The importance of pre-welding heating and post-weld heat treatment for pressure vessels

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Preheating and post-heating, including overall heat treatment after welding, are very important processes in the manufacturing and installation of pressure vessels. If not handled properly, they can have a serious impact on the overall quality of these vessels; therefore, it is essential to pay close attention to preheating and post-heating procedures. Preheating before welding: Preheating before welding and post-weld heat treatment are very important for ensuring welding quality. Welding of critical components, welding of alloy steels, and welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows: (1) Preheating can slow down the cooling rate after welding, which facilitates the escape of diffused hydrogen from the weld metal and helps to prevent hydrogen-induced cracks. It also reduces the hardening degree of the weld and heat-affected zone, thereby improving the crack resistance of the welded joint. 　　(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as the temperature gradient) between the workpieces in the welding area. In this way, on the one hand, welding stress is reduced, and on the other hand, the welding strain rate is decreased, which helps to prevent the formation of welding cracks. 　　(3) Preheating can reduce the constraint on the welded structure, and this effect is particularly evident in reducing the constraint on fillet joints; as the preheating temperature increases, the crack incidence decreases. 　　The selection of the preheating temperature and interpass temperature depends not only on the chemical composition of the steel and welding electrodes, but also on factors such as the rigidity of the welded structure, the welding method, and the ambient temperature; these factors should be taken into consideration comprehensively before making a decision. Furthermore, the uniformity of the preheating temperature in the thickness direction of the steel plate and in the weld area has a significant impact on reducing welding stress. The width of local preheating should be determined based on the degree of restraint of the workpiece to be welded; it generally should be three times the wall thickness around the weld area, and must not be less than 150–200 millimeters. If the preheating is uneven, it not only fails to reduce welding stress but may even increase it. Post-weld heat treatment serves three purposes: removing hydrogen, eliminating welding stress, and improving the microstructure and overall properties of the weld. 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 zone and the 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 internal constraints in the components or external constraints applied, welding stresses always arise in the components after the welding is completed. The presence of welding stress in the components reduces the actual load-bearing capacity of the welded joint area, causes plastic deformation, and in severe cases, can even lead to the failure of the components. 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 part as a whole into a heating furnace, slowly heating it to a certain temperature, holding 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 stresses and smoothing out the stress distribution, thus partially eliminating the welding stresses. 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.