What precautions should be taken when cooling stainless steel pipes?

As a very common metal material, stainless steel pipes are widely used due to the rapid progress in material processing technology. These better processing technologies help enhance the performance of stainless steel pipes to meet demand. However, after a series of processing processes, especially after heat treatment, the stainless steel pipe must be cooled.
The cooling process is very important for the use of stainless steel pipes, because once there is a mistake or error in any process, it will cause quality problems of stainless steel pipes and affect the use, because there are several issues that need to be paid special attention to when performing the cooling process of stainless steel pipes.
The first is that the supercooled austenite of stainless steel pipes is very stable and has great hardenability. Even if it is air-cooled, it can obtain a martensite structure. However, air-cooling will cause oxidation on the surface of stainless steel cutting tools and other products, and may also cause Eutectoid carbides are precipitated, so the air cooling process is now avoided as much as possible.
For larger stainless steel pipes with complex structures, in order to reduce the degree of deformation and cracking, pre-cooling can be done during quenching, but the time needs to be controlled, usually between a few seconds and tens of seconds depending on the shape of the stainless steel pipe, etc., but The premise is that secondary carbides cannot be precipitated to reduce the hardness and red hardness of the tool, and corrosion pits, etc. cannot be formed.
The third is that the graded quenching temperature should not be higher than 650°C to avoid the transformation of pearlite and the precipitation of carbides.
The fourth is to consider that the transformation temperature of lower bainite is 320-250°C, and its fastest transformation temperature is 260 to 300°C, so 260-280°C isotherm is used for 2 to 4 hours in production. Because the retained austenite temperature after austempering is relatively high and stable, four temperings are required.
The fifth is that high-temperature graded quenching should be carried out in a neutral salt bath at 580 to 620°C. Nitrate salt at 500 to 550°C cannot be used for graded cooling. The reason is that nitrate salt will cause oxidation and corrosion of hot tools. Even the sharp corners of the tool are burned.
The last point to note is that the stainless steel pipe should be placed vertically into the cooling medium during the quenching operation to reduce the deformation of the stainless steel pipe.