The welding residual stress is caused by the uneven temperature distribution of the weldment caused by welding, the thermal expansion and contraction of the weld metal, etc., so the residual stress will inevitably be generated along with the welding construction.
The most common method to eliminate residual stress is high temperature tempering, that is, the weldment is heated to a certain temperature and held for a certain period of time in a heat treatment furnace, and the yield limit of the material is reduced at high temperature to generate plastic flow in the place with high internal stress. The elastic deformation gradually decreases, and the plastic deformation gradually increases to reduce the stress.
01 Choice of heat treatment method
The effect of post-weld heat treatment on the tensile strength and creep limit of the metal is related to the temperature and holding time of the heat treatment. The effect of post-weld heat treatment on the impact toughness of weld metal varies with different steel grades.
The post-weld heat treatment generally adopts single high temperature tempering or normalizing and high temperature tempering. For gas welding joints, normalizing and high temperature tempering are used. This is because the grains of the gas welding seam and the heat-affected zone are coarse, and the grains need to be refined, so normalizing is used.
However, single normalizing cannot eliminate residual stress, so high temperature tempering is required to eliminate stress. A single medium temperature tempering is only suitable for the assembly and welding of large ordinary low carbon steel containers assembled on site, and its purpose is to partially eliminate residual stress and remove hydrogen.
In most cases, a single high temperature tempering is used. The heating and cooling of the heat treatment should not be too fast, and the inner and outer walls should be uniform.
02 Heat treatment methods for pressure vessels
There are two types of heat treatment methods used in pressure vessels: one is heat treatment to improve mechanical properties; the other is post-weld heat treatment (PWHT). Broadly speaking, post-weld heat treatment is the heat treatment of the welded area or welded components after the workpiece is welded.
The specific contents include stress relief annealing, complete annealing, solid melting, normalizing, normalizing and tempering, tempering, low temperature stress relief, precipitation heat treatment, etc.
In a narrow sense, post-weld heat treatment only refers to stress relief annealing, that is, in order to improve the performance of the welding zone and eliminate the harmful effects of welding residual stress, so as to uniformly and fully heat the welding zone and related parts below the temperature point of metal phase transition 2. , and then the process of uniform cooling. In many cases the post-weld heat treatment discussed is essentially post-weld stress relief heat treatment.
03 Purpose of post-weld heat treatment
1. Relax welding residual stress.
2. Stabilize the shape and size of the structure and reduce distortion.
3. Improve the performance of base metal and welded joints, including:
a. Improve the plasticity of the weld metal.
b. Reduce the hardness of the heat affected zone.
c. Improve fracture toughness.
d. Improve fatigue strength.
e. Restore or increase the yield strength reduced in cold forming.
4. Improve the ability to resist stress corrosion.
5. Further release the harmful gases, especially hydrogen, in the weld metal to prevent the occurrence of delayed cracks.
04 Judgment of the necessity of PWHT
The necessity of post-weld heat treatment for pressure vessels should be clearly specified in the design, and the current design specifications for pressure vessels require this.
In the welded pressure vessel, there is a large residual stress in the welding area, and the residual stress has an adverse effect. manifested under certain conditions. When the residual stress combines with the hydrogen in the weld, it will promote the hardening of the heat-affected zone, resulting in the development of cold 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 effect of the medium, it may cause crack-like corrosion, that is, the so-called stress corrosion. Welding residual stress and the hardening of the base metal caused by welding are important factors for stress corrosion cracking.
The research results show that the main effect of deformation and residual stress on metal materials is to change the metal from uniform corrosion to localized corrosion, that is, to intergranular or transgranular corrosion. Of course, both corrosion cracking and intergranular corrosion of metals occur in media that have certain properties for that metal.
In the presence of residual stress, it varies according to the composition, concentration and temperature of the aggressive medium, as well as the difference in the composition, structure, surface state, stress state, etc. of the base metal and the weld zone, so that the corrosion The nature of the damage may change.
Whether the welded pressure vessel needs to be subjected to post-weld heat treatment should be determined comprehensively from the application, size of the vessel (special edge is the thickness of the wall plate), the properties of the materials used and the working conditions. In one of the following situations, post-weld heat treatment should be considered:
1. The use conditions are harsh, such as thick-walled containers that are at risk of brittle fracture when working at low temperatures, and containers that bear large loads and alternating loads.
2. Welded pressure vessels with thickness exceeding a certain limit. Including boilers, petrochemical pressure vessels, etc. with special regulations and specifications.
3. For pressure vessels with high dimensional stability.
4. Containers made of steel with a high tendency to harden.
5. Pressure vessels at risk of stress corrosion cracking.
6. Other pressure vessels specified by special regulations, specifications and drawings.
In steel welded pressure vessels, residual stresses up to the yield point develop in the region close to the weld. The generation of this stress is related to the transformation of the structure mixed with austenite. Many researchers pointed out that in order to eliminate the residual stress after welding, the tempering of 650 degrees can have a good effect on the steel welded pressure vessel.
At the same time, it is believed that without proper heat treatment after welding, corrosion-resistant welded joints cannot always be obtained.
It is generally believed that stress relief heat treatment is a process in which the welding workpiece is heated to 500-650 degrees and then slowly cooled. The reduction in stress is due to creep at high temperature, starting from 450 degrees in carbon steels and from 550 degrees in molybdenum-containing steels.
The higher the temperature, the easier it is to relieve the stress. But once the original tempering temperature of the steel is exceeded, the strength of the steel will decrease. Therefore, the heat treatment of stress relief must master the two elements of temperature and time, which are indispensable. However, in the internal stress of the weldment, tensile stress and compressive stress are always accompanied, and stress and elastic deformation exist at the same time. When the temperature of the 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 relieved. However, when the temperature is too high, the surface of the steel will be seriously oxidized. In addition, for the PWHT temperature of quenched and tempered steel, the principle should not exceed the original tempering temperature of the steel, which is generally about 30 degrees lower than the original tempering temperature of the steel, otherwise the material will lose the quenching and tempering effect, and the strength and fracture toughness will be reduced. reduce. This point should be paid special attention to for heat treatment workers.
The higher the post-weld heat treatment temperature for eliminating internal stress, the greater the degree of softening 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 K=0.4×melting temperature (K). The closer the heat treatment temperature is to the recrystallization temperature, the more effective it is to eliminate residual stress.
05 Consideration of the comprehensive effect of PWHT
Post-weld heat treatment is not absolutely advantageous. In general, post-weld heat treatment is beneficial to relieve residual stress, and it is only carried out when there are strict requirements for stress corrosion. However, the impact toughness test of the specimen shows that the post-weld heat treatment is not good for the toughness of the deposited metal and the welding heat-affected zone, and sometimes intergranular cracking may occur within the grain coarsening range of the welding heat-affected zone.
Furthermore, PWHT relies on the reduction of material strength at high temperature to achieve stress relief. Therefore, during PWHT, the structure may lose rigidity. For the structure of overall or partial PWHT, the weldment at high temperature must be considered before heat treatment. bearing capacity.
Therefore, when considering whether to carry out post-weld heat treatment, the advantages and disadvantages of heat treatment should be comprehensively compared. From the perspective of structural performance, there are aspects of improving performance and reducing performance. Reasonable judgments should be made on the basis of comprehensive consideration of the two aspects.
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