CONCLUSIONS
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.
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