MEASURING THE EXTENT OF STRESS RELIEF
Until recently there was no reliable method for the precise measurement
of residual stress, that not only originated from welding, but also
from forging, cold drawing and other types of metal working. Now, with
the use of diffractometry with x-rays, the problem has been solved.
In the past, the only way of checking if residual stress had been
reduced to an acceptable level was by analogy with hardness. It is a
well known fact that materials get harder when submitted to stress.
Experience acquired over the years, upon which the applicable standards
are still based, demonstrated that if the hardness measured after
stress relief had been performed was lower than a given empiric value,
the treatment had been successful.
This
condition was especially important if the weld was to be in contact
with corrosive environments, as is the case of the chemical industry.
It is well known that, depending on the environment, metals show less
corrosion resistance if submitted to residual stress.
This fact is taken into account in ASME/ANSI B 31 Code for Pressure
Piping. Section B 31.3, which is used in process industries such as oil
refineries and chemical plants, states the maximum allowable hardness
in welds after stress relief. This requirement does not exist in
Section B 31.1, which is used in power plants, where the possibility of
corrosion is much less.
Until very recently,
diffractometry by x-rays presented a serious operational problem. The
equipment was too large to be taken to the point of use, and in many
cases the weld could not be taken to the equipment. This situation has
been overcome by the development of equipment small enough to be moved
from one place to another. The applicable codes, however, continue to
consider Hardness as the parameter ruling the approval of stress
relief, and this is why we have adopted it in the Experimental Section.
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