Applications of preheating, post-heating, and post-weld heat treatment

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During the welding process, the base material undergoes a welding heat cycle, which causes changes in the microstructure of the weld and the heat-affected zone; this directly affects the mechanical properties of the welded joint. Therefore, to ensure the performance and quality of welded structures and prevent the occurrence of welding defects, process measures such as preheating, post-heating, and heat treatment after welding are often required in welding operations, especially when welding thick plates, hardening-prone steels, and highly rigid structures. Preheating is a process measure in which the entire or part of the weldment is heated to a specified temperature prior to welding. Welding of critical components, welding of alloy steels, and welding of thick parts all require preheating prior to welding. 1. Preheating effect: ◆ ◆ It slows down the cooling rate after welding, thereby effectively preventing the formation of cracks. By appropriately extending the cooling time in the 800–500°C range, it facilitates the escape of diffused hydrogen from the welded metal, helping to avoid hydrogen-induced cracks. It also reduces the degree of hardening in the weld and heat-affected zone, thereby improving the crack resistance of the welded joint. ◆ ◆ Local or overall preheating to reduce welding stresses uniformly can decrease the temperature difference (also known as the temperature gradient) between different parts of the workpiece to be welded. This not only reduces welding stresses but also lowers the welding strain rate, thereby helping to prevent the formation of welding cracks. ◆ ◆ Reducing the degree of constraint in welded structures: Preheating is particularly effective in reducing the constraint on fillet joints; as the preheating temperature increases, the crack incidence decreases. 2 Preheating Requirements ◆ ◆ Preheating Temperature The appropriate preheating temperature should be determined by taking into account factors such as the chemical composition of the base material, its weldability, thickness, the degree of constraint on the welded joint, the welding method and environment, as well as relevant technical standards for the product. For important structures, crack tests must be conducted to determine the lowest preheating temperature that prevents crack formation. The higher the preheating temperature, the better its effect in preventing crack formation; however, exceeding the necessary preheating temperature will cause the metal grains in the vicinity of the fusion zone to become coarser, reducing the quality of the welded joint and also worsening the working conditions. ◆ ◆ For overall preheating, various types of furnaces are typically used for heating. However, in actual welding processes, it is difficult or even impossible to carry out overall preheating for many large structures, such as large spherical tanks and pipelines. Therefore, local preheating is often employed to prevent cracks from forming; local heating is generally achieved using gas flame heating or infrared heating. The preheating temperature is commonly measured with a surface thermometer. 3 Precautions ◆ ◆ When welding different steel grades together, the preheating temperature should be set according to the requirement of the steel grade with the higher preheating needs ； ◆ ◆ When local preheating is used, excessive local stress should be prevented. The preheating range should be at least 3 times the thickness of the workpiece on each side of the weld, and shall not be less than 100 mm ； ◆ ◆ The temperature of the weldments that require preheating should remain at or above the preheating temperature throughout the entire welding process ； ◆ ◆ Preheating requirements are also considered when cutting, grooving, root cleaning, slotting, or applying temporary welds using hot working methods. Post-heating refers to the process measure of immediately heating the entire or a portion of the welded structure after welding, maintaining that temperature for a certain period of time, and then allowing it to cool naturally. The use of post-heating in production allows for achieving the same results as preheating, even by reducing the preheating temperature or eliminating it altogether. ●Post-heating ◆ ◆ accelerates the escape of diffused hydrogen and prevents the formation of delayed cracks. Post-heating is particularly effective in preventing delayed cracks in low-alloy steels with higher strength grades as well as in welded structures under high stress conditions; therefore, post-heating is also known as hydrogen-removal treatment. ◆ ◆ It helps to reduce the preheating temperature. In the actual welding of some low-alloy steels containing high amounts of alloying elements, if preheating is relied on solely to prevent hydrogen-induced cracks, a higher preheating temperature must be chosen, which deteriorates the operating conditions and can sometimes lead to other adverse effects such as thermal stress cracks or crystalline cracks. However, applying post-heating can avoid such effects. ●Precautions ◆ ◆ The temperature for post-heating and the holding time depend on the thickness of the workpiece; generally, the post-heating temperature is set at 200–350°C, with a holding time of no less than 0.5 hours. ◆ ◆ Since hydrogen removal can be achieved during heat treatment, weldments that require heat treatment immediately after welding do not need to undergo post-heat treatment. However, if heat treatment cannot be carried out immediately after welding and the welded part must be dehydrogenated, post-weld heat treatment must be performed right after welding; otherwise, delayed cracks may occur during the period before heat treatment. Post-weld heat treatment is a process in which solid metal is subjected to heating, holding at a certain temperature, and cooling in order to improve its internal structure and achieve the desired properties. ●Effect of heat treatment ◆ ◆ Reduces or eliminates welding residual stresses ； ◆ ◆ Eliminate the hardened structure in the weld heat-affected zone, and improve the microstructure and properties of the welded joint ； ◆ ◆ Promoting the escape of residual hydrogen helps prevent delayed cracking ； ◆ ◆ Improve the geometric stability of the structure ； ◆ ◆ Enhances the resistance of the component to stress corrosion. ●Common processes ◆ ◆ Stress-relief annealing: The heating temperature range for stress-relief annealing is the same as that for high-temperature tempering. Generally, the welded part is heated as a whole or in certain areas to 550–650°C, and then cooled slowly after maintaining that temperature for an appropriate period of time. The holding time is generally calculated at 2.5 minutes per millimeter of thickness for steel, but it must not be less than 15 minutes. For thicknesses over 50mm, an additional 15 minutes is added for every 2mm increase. Overall stress-relief annealing is generally carried out in a furnace, and it can eliminate 80%-90% of the residual stress. Local stress-relief annealing can be carried out using infrared heaters, power-frequency induction heaters, etc. for local heating, but it is necessary to ensure an adequate heating width. Local stress-relief annealing can essentially achieve the same results as full stress-relief annealing. ◆ ◆ Post-weld heat treatment such as normalizing or normalizing followed by tempering is generally applicable to electroslag welded structures to improve the microstructure and properties of the joints. Normalizing involves heating the steel above Ac3; the holding time is calculated at 2 minutes per millimeter of thickness, but it must be no less than 30 minutes, after which the steel is taken out of the furnace and cooled in air. Since it is a recrystallization process, a finer-grained structure can be obtained, thereby improving the mechanical properties. Normalizing followed by tempering involves performing tempering after normalizing. The purpose of tempering is to eliminate the structural stresses resulting from the normalizing cooling process, thereby further improving the overall properties of the steel or welded joints. ◆ ◆ Quenching and tempering, this type of post-weld heat treatment process, is suitable for quenched and tempered steel or other welded structures that require such treatment after welding. After quenching and tempering, steel or welded joints can obtain mechanical properties with a good balance between strength and toughness. Quenching is a heat treatment process in which steel is heated to 30–50°C above its critical points Ac1 or Ac3, held at that temperature for a certain period of time, and then rapidly cooled in water or oil to obtain a highly hard microstructure. ●Precautions ◆ ◆ The heat treatment temperature after welding for quenched and tempered steel should be lower than the tempering temperature used in the quenching and tempering process. When welding pieces made of different steel grades together, the heat treatment specifications and temperatures must be set at higher levels as required; however, these temperatures should not exceed the lower critical point Ac1 of either steel grade ； ◆ ◆ When performing post-weld heat treatment on steels prone to reheat cracking, care should be taken to prevent reheat cracking ； ◆ ◆ The post-weld heat treatment for electroslag welds should follow the normalizing + tempering process specification ； ◆ ◆ Austenitic high-alloy steel pressure vessels generally do not undergo heat treatment for stress relief after welding ； ◆ ◆ Post-weld heat treatment should be carried out after patch welding and before the pressure test ； ◆ ◆ Overall heat treatment should be employed as much as possible ； ◆ ◆ During extracorporeal heat treatment, efforts should be made to ensure uniform temperature across the inner and outer walls as well as on both sides of the welds; at all times within the heating area, the temperature difference between any two measurement points should be below 50°C. When the thickness is greater than 10 mm, induction heating or resistance heating should be used ； ◆ ◆ When segmented heat treatment must be used, the length of the overlapping heating area should be at least 1500 mm. During patch welding and local heat treatment of circumferential seams, the width of the heating zone on each side of the weld shall not be less than 2 times the thickness of the container; whereas during heat treatment of the entire circumferential weld where the nozzle is welded to the container, the width of the heating zone shall not be less than 6 times the thickness of the shell. Measures should be taken to prevent harmful temperature gradients outside the heating zones for segmental and local heat treatment ； ◆ ◆ Temperature measurement during post-weld heat treatment must be accurate and reliable; automatic temperature recording should be employed, and the instruments, thermocouples, and their accessories used must have passed calibration verification. Temperature measurement points should be arranged reasonably; for example, in the case of pipeline heat treatment, these points should be placed symmetrically on both sides of the weld, with no fewer than two points. The temperature measurement points in horizontal pipes should be arranged symmetrically up and down ； ◆ ◆ After heat treatment, proper records and markings should be made, such as applying a stamp with the heat treatment operator’s code.