1. The conditions of use are harsh, such as thick-walle […]
1. The conditions of use are harsh, such as thick-walled vessels that are in danger of brittle fracture when working at low temperatures, and vessels that bear larger loads and alternating loads.
2. Welded pressure vessels whose thickness exceeds 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 with stress corrosion cracking risk.
6. Other pressure vessels with special regulations, specifications and drawings.
In steel welded pressure vessels, a residual stress that reaches the yield point is formed in the area 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, 650 degrees tempering can have a good effect on steel welded pressure vessels.
At the same time, it is believed that if proper heat treatment is not performed after welding, corrosion-resistant welded joints will never be obtained.
It is generally believed that the stress relief heat treatment is a process in which the welding workpiece is heated to 500-650 degrees and then slowly cooled. The decrease in stress is caused by creep at high temperature, which starts at 450 degrees in carbon steel and 550 degrees in molybdenum-containing steel.
The higher the temperature, the easier it is to eliminate the stress. However, once the original tempering temperature of the steel is exceeded, the strength of the steel will decrease. Therefore, the heat treatment for stress relief must master the two elements of temperature and time, and neither of them is indispensable. However, the internal stress of the weldment is always accompanied by tensile stress and compressive stress, 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 fully the internal stress is eliminated. But when the temperature is too high, the surface of the steel will be severely oxidized. In addition, the PWHT temperature of quenched and tempered steel should not exceed the original tempering temperature of the steel as a principle, 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 heat treatment workers.
The higher the post-weld heat treatment temperature to eliminate internal stress, the greater the softening of the steel. Generally, 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=0.4X the melting temperature (K). The closer the heat treatment temperature is to the recrystallization temperature, the more effective it is to eliminate residual stress.
Consideration of the comprehensive effect of PWHT
Post-weld heat treatment is not absolutely advantageous. In general, post-weld heat treatment is helpful to alleviate residual stress, and only when there are strict requirements for stress corrosion. However, the impact toughness test of the specimen shows that the post-weld heat treatment is unfavorable to the toughness improvement of the deposited metal and the welding heat-affected zone, and sometimes intergranular cracking may occur in the grain coarsening range of the welding heat-affected zone.
Furthermore, PWHT relies on the reduction of material strength at high temperatures to achieve stress relief. Therefore, during PWHT, the structure may lose rigidity. For structures that adopt overall or partial PWHT, the weldment at high temperature must be considered before heat treatment. Supporting capacity.
Therefore, when considering whether to perform post-weld heat treatment, the advantages and disadvantages of heat treatment should be comprehensively compared. From the structural performance point of view, there is a side to improve performance, but also a side to reduce performance. Reasonable judgments should be made on the basis of comprehensive consideration of the two aspects.