Vibratory Stress Relief 2

[coolbreeze]

After reading an earlier thread on vibratory stress relief (VSR), I am intrigued about the application as it applies to cold worked low-alloy steel pipe. Meta-Lax is one company that has applied VSR with repeated commercial success. Their applications are specific, that is for relieving thermally stressed parts and they stop short of recommending this as a method for relieving residual stresses induced from cold working. Does anyone know of a VSR process that is proven to reduce residual stresses from cold working?

 

[metengr]

Cold work results in the movement or slip of crystal lattice imperfections that create dislocations in metals. Cold working increases the density of dislocations, which ultimately reduces available slip systems, and results in the strain hardening of metals. During this process, the crystal lattice of a metal becomes strained as a result of added dislocations.

To reduce the dislocation density and crystal lattice strains requires external energy to overcome the initial energy absorbed by the metal from cold work. Think of this concept as a typical thermal barrier or roller coaster physics problem - it takes a certain amount of external energy applied to a system to overcome the existing energy barrier. Once the external energy exceeds this energy barrier, dislocations are then free to move and combine, thereby reducing the total strain energy of the metal and crystal lattice strains.

So, to answer your question, I do not see a vibratory stress relief as a means of providing the necessary external energy to reduce dislocation density and crystal lattice strains from cold work.

The vibratory stress relief probably works on strains that are elastic in nature. In this case, slip or permanent yielding has not occurred, and elastic strains in the crystal lattice could probably be redistributed over time from vibratory stresses (a sort of mechanical stress relaxation) because of the inherent relationship between elastic stress and strain with Hooke's Law.

 

[mcguire]

Not necessarily. Vibration simply works to reduce residual stress by localized plastic deformation. If the amplitude is of the elastic wave is sufficient when added to the residual stress to induce yielding, the material deforms to accomodate the residual stress and the residual stress diminishes, even though softening has not occurred.
An analogous way to relieve residual stress is to stress the entire body over the yield point. Sheet metal is stretched to flatten it and relieve residual stress without any removal of dislocations and without removing cold work.

 

[bklauba]

As a manufacturer of the equipment that performs the VSR Process, and developer and frequent practicioner of the process, I would like to offer the conclusions of our experience:

First, we do see a change in the resonance patterns (growth of resonance peaks being the stronger, shift to lower frequency being the weaker) of cold-worked pipe products. But we have had a mixed level or success in solving problems that the material suffers, such as dimensional instability. In almost all situations, there is improvement, but at times it is marginal.

Second, the vast majority of applications of the VSR Process remain large welded structures, castings and hot-rolled or formed products. Within these categories (and it would appear virtually independent of chemistry), the VSR Process can repeatedly and predictably render a wide range of components dimensionally stable and very well behaved during machining, assembly, field use. Often such parts are more stable than what can be accomplished with thermal, which is why so many machine tool builders use the VSR Process. A typical example: An 18 meter longer gantry for a milling machine, VSR Processed before and after rough machining, has a TIR envelope of straightness of about 1/10 of a mmm (0.004") full length. The gantry displayed the same accuracy (as did also a twin to it) on-site, after installation, hundreds of miles from the machine tool manufacturer.

So, I would approach such applications experimentally. Make sure the gear used has very good vibrator speed regulation, variable unbalance, is capable of speeds that can resonante the components involved, and instrumentation that is capable of tracking BOTH the types of changes outlined above. Since pipe has to be fixtured, proper design of the fixture, so as to allow resonanting, but efficient transfer of vibration signal (somewhat cross-purpose design goals) is also important.

Hope this helps.

 

[bklauba]

An additional point that should not be overlooked, and perhaps within the context of the originator of this thread: A key requirement needed to make vibratory stress relieving be fully effective is freedom of movement. The workpiece(s), if held stiffly in place, such as in an installed condition, typically are prevented from going thru the kind of flexure that will allow the process to work.

Massive components, such as press frames after extensive weld repair (think thousands of cubic inches of weld fill) can be very effectively vibratory stress relieved, providing they are fully released from their foundation and surrounding physical contacts, jacked up so as to have rubber load cushions placed beneath them (positioning these is important, keep them AWAY from the corners of the frame). Freedom of movement is mandatory.

I have done a good number of these, and seen more of them done, with excellent results. The frames typically are in the 100 - 200 ton range (net weight of press frame).

But "in-situ" components are tough to do using this technique, at least at this point.

BK