The existing methods for relieving residual stress from welds are: mechanical, heat and electromagnetic.
The mechanical method may be performed by hammering or vibration. The heat method consist of heating the whole welded piece or each weld, one by one. The electromagnetic method uses the electromagnetic hammer technique.
In the heat treatment the part is heated until the yield point is reduced to less than the residual stress, which in turn causes local plastic distortion, decrease of the residual stress intensity and reduction of hardness.
The vibration method introduces energy into the part by means of vibrations. For the stressed atomic structure there is no difference between the energy introduced through heat and the energy introduced through vibrations. The applied energy reorganizes the crystalline structure, relieving stress and stabilizing the piece, without distortion.
This process is especially useful for stress relieving of big structures, for which the cost of treatment by electrical means would be high, and for parts with severe dimensional tolerances, in which heat treatment could cause distortions that would exceed them.
The vibrators generally used have a frequency band of 0 to 100 Hz. They are connected to the structure, which should be supported on rubber blocks. Frequency is gradually increased until the first resonance is reached. This resonance is maintained for a specific period of time and then the frequency is increased again until the second resonance is reached and so on.