Welding residual stress is caused by uneven temperature distribution in the welded parts, thermal expansion and contraction of weld metal, etc., therefore, the associated welding construction will inevitably generate residual stress.
The most common method of eliminating residual stresses is high-temperature tempering, that is, heating the welded parts to a certain temperature and keeping them for a certain time in a heat treatment furnace.
By reducing the yield strength of the material at high temperature, plastic flow occurs in areas with high internal stress, the elastic deformation gradually decreases, and the plastic deformation gradually increases, thereby reducing the stress.
1. Selection of heat treatment method
The effect of post-weld heat treatment on the tensile strength and creep limit of metals is related to the temperature and retention time of the heat treatment. The impact resistance of weld metal after heat treatment varies with different types of steel.
Generally, single high-temperature tempering or normalizing plus high-temperature tempering is selected for post-welding heat treatment. For gas welding joints, high-temperature normalizing and tempering heat treatment is used because the grains in the weld and heat-affected zone of gas welding are coarse and need to be refined by normalizing.
However, a single normalizing treatment cannot eliminate residual stress, so high-temperature tempering is required to eliminate stress. Single intermediate tempering is only suitable for the assembly welding of common large low-carbon steel containers on the construction site, with the aim of partially eliminating residual stresses and removing hydrogen.
In most cases, a single high-temperature temper is selected. Heating and cooling during heat treatment should not be too fast, and the inner and outer walls should be heated evenly.
2. Heat treatment methods used for pressure vessels
There are two types of heat treatment methods used for pressure vessels: one is heat treatment to improve mechanical properties and the other is post-welding heat treatment (PWHT).
In general terms, PWHT is a heat treatment carried out on the welded area or welded components after welding the part.
Specific content includes stress relief annealing, full annealing, solid solution treatment, normalization, normalization plus tempering, tempering, low temperature stress relief, precipitation treatment, etc.
In a strict sense, PWHT only refers to stress-relieved annealing, which uniformly and sufficiently heats the welded area and related parts below the metal phase transition temperature in order to improve the performance of the welded area and eliminate harmful effects of welding residual stress, followed by uniform cooling.
In many cases, the heat treatment discussed for PWHT is essentially stress relief annealing after welding.
3. Objectives of Post-welding Heat Treatment
(1) Relax welding residual stress.
(2) Stabilize the shape and size of the structure, reduce distortion.
(3) Improve the performance of base metal and welded joints, including:
- The. Improve the plasticity of weld metal.
- B. Reduce the hardness of the heat-affected area.
- w. Improve fracture resistance.
- d. Improve fatigue resistance.
- It is. Restore or improve the reduced yield strength during cold forming.
(4) Improve stress corrosion resistance.
(5) Further release harmful gases, especially hydrogen, into the weld metal to prevent delayed cracking.
4. Assessment of the need for PWHT
Whether post-weld heat treatment is required for pressure vessels must be clearly specified in the design, and current pressure vessel design standards have requirements for this.
The welded area of a pressure vessel has significant residual stress and the adverse effects of residual stress only manifest themselves under certain conditions. When residual stress combines with hydrogen in the weld, it will cause hardening of the heat-affected zone, leading to the occurrence of cold cracking and delayed cracking.
Static stress existing in the weld or dynamic loading stress during operation combined with corrosion of the medium can cause stress corrosion cracking, known as SCC.
Welding residual stress and martensitic hardening caused by welding are important factors in the generation of stress corrosion cracking.
The research results showed that the main effect of deformation and residual stress on metallic materials is to transform uniform corrosion into localized corrosion, namely intergranular or transgranular corrosion. It is clear that both corrosion cracking and intergranular corrosion occur in media with certain characteristics for that specific metal.
In the presence of residual stress, depending on the different composition, concentration and temperature of the corrosive medium, as well as differences in the composition, structure, surface state and stress state between the base metal and the welded area, the nature of corrosion damage may change.
Whether post-welding heat treatment is required for welded pressure vessels should be determined by considering the purpose and size of the vessel (especially wall panel thickness), the performance of the materials used, and the working conditions. If any of the following situations occur, post-weld heat treatment should be considered:
- Vessels that work in adverse conditions, such as those at risk of brittle fracture at low temperatures, thick-walled vessels subjected to heavy and alternating loads.
- Welded pressure vessels with a thickness exceeding a certain limit, including boilers, petrochemical pressure vessels, etc., which have specific regulations and standards.
- Pressure vessels with high dimensional stability requirements.
- Vases made from steel with a high tendency to harden.
- Pressure vessels at risk of stress corrosion cracking.
- Other pressure vessels with dedicated regulations, specifications and drawings.
Residual stresses that reach the yield point are formed in the vicinity of the weld seam in welded steel pressure vessels. The generation of this stress is related to the transformation of the austenite-containing structure.
Many researchers have pointed out that a tempering process at 650°C can effectively eliminate the residual stress after welding steel welded pressure vessels.
At the same time, it is believed that without adequate post-weld heat treatment, a corrosion-resistant welded joint cannot be obtained.
It is generally believed that stress relief heat treatment refers to the process in which the welded part is heated to 500-650°C and then slowly cooled. The stress reduction is due to high-temperature creep, which begins at 450°C in carbon steel and 550°C in molybdenum-containing steel.
The higher the temperature, the easier it is to eliminate stress. However, once the original tempering temperature of the steel is exceeded, the strength of the steel will decrease. Therefore, it is necessary to control the temperature and time in stress relieving heat treatment.
However, in the internal stress of welding, tensile stress and compressive stress always coexist, and elastic stress and deformation exist simultaneously.
As the temperature of the steel increases, the yield strength decreases and the original elastic deformation becomes plastic deformation, resulting in stress relaxation.
The higher the heating temperature, the more completely internal stress can be eliminated. However, when the temperature is too high, the steel surface will be seriously oxidized.
In addition, for the PWHT temperature of quenched and tempered steels, the principle should be not to exceed the original tempering temperature of the steel, generally about 30 degrees lower than the original tempering temperature of the steel.
Otherwise, the material will lose its tempering effect and the strength and fracture toughness will decrease. This point should receive special attention from heat treatment workers.
The higher the post-welding heat treatment temperature for stress relief, the greater the degree of softening of the steel, generally heated to the steel's recrystallization temperature, and the internal stress can be eliminated. The recrystallization temperature is closely related to the melting temperature.
Generally, recrystallization temperature K = 0.4X melting temperature (K). The closer the heat treatment temperature is to the recrystallization temperature, the more effective the residual stress relief will be.
4. Consideration of the overall effectiveness of the PWHT
Post-weld heat treatment is not always advantageous. Generally, post-weld heat treatment is beneficial to mitigate residual stress and is only performed in cases where stringent requirements for stress corrosion cracking are necessary.
However, the impact strength test of the samples shows that post-welding heat treatment is detrimental to improving the toughness of the weld metal and the heat-affected zone, and intergranular cracks may sometimes occur within the grain coarsening range of the heat-affected area.
Furthermore, PWHT relies on decreasing material strength at high temperatures to achieve stress relief. Therefore, during PWHT, the structure may lose rigidity.
For structures adopting full or partial PWHT, the bearing capacity of the welded joint at high temperatures must be considered before heat treatment.
Therefore, when considering whether to carry out post-welding heat treatment, both the advantages and disadvantages of heat treatment must be comprehensively compared.
From a structural performance perspective, there are aspects that can improve performance and aspects that can reduce performance. Reasonable judgments must be made based on a comprehensive consideration of both aspects.
