace with shielding gas, which can obtain a non-oxidized bright surface, thereby eliminating the traditional pickling process. The protective gases that can be used are high-purity hydrogen, decomposed ammonia and other protective gases. Because stainless steel welded pipes contain chromium, it is impossible to perform bright heat treatment in the usual shielding gas (such as hydrocarbon decomposition gas, etc.), and it is best to perform it in a vacuum environment. However, for the heat treatment of on-line stainless steel welded pipes, a vacuum environment is not possible, and an inert gas (such as argon) can be used. Although the use of inert gas as the shielding gas for the heat treatment of austenitic stainless steel welded pipes has the characteristics of not participating in chemical reactions, simple operation, safe and reliable, but it does not have reducing properties, so that the heat treatment effect cannot meet the ideal bright heat treatment quality requirements. It is silver-gray. Furthermore, the cost of inert gas is relatively high, and it is not suitable for large-scale production. According to the research on the heat treatment process and the analysis and repeated tests on the quality of the stainless steel welded pipe after bright heat treatment, the method of first using inert gas to drain the air in the heat treatment furnace, and then replacing the inert gas with hydrogen, has proved that the bright heat treatment has been achieved. Quality requirements.

The main factors affecting the on-line bright heat treatment of stainless steel welded pipe made by directly forming to square pipe mill are the influence of heat treatment temperature. ②The influence of protective gas. ③The influence of cooling temperature. ④The influence of welded pipe surface. ⑤The influence of heat treatment furnace sealing. ⑥The influence of other factors on bright heat treatment.