The welding stresses introduced into the structures due to welding sometimes increase to higher values as the yield strength of the material..
The problem of welding stress was not taken seriously until the construction of large structures by fusion welding. However unwanted internal stresses may reduce the endurance limit of individual parts.
But these failures were not a spectacular one, even disastrous.
But many accidents in the past such as bridge collapse, ship collapse etc. attracted attention and research and studies started on it.
Undoubtedly, the welding community has been at the forefront of studying this phenomenon of high stress.
The stresses introduced into the structures due to welding sometimes increase to higher values as the yield strength of the material and when combined with the stresses from external service loads, the resulting (stress) values in the design May likely exceed stress. Besides, the heat-affected zone may be harder or more brittle than the base metal. These have the effect
1- To increase susceptibility to brittle fracture,
2- To increase susceptibility to stress-corrosion cracking,
To reduce 3- dimensional stability.
Similarly, these residual stresses are thought to cause deformation of the structure, especially if it is to be mechanized.
Removal of some surface metal may cause a redistribution of these induced stresses. As a result, some parts become out of shape, making it impossible to achieve the desired alignment.
Techniques for minimizing welding stress
welded structures such as press frames are subjected to vibrations to relieve residual stress.
In this method of stress-relieving, an oscillating or rotating wave generator is mechanically coupled to the part to be stress-relieved.
The welded structure is placed on a platform that vibrates and in turn, the welded structure vibrates at one of its natural(resonant)frequencies.
For such a system to be effective, it must produce plastic yielding in the region to stress relief.
If not all the welding stresses, the vibratory method can at least reduce the magnitude of peak stress. with this method, up to 25% of residual stresses may be relieved.
Even this reduces distortion in parts machined after welding and improves counteraction to brittle fracture also.
Since vibratory stress relief treatment does not change the metal surgical structure of welds or heat affected zone it does not alter mechanical properties lie the strength or toughness of the weldment.
2-Thermal treatment of welding stress
Thermal treatment proves to be a better option relatively because it improves weld strength by making changes in microstructure.
Thermal stress relief treatment requires the parent metal to heat uniformly up to an appropriate temperature (mostly in a furnace), at which temperature it is held for a set time and then followed by a uniform cooling process.
See Detail about Thermal treatment
The first use of peening in 1926, peening became an accepted process until 1950. The process of elongating the outer fibres of the weld with the help of a mechanical hammer is called peening. Instead of the hand, peening can be better controlled if a pneumatic chisel with a blunt round edge is used. Reduces internal stress of any very low intensity affected by heating below the critical point.
Excessive peening should be avoided, as it can also cause hardening of the weld metal, bending and cracking of the weld.
4-overstressing technique for welding stress relief
This method can be used for thin welds and ductile steel. All are necessary to bring the applied stresses above the material’s yield stress. Sometimes it is not necessary to load that much. In a structure where the internal stresses are high, it is only necessary to apply such a load, as the aggregate will bring about to relieve itself without deformation.
5-Thermo-mechanical stress relief-treatment
In addition, welding stress relief is also done by the thermomechanical method. The purpose of this technique is to use thermal expansion so that mechanical forces are provided to counter another residual stress system and thereby cancel the original already set-up due to welding.
To be continued…