After the pressure vessel is welded, a residual stress is produced in the structural weld zone. This is because the uneven heating temperature field during welding causes the internal stress to reach the yield point of the material, leading to plastic deformation in local areas. Even when the temperature returns to the original uniform state, internal stress remains in the structure, hence the term “residual stress”.
The peak value and distribution of welding residual stress have a direct negative impact on fatigue failure and stress corrosion cracking of vessels.
Related reading: What is welding voltage?
Research shows that once a vessel is welded, residual stress will inevitably follow it.
Although the mechanism of generating residual stress in pressure vessels has been preliminarily understood, the level of residual stress varies greatly due to differences in external dimensions, welding processes, welding procedures, and restriction size. Furthermore, the residual stress distribution can be very complex.
Therefore, it is necessary to develop reasonable countermeasures to eliminate or reduce welding residual stress in order to ensure economically reasonable quality and safe operation during service, thereby avoiding accidents.
Related Reading: The Ultimate Guide to Welding
Overload method
Under controlled conditions, apply a slightly greater external load to the vessel, one or several times, than under its working condition.
The voltage formed by the load is superimposed on the welding residual stress existing locally in the vessel.
When the resulting stress is less than the yield strength of the material, the material is in an elastic state and the relationship between stress and strain is linear.
When the compound stress reaches the yield point of the material, plastic deformation occurs in local areas.
As the value of external stress increases, the range of composite stress that reaches the yield point also increases, and the range of plastic deformation increases correspondingly, but the stress value does not increase or increases only slightly.
As the container itself is continuous, during the removal of the external load, the yield deformation area and the elastic deformation area simultaneously recover to an elastic state. The welding residual stress existing in the container is partially eliminated, and the magnitude of the eliminated residual stress is equal to the value of the voltage generated by the external load.
Full heat treatment
The entire welded container must be heated to a temperature of 500°C~Ac1 at a specific heating rate and held at that temperature for a period of time to allow recrystallization of the deformed metal, resulting in the formation of new equiaxed grains.
This process eliminates all types of crystal defects, reduces metal strength and improves toughness, which in turn relaxes and releases residual stress from welding.
Because pressure vessels are typically large, they cannot be heat treated in a furnace like smaller equipment or mechanical parts.
To resolve this, the outer wall of the container can be covered with a layer of thermal insulation or, alternatively, the container can be heat treated using electrical heating or injecting fuel to create a high temperature inside the container.
Local heat treatment
The principle of local heat treatment is similar to that of general heat treatment. Currently, the welding area is heated mainly with infrared plate heaters or Caterpillar resistance heaters.
Due to localized heating, the elimination of residual stress is not as effective as global heat treatment. Local heat treatment can only reduce the maximum value of internal stress, resulting in relatively smooth stress distribution but not complete stress elimination.
However, local heat treatment can improve the mechanical properties of welded joints. It should be noted that this treatment is generally limited to simpler welded joints.
Temperature difference stretching method
The thermal effect of the temperature difference can be used to eliminate residual stress in the weld zone, forming a reverse voltage field.
The key to the success of this method lies in the selection of the appropriate temperature difference Δt, which depends on the material's yield strength σs, modulus E and coefficient of thermal expansion β.
By selecting the appropriate heating zone and Δt, this method can achieve effective stress relief without causing plastic deformation or loss of plastic reserves, or affecting the metallographic structure of the metal.
The stress-relieving effect can be significant, ranging from 50% to 70%.
This method is particularly useful for plate and shell structures with regular welds and moderate thickness.
Hammer
After rapid and uniform hammering, the weld metal will undergo transverse plastic extension, which can, to some extent, compensate for the weld shrinkage. Furthermore, the elastic deformation caused by the tensile residual stress in this area can be relaxed, partially eliminating the welding residual stress.
Explosive method
When detonating the explosive belt arranged at or near the weld seam, the shock wave of the instantaneous explosion interacts with the residual stress in the metal, resulting in an appropriate amount of plastic deformation and relaxation of the residual stress in the weld seam area.
Explosive treatment not only effectively eliminates the residual stress of welding, but also generates a certain amount of compressive stress in the treatment area, thereby improving the damage resistance of the welded joint under tensile stress.
Therefore, heat treatment is ineffective in achieving this result.
The explosion method is unique in eliminating residual stresses in welding seam repair engineering during in-service pressure vessel inspection.
