Vibratory Stress Relief (VSR)
VSR is a relatively simple method that induces high-amplitude-harmonic or sub–harmonic vibrations inside a material. All metals have a natural harmonic frequency. Vibratory stress relief relies on this fact, and it affects a material similar to that of a hammer blow - sending a wave of vibrational energy into the crystal lattice until all the energy is dissipated through internal friction.
The difference is the source of vibration. Vibratory stress relief typically employs an electrical motor to supply a constant source of fixed- (or variable-) energy vibrations. Enough energy results from these vibrations for the lattice dislocations to reposition themselves and return to a lower stress state. In this way the residual stresses are reduced in the material.
Vibratory and thermal stress relief is similar in that both processes rely on internal accelerated motion to accomplish stress relief and both processes require a soak or dwell period. A key difference, however, is that if the setup or vibratory conditions are not proper or change over time, vibratory stress relief may produce no benefit at all.
In order to decrease residual stresses caused by extensive welding operations, bend straightening can be used. It is typically performed using hydraulic pressure in which the component part is fixtured in such a way as to allow it to be bent in four directions – two bends in the horizontal plane, two in the vertical plane. An example of a component for which this technology might be used is an axle housing.
Stress relief is typically performed at temperatures lower than post-weld heat treatment (PWHT). The purpose is to reduce residual stresses from fabrication, exclusive of welding. Post-weld heat treatment is a higher-temperature thermal treatment to reduce residual stresses from welding and to temper or soften the heat-affected zone of the base metal.