What is Vibrational Distortion Control ?

[tc7]

This is a general question pertaining to vibration or sub-harmonic systems for distortion control for welds or machining of MILD steels of the ASTM A36 variety:

One feature that a vendor mentions in regard to their vibration or sub-harmonic system is for "distortion control". They seem to distinguish "distortion control" from stress relief. This may be an important distinction when it comes to weld procedures (ASME or AWS). If viewed as a method to control distortion then perhaps it would not need to be addressed as an essential variable whereas a method of post weld stress relief would most certainly have to be.

But my first question is - what is distortion control? is it a means to prevent stress in weldments thereby precluding the need for PWHT?

Secondly, if a qualified weld procedure on A36 mild steel called for no PWHT and a vibrational stress relief was applied anyway, does this violate the WPS?

Thankyou.

 

[7Mark]

Back in the 70's we had a job in our custom fab shop to build a small tower for a chemical company that had a number of horizontal, circular trays (about 48" dia) made of thin (perhaps 1/8 max thickness or a little less) sheets of stainless steel. Each tray had a one inch high vertical lip around the circumference (also quite thin) and a large number of what looked like stainless steel "mushrooms" welded to the tray. These mushrooms were thin gauge assemblies of multiple piece construction with one piece moveable vertically. If my recollection is correct, the "mushrooms" were designed to permit a liquid to accumulate on the trays up to a certain level (about 1/2 inch) and then allow the liquid to spill off through the mushroom to the tray below. Each "mushroom" had a loose fitting cap (which is what made it look like a mushroom) to prevent fluid falling down from the tray above to fall directly through.

Each tray had a hole in its center and after being positioned at the proper elevation was welded to a vertical pipe perhaps 10 or 12 inches in diameter. There were many of these trays. The requirement was that all the trays had to be level. Our first thought was that the trays had an overwhelming probability of turning into potato chips because of all the welding. Stress relieving was not an option because of the stainless steel and sensitization issues. A coworker showed me an article he had just read about vibratory stress relief and we thought we would give this a try. Although it made a tremendous amount of noise because of all the loose parts it actually did a pretty good job. The customer was satisfied with the results and we went on to other things.

 

[MintJulep]

A bit on vibrational stress relief here (a bit more than half-way down):

http://www.welding-advisers.com/PRACTICAL_WELDING_LETTER-PracticalWeldingLetterNo15.html

It notes that VSR has not been approved by pressure vessel codes.

Perhaps your vendor is playing with semantics.

 

[7Mark]

I just realized I left out an important part of the vibrational stress relief of the "mushroom" trays assembly. The pieces were not vibrated after welding - they were all vibrated during welding. It made for some specialized fixturing and, as I mentioned earlier, it worked.

 

[bklauba]

One of the most confusing aspects of this subject is the use of the term VSR or Vibratory Stress Relief, when what is actually being discussed is Vibration During Welding, or VDW. Since the exact procedure(s) and benefits of VDW are far from established, it is hard to justify labeling it Vibratory Distortion Control, or Vibratory Conditioning. Most of the time VDW is used in an attempt to control distortion.

In the VSR On-Line Research Library [ which can be found at

http://vsrtechnology.net/

) there are three articles on VDW (two of the teams of authors also being guilty of using the term VSR to describe VDW):

1. Reduction of Residual Stress by Ultrasonic Surface Vibration by Hirai & Aoki

2. Modification Of Welding Stresses By Flexural Vibration During Welding by Munsi, Waddell & Walker

3. Effect of vibratory stress relief during welding of thick stainless steel plate by Spooner, David, Wang, Hubbard, Holden & Root

The first article describes use of ultrasound to limit the production of stresses while welding. The second describes how VDW (they call it VSR) can either descrease OR increase the amount of stress generated during welding, depending on location relative to the weld (toes of welds were sites found to suffer increases in stress, due to VDW), and the the third failed to find any benefits from VDW, although the material (stainless steel) is more prone to distortion during welding.

If the term VSR is used to mean the use of vibration to reduce stress, most commonly done post-weld, but can also be done upon castings, forgings, plate or bar stock, etc., and VDW is used to to mean Vibration During Welding, confusion would likely be reduced (and maybe even a bit more stress, down the road.)

It is also worth pointing out that to perform VDW, vibratory stress relief equipment is not needed. There is no need to have automatic scanning and recording of amplitude vs. vibrator speed, tight speed regulation, or numerous other features of a high-performance vibratory stress relief system if only VDW is being done. Practioners of VDW have found that using rotary air vibrators work quite well, and they have the benefits of being self-cooling, have wider speed range, and do not suffer the complications of having a grounded electrical device connected to arc-welding.

BK