First, it is noted that while the specimens that had been heat treated showed a decrease in tensile strength and an increase in elongation, as was to be expected, the vibration method practically does not alter those values, for it does not temper, normalize or anneal nor does it modify the mechanical properties of the material.
Second, the energies absorbed in the impact test are practically the same. None of the specimens broke, indicating that the welded material is ductile because it has a considerable amount of ferrite, as can be observed in the micrographs. The decrease in hardness resulting from both treatments was similar, indicating an effective reduction in residual stress.
Stress Relief Engineering Co.: Resonant vibration method for reducing residual stresses in welded or machined fabrications.
The conclusion that could be reached would be that heat and vibration methods are equivalent for practical purposes. The authors agree, however, that a test conducted with only one sample piece treated with each method is not sufficiently representative to reach valid conclusions. Nonetheless, the authors believe that the preliminary conclusion of this paper can encourage other researchers to seriously consider further investigation of the vibration method of stress relief.
The fact that the American standards, widely used not only in the U.S.A. but also in many other countries, do not formally accept vibratory stress relief, raised certain doubts regarding this method. However, the method could be used with a specific piece of material, not designed and/or manufactured according to those standards. It is the case, for example, of the supporting
structure of paper machines, called Beloitmachine in the paper industry slang. As the structure makes part of the machine itself, the welds do not need to follow standard AWS D.1, as would be the case of the structural steel of a football stadium. Consequently, the machine manufacturer is free to choose the stress relief method most convenient for him, provided that the final quality of the product is maintained. In the case of the Beloit machines, the reason for the choice is clear: the machine must be installed within severe level tolerances and stress relief by heat could produce distortions that, even if very small, would exceed the mentioned tolerances. Relief by vibration, on the other hand, does not introduce dimensional changes in the parts.
In some cases, stress relief by vibration may be used as a more convenient option to heat treatment, as for example when the piece is too big to be transported and placed in a furnace. In these cases, the possibility of carrying out the stress relief on site with no need to move the piece is a clear advantage. An example would be a petrochemical tower.
On the other hand, pieces in big quantities and easy to handle, such as prefabricated piping sections, may be conveniently heat treated in batches in a furnace.