By: Weite Wu, Distinguished Professor, Dept. of Materials Sci. and Eng.,
National Chung University, Taiwan
Excessive Residual stress is an invisible killer for
materials. In some serious cases, such as workpiece fracture, pressure vessel
or chemical tank pitting corrosion will cause significant damage to public
security or properties. In slight cases, it will cause workpieces deformation
and also result in the loss of structural precision. Therefore, residual stress
is an important issue that cannot be ignored in industrialized society.
Manufacturing and fabricating processes —turning,
forging, welding, and heat treatment — introduce massive residual stress into
the metal body. Especially, welding is the greatest of all metalworkings,
because the residual stress contains solidification shrinkage stress, thermal
stress caused by temperature gradient, phase transformation stress, as well as
plastic stress.
Therefore, welders know that it is necessary to knock
the weld bead after welding, not only for removing welding slag but also
relieving the massive residual stress caused by welding. But is knocking
effective? How much force is needed to apply? When to do it? I believe those questions
have always appeared in welders mind.
You may have heard about the experience of our
ancestors. When things do not hurry to be used, it should be placed outside to
be worn and torn of the weather and absorb the essence of nature (exposure
method, currently used in steel plants). Besides, there are also an action of
striking trees while sword processing. Both are conducting stress relief, the
former use thermal cycling, and the later use force cycling. Even the ancestors
knew such inefficient ways to relieve stress, how can we modern people don’t
know and doubt this intelligence method?
The reason is that you can’t see and feel the residual
stress, but when you apply a force to the material and let the internal stress
over its elastic limit, the material will vent the excessive stress by stress
relief, this will lead to two results, one is material deformation, and the
other is material failure. The results of both will cause problems when using
the workpiece. After deformation, the object will lose the accuracy of original
design. In slight cases, it will need to be re-processed, more seriously, to be
scrapped and redo. However, both of them won’t jeopardize industrial safety.
But when the internal stress comes large, coupled with
the external force, it may cause the material to fracture. The tragedy may
happen in anytime, and this is what we have been always cautious about.
Therefore, if you can eliminate the excessive residual stress before using, it
is like a talisman that can protect the object. Hence, is the engineering you
do safe? Have you done residual stress relieved? If not, then you are a
dangerous construction project supervisor, or even you have no notion of stress
relief, then you are not fit to hold this important engineering contract.
Stress relief
Stress relief of metals can be divided into two
categories, one is heat treatment, and another is adding mechanical force. This
can be simply explained by hot bath, and massage. When a person get nervous,
after taking a hot bath, he will feel relax and a little bit sluggish, this is
the phenomenon of releasing stress. But in this moment, you won’t want to work.
Sleeping is better.
In addition, some people may choose massage to
eliminate stress, during the course with a proper strength, it will make you
feel a little pain, but not too hurt. After the toss, with a refreshing body,
you can have a fully vitality while working. However, what is the difference
between the two? It depends on how much you eliminate the stress. When
releasing too much stress, it will make you feel lazy and weak, while with a
moderate stress relief, it is like taking a good rest and having an energy
drink.
This is same to the metals, the heat treatment can
remove most of the residual stress inside the metal, but it will also reduce
the strength or hardness of the metal. Some latest reported shows that
alternating compressive and tensile stresses can release high stress
concentration area, which means that by applying a fluctuating force (e.g.
vibration wave) to make the material vibrate in a very high frequency, not only
can avoid being damaged by residual stresses but can also maintain the original
physical and chemical properties of metal.
Currently, the methods of residual stress relief are
only heating or using mechanical force. Heat method is the most commonly used,
but not every objects are suitable. For example, after heating the material may
deteriorate (e.g. stainless steel will get sensitized and begin to corrosion
after stress-relief annealing, as shown in figure 3), become soft (e.g.
martensite in hardened steel will disappear and decrease in hardness), have
great deformation (after stress-relief annealing, the workpiece will deform
because of mass stress relieving), get oxidized easily (due to the material
easy to catch oxygen in the high temperature), or the workpiece is too large,
as show in figure 4.
In addition, the heat treatment equipment is expensive
(especially with a vacuum or gas protective function), need a long-time
processing (about four times more than the vibration), treatment space is
limited (furnace cannot be brought to the construction site), etc. Then the
vibratory stress relief (VSR) is the best option for these cases.
Appling a proper
vibration can get a significant result of stress relief, if not, not only can’t
release the residual stress effectively, but do harm to the materials or
workpieces. The most common damage is fatigue failure, which will reduce the
using span. When using vibration method, you have to know specific frequency
and vibration mode for all kinds of materials or workpieces, just like everyone
has their special acupuncture point in their body, only by using the right
massage methods and strength can you get completely relax.
Such vibration mode of stress relief is the best in
the world and has the U.S. patent filed. Currently, the market is full of VSR
methods without theoretical basis, which is not only ineffective but doing harm
to workpieces. But how to judge it is a swindler or a professional masseur?
It’s very simple: just directly using equipment to measure the stress. If you
have any needs or problems, National Chung Hsing University vibratory stress
relief team is willing to provide such assistance.
Vibratory stress relief technique applied to stainless steel component
Vibratory stress
relief technique applied to water pipe
