Research Of Heat Treatment For Stress Relieving Of Thin-wall304Stainless Steel Welded Pipe

304stainless steel (0Cr18Ni9in GB/T1220—92) is one type of austenitic stainless steel.304stainless steel which has good performance of corrosion resistance, heat resistance, welding, forming, bending is widely used in heat exchangers, condensers, water pipes and nuclear power plant pipes. Although304stainless steel has good comprehensive properties, it may be corroded and crack with the influence of temperature, stress and corrosive medium.Statistics show that the corrosion failures especially stress corrosion cracking occur most often in equipment made by austenitic stainless steel. Stress corrosion not only may cause a large number of economic losses, even casualties to enterprise. Thin-wall304stainless steel welded pipe produced by a factory of shanghai was tested that there was large residual tensile stress on the surface of pipes. In the stress corrosion test, stress corrosion cracking occurred on the pipes about52hours later.For this reason, based on the study of the anterior research fruit of other people,, the author focused on the following research:surface residual stress test, heat treatment process, establishment for stress relief, stress corrosion test, metallographic analysis, deformation degree of steel pipe diameter measurement and mechanical properties test. The heat treatment process for eliminating the residual tensile stress on the surface of steel pipes was determined. The result of this study can be used to improve heat treatment process for relieving residual stress of thin-wall304stainless steel welded pipe.Main research work and the results include:1. Surface residual stress test. The method of X-ray diffraction was used to test the residual stress of thin-wall304stainless steel welded pipe which was produced by a factory of shanghai. The results show that there is high residual tensile stress on the surface of steel pipe. This is the main reason to cause stress corrosion cracking of steel pipes in the practical application.2. Heat treatment for stress relief. The thin-wall304stainless steel pipes were heated in950degrees for30minutes and put into5%,10%,15%,20%PAG solution to cool, then the surface residual stress of pipes was measured and compared with which before heat treatment. The results showed that:After heat treatment, the residual tensile stress on the surface of steel pipes reduces in varying degrees and it is effective for steel pipes to improve corrosion resistance.3. Stress corrosion test. The pipes are tested according to YB/T5362-2006"Test method for stress corrosion-cracking resistance of stainless steels in a boiling magnesium chloride solution". In test, the pipes after heat treatment didn’t crack150hours later, while those pipes without heat treatment cracked52hours later. The results show that the heat treatment process defined in this article can efficiently improve the resistance to stress corrosion cracking.4. The deformation degree of steel pipes diameter measurement. Treating the steel pipes with heat treatment processes which defined in this article and created by senior student. After that, measuring the steel pipe’s diameters, and then comparing their deformation. The results demonstrate that the process that defined in this article can reduce the deformation of steel pipes diameter. Also, the higher the concentration of PAG solution is, the less the deformation of steel pipes is.5. Metallographic analysis. Comparing the metallographic of steel pipes before and after heat treatment, the conclusions below can be reached. After heat treatment, there were no precipitated carbides along the austenite boundary of steel pipes, and there were part of grains growing up.6. Microhardness and mechanical properties test. The microhardness and tensile strength of steel pipes after heat treatment slightly decreased. The elongation at break slightly increased. They all conform to national standard.