Stress-relief heat treatment for circumferential welds of pipes

The purposes of post-weld heat treatment are three: hydrogen removal, elimination of welding stress, and improvement of the weld microstructure and overall properties. 　　Post-weld dehydrogenation treatment refers to low-temperature heat treatment carried out after welding is completed, before the weld has cooled below 100°C. The general practice is to heat to 200–350°C and maintain that temperature for 2–6 hours. The main purpose of post-weld hydrogen removal treatment is to accelerate the escape of hydrogen from the weld and heat-affected zone, and it is highly effective in preventing welding cracks that occur during the welding of low-alloy steel. 　　During the welding process, due to uneven heating and cooling, as well as constraints arising from the components themselves or external forces, welding stresses always occur in the components after the welding is completed. The presence of welding stress in components reduces the actual load-bearing capacity of the welded joint area, causes plastic deformation, and in severe cases, can lead to the failure of the component. 　　Stress-relief heat treatment involves subjecting the welded workpiece to high temperatures in order to reduce its yield strength, thereby relieving the welding stresses. There are two common methods: one is overall high-temperature tempering, which involves placing the welded piece as a whole into a heating furnace, slowly heating it to a certain temperature, holding it at that temperature for a while, and then cooling it in air or within the furnace. This method can eliminate 80%–90% of welding stress. Another method is local high-temperature tempering, which involves heating only the weld area and its surrounding region before cooling it slowly, thereby reducing the peak value of welding stress and smoothing out the stress distribution, thus partially eliminating the welding stress. 　　Some alloy steel materials develop a hardened microstructure in their welded joints after welding, which deteriorates the mechanical properties of the material. Furthermore, this hardened structure may lead to joint failure under the influence of welding stresses and hydrogen. If the microstructure of the joint is improved after heat treatment, the plasticity and toughness of the welded joint are enhanced, thereby improving the overall mechanical properties of the welded joint.