6 Methods to Eliminate Welding Residual Stress

After the pressure vessel is welded, residual stress will be produced in the structural weld zone.

It is due to the fact that the internal stress caused by uneven heating temperature field during welding reaches the material yield limit, causing plastic deformation in local areas.

When the temperature returns to the original uniform state, the internal stress is still left in the structure, so it is called residual stress.

The peak value and distribution of welding residual stress have a direct negative impact on the fatigue failure and stress corrosion cracking of vessels.

Related reading: What Is Welding Stress?

The research shows that once the vessel is welded, the residual stress will inevitably accompany at the same time.

6 Methods to Eliminate Welding Residual Stress 1

Although its generation mechanism has been preliminarily understood, due to the difference of the external dimensions, welding processes, welding procedures and constraint size of the pressure vessel, the level of the residual stress is also very different, and the distribution is very complex.

Therefore, it is necessary to determine reasonable countermeasures to eliminate (or reduce) the welding residual stress, so as to ensure the quality is economically reasonable;

During service, it shall operate safely and avoid accidents.

Related reading: The Ultimate Guide to Welding

Overload method

Under controlled conditions, apply a slightly greater external load to the vessel once (or several times) than that under its working condition.

The stress formed by the load is superposed with the welding residual stress existing locally in the vessel.

When the resultant stress is lower than the material yield limit, it is in an elastic state, and the relationship between the stress and the stress is linear;

When the composite stress reaches the material yield limit, plastic deformation will occur in local areas.

With the increase of the external stress value, the range of the composite stress reaching the yield limit will increase, and the range of plastic deformation will also increase correspondingly, but the stress value will not increase (or not increase much).

Because the container itself is continuous, during the removal of external load, the yield deformation area and elastic deformation area recover in an elastic state at the same time, and the welding residual stress existing in the container is partially eliminated, and the magnitude of the eliminated residual stress is equal to the stress value generated by the external load.

Integral heat treatment

Heat the whole welded container to a temperature of 500 ℃~Ac1 at a certain heating rate, and keep it warm for a period of time to recrystallize the deformed metal to form new equiaxed grains.

All kinds of crystal defects are basically eliminated, the metal strength is reduced, and the toughness is improved, so that the welding residual stress is relaxed and released.

Pressure vessels are generally large in structure and cannot be put into the heating furnace for heat treatment like other small equipment or mechanical parts.

The external wall of the container can be covered with a thermal insulation layer, and the internal combustion method that injects fuel burning inside the container to form a high temperature for heating can also be used to conduct the overall heat treatment of the container by electric heating.

6 Methods to Eliminate Welding Residual Stress 2

Local heat treatment

Its principle is the same as that of overall heat treatment.

At present, infrared plate heaters or caterpillar resistance heaters are mostly used to heat the weld area.

Because of local heating, the effect of eliminating residual stress is not as good as that of overall heat treatment, which can only reduce the peak value of internal stress, so that the stress distribution is relatively gentle, but not fundamentally eliminated.

However, local heat treatment can improve the mechanical properties of welded joints, and the treatment objects are often limited to simpler welded joints.

Temperature difference stretching method

According to the distribution of residual stress in the weld zone, a reverse stress field is formed by using the thermal effect of temperature difference to eliminate the residual stress.

The key to eliminate the effect of this method lies in the selection of temperature difference Δt, which is related to material yield limit σs, modulus E and thermal expansion coefficient β.

As long as the heating zone and Δt are properly selected, it will not cause plastic deformation and loss of plastic reserves, nor affect the metallographic structure of the metal.

It can achieve better stress relief effect, which can reach 50%~70%.

This method has certain application value in plate and shell structures with regular welds and not too large thickness.

Hammer

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Under the rapid and uniform hammering, the weld metal will produce transverse plastic extension, so that the weld shrinkage can be compensated to some extent, the elastic strain of the tensile residual stress in this part can be relaxed, and the welding residual stress can be partially eliminated.

Explosive method

By detonating the explosive belt arranged at and near the weld seam, the interaction between the shock wave of the instantaneous explosion of the explosive in the weld seam area and the residual stress can make the metal produce an appropriate amount of plastic deformation, and the residual stress can be relaxed.

The explosive treatment can not only effectively eliminate the welding residual stress, but also produce a certain amount of compressive stress in the treatment area, so that the welded joint can improve the damage resistance related to tensile stress.

Therefore, heat treatment is powerless.

The explosion method is unique in eliminating the residual stress in the welding seam repair engineering during the inspection of the in-service pressure vessels.

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