Heat treatment of pressure vessels
1. Pressure vessels will cause the following problems during the manufacturing process:
Cold work hardening due to excessive cold rolling and cold correction. Changes in microstructure and properties of welds caused by welding. Due to welding residual stress and the resulting stress corrosion cracking and development. When pressure vessel is welded, a sharp temperature gradient with a temperature difference of more than 100 degrees is generated in the adjacent region of the base metal, resulting in an inhomogeneous plastic strain in ferritic steel or other similar materials, and in the subsequent cooling process, a residual stress field with a peak stress reaching the yield point will be generated. In addition, because of the inhomogeneous plastic strain in pressure vessel manufacturing, the residual strain can be produced in elastic-plastic materials, and the residual strain can be from mechanical (mainly cold coil, cold rectification and other cold processing) thermal (mainly from the welding process), or both reasons, that is, the original thermal machinery. Because of. Therefore, the residual elastic strain field and the corresponding elastic residual stress will be left in the final product processed by the pressure vessel. The existence of residual stress will affect the service performance of pressure vessels. In order to eliminate the peak strain in the welding zone and achieve uniform distribution of internal strain, many methods can be adopted, such as mechanical vibration method, post-weld heating method and so on. However, because many potential problems in pressure vessels are mainly caused by metallurgical damage in the weld zone, mechanical methods to reduce internal strain are not enough to prevent many problems that may occur in the future operation process. In addition, the hydrogen embrittlement of metals has attracted much attention. After the hydrogen enters the steel, the mechanical properties will deteriorate obviously. The strength and plasticity of the steel decrease obviously. The hydrogen dissolved in the metal lattice causes brittle failure of the steel during slow deformation. Hydrogen in metal materials can be absorbed during the production process of metal materials, such as the hydrogen absorbed by liquid metal during welding is retained in the weld, or the hydrogen absorbed by the material in the hydrogen environment. For the hydrogen absorbed in the weld, the more effective method is to carry out post-weld heat treatment, which can relax and relax the welding residual stress, improve the weld heat affected zone hardened and embrittled by welding, improve the ductility and fracture toughness of the weld metal, and also make the welding zone and the vicinity of hydrogen and other harmful gases. Diffusion escapes. There are two kinds of heat treatment methods for pressure vessels: one is heat treatment to improve mechanical properties, the other is post-weld heat treatment (PWHT). Generally speaking, post-weld heat treatment is the heat treatment of the welded area or welded components after the workpiece is welded. It includes stress relief annealing, complete annealing, solid fusion, normalizing, normalizing plus tempering, tempering, low temperature stress relief, precipitation heat treatment and so on. In a narrow sense, post-weld heat treatment only refers to the process of eliminating stress annealing, that is, in order to improve the performance of the welding zone and eliminate harmful effects such as welding residual stress, so that the welding zone and related parts are uniformly and adequately heated below the temperature point of metal phase transition 2, and then cooled uniformly. In many cases, post weld heat treatment is essentially post weld stress relief heat treatment.
Two. Post weld heat treatment (PWHT):
1. relaxation welding is involved in stress.
2. stabilize the shape and size of the structure to reduce distortion.
3. improve the properties of base metal and welding zone, including A. to improve the plasticity of weld metal. B. reduces heat affected zone hardness. C. improves fracture toughness. D. improves fatigue strength. E. can restore or improve the yield strength in cold forming.
4. improve the ability to resist stress corrosion.
5. further release harmful gas in weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.
Three. The judgement of the necessity of PWHT:
Whether there is a need for post weld heat treatment of pressure vessel should be clearly defined in the design, which is required by the current design code of pressure vessel. There is a large residual stress in the welding zone of the welded pressure vessel, and the adverse effect of the residual stress appears only under certain conditions. When the residual stress is combined with hydrogen in the weld, the HAZ will harden and lead to cold cracks and delayed cracks. When the residual static stress in the weld or the dynamic load stress in the load operation is combined with the corrosion action of the medium, it is possible to cause crack corrosion, that is, stress corrosion. Welding residual stress and harden hardening caused by welding are important factors for producing stress corrosion cracking. The results show that the main effect of deformation and residual stress on metal materials is to change the metal from uniform corrosion to local corrosion, i. e. to intergranular or transgranular corrosion. Of course, corrosion cracking and intergranular corrosion of metals occur in the medium with certain characteristics of the metal. In the case of residual stress, the properties of corrosion damage may be changed according to the composition, concentration and temperature of the corrosive medium, the composition, microstructure, surface state and stress state of the base metal and the welding zone. Whether the welded pressure vessel needs post-weld heat treatment or not should be decided by the use, dimension of the vessel (especially the thickness of the wall plate), the performance of the material used and the working conditions. Welding heat treatment should be considered in one of the following situations:
1. The service conditions are harsh, such as the thick-walled vessels which are in danger of brittle fracture when working at low temperature, and the vessels which bear large loads and alternating loads.
2. welding pressure vessel with a thickness exceeding a certain limit. It includes special regulations and specifications for boilers, petrochemical pressure vessels, etc.
3. pressure vessels with high dimensional stability.
4. containers made of hardened steel.
5. pressure vessels with stress corrosion cracking risk.
6. other pressure vessels with special regulations, specifications and drawings. In steel welded pressure vessels, residual stress at yield point is formed near the weld area. This stress is related to the transformation of the structure with austenite. Many researchers have pointed out that 650 degree tempering has a good effect on steel welded pressure vessels in order to eliminate the residual stress after welding. At the same time, it is believed that if proper heat treatment is not carried out after welding, the corrosion-resistant welded joint can never be obtained. It is generally believed that stress relief heat treatment is a process in which the welded workpiece is heated to 500-650 degrees and then slowly cooled. The stress reduction is due to creep at high temperatures, starting at 450 degrees in carbon steels and starting at 550 degrees in molybdenum-containing steels. The higher the temperature, the easier the stress is eliminated. But once the original tempering temperature exceeds the steel, the strength of the steel will be reduced. Therefore, the elimination of stress heat treatment must grasp the temperature and time two elements, indispensable. However, tensile stress and compressive stress are always accompanied in the internal stress of weldments, and stress and elastic deformation exist simultaneously. When the temperature of steel increases, the yield strength decreases, and the original elastic deformation becomes plastic deformation, which is stress relaxation. The higher the heating temperature, the more sufficient the internal stress is eliminated. But when the temperature is too high, the steel surface will be severely oxidized. In addition, the PWHT temperature of quenched and tempered steel should not exceed the original tempering temperature of steel as the principle, generally lower than the original tempering temperature of steel about 30 degrees, otherwise the material will lose the quenching and tempering effect, strength and fracture toughness will be reduced. This is a special concern for heat treatment workers. The higher the post-weld heat treatment temperature, the greater the softening degree of the steel. Usually, the internal stress can be eliminated by heating to the recrystallization temperature of the steel. The recrystallization temperature is closely related to the melting temperature. Generally, the recrystallization temperature is K=0.4X melting temperature (K). The closer the heat treatment temperature is to recrystallization temperature, the more effective it is to eliminate residual stress.
Four. Considering the comprehensive effect of PWHT, post weld heat treatment is not absolutely beneficial.
In general, post-weld heat treatment is conducive to relieve residual stress, and stress corrosion has a strict requirement for the case. However, the impact toughness test shows that post-weld heat treatment is not conducive to the toughness improvement of deposited metal and HAZ, and sometimes intergranular cracking may occur within the grain coarsening range of HAZ. Furthermore, PWHT relies on the reduction of material strength at high temperature to eliminate stress. Therefore, the structure may lose rigidity in PWHT. For the structure with whole or part PWHT, the supporting capacity of weldments at high temperature must be considered before heat treatment. Therefore, the advantages and disadvantages of heat treatment should be compared comprehensively when considering whether to carry out post weld heat treatment. From the point of view of structural performance, there are some aspects that can improve the performance, but also can reduce the performance. We should make a reasonable judgment on the basis of comprehensive consideration of the two aspects of basic work.