Relaxation heat treatment of stainless steel pipe

For general stainless steel pipes, it is only necessary to carry out the load test before shipment (compression or expansion to the limit position, at which time the load is about twice the working load) and after the load test. (or stretched) stainless steel pipe can be used if the size meets the requirements with certain deformation (generally not more than a few millimeters). If it is suitable for the test, the stainless steel pipe has a sufficiently high elastic limit and will not deform obviously under the working load. However, for some high-precision stainless steel pipes (such as stainless steel pipes for measuring instruments), the dimensional stability of stainless steel pipes is required to be higher. A small amount of deformation will occur due to stress relaxation when stainless steel pipes are used to bear loads for a long time. Even high-precision stainless steel pipes are unacceptable. Rest and heat can overcome these shortcomings. The treatment method is to pre add deformation (elastic deformation) exceeding the working load on the stainless steel pipe, and then fix and heat it. The temperature is slightly higher than the working temperature of thin-walled stainless steel pipe. The insulation lasts for 8-24 hours. In this process, the prestress relaxation and deformation of the stainless steel pipe occur, which greatly reduces the relaxation phenomenon during later work and achieves the purpose of dimensional stability. For high temperature stainless steel pipes, stress relaxation is more serious at high temperature, so more relaxation heat treatment is required.

Inspection after heat treatment: generally, hardness and load tests shall be carried out for stainless steel pipes after heat treatment, except for those whose dimensions meet the requirements of the drawings. Key stainless steel pipes need self inspection after tempering, and small cracks that are not easy to find are found. In order to check the quality of stainless steel pipes, it is sometimes necessary to carry out a certain number of fatigue tests. Generally, the fatigue life (number of stress changes) is measured in a short time by applying a stress exceeding the working load. It is time to assess the quality of the heat treatment. Some samples were subjected to fatigue test for a certain period of time under the same conditions as the actual working environment. In order to control the quality of the stainless steel pipe, the stainless steel pipe will not break during the test.

Some methods to improve the quality of stainless steel pipe:

At present, short iron sandblasting: to improve the fatigue life of stainless steel pipe, mainly to improve the surface condition of stainless steel pipe. The widely used one is shot blasting. The centrifugal shot blasting machine shoots the self sealing iron at a speed of about 50-60 meters per second (or hardens the steel ball to obtain better effect, using glass beads) shoot. The spring causes the plastic deformation of the surface layer and bears the compressive stress. Therefore, the influence of various defects and stress concentration areas on the fatigue life of the stainless steel pipe surface is greatly reduced and the service life of the fatigue stainless steel pipe is greatly improved. It is better to apply tensile stress on the surface of 304 stainless steel pipe and then conduct shot blasting. Generally, the fatigue life of stainless steel pipe after shot blasting can be increased several times by high numbers.

Thermomechanical heat treatment: This article introduces the general principle and function of thermomechanical heat treatment, which can be used to further improve the strength and plasticity of 304 stainless steel pipe. The process characteristics of stainless steel tubes are beneficial to the heat treatment of high temperature deformed stainless steel tubes. That is, the sheet or bar is formed immediately after hot rolling and deformation, and then quenched. Thermomechanical heat treatment is applied to the production of automobile leaf springs. The fatigue life of leaf springs after thermomechanical heat treatment under 95kg/mm2 test stress reaches 1.45 million cycles, which is three times that of leaf springs. Compared with general heat-treated leaf springs, heat-treated leaf springs have improved mechanical properties in strength, yield limit, plasticity, toughness, especially impact toughness.