Vibratory Stress Relief 1

[21121956]

Hello everybody:

For a hydroelectric project several control gates will be constructed. Among them, there are two bottom outlets, slide flat gates with 6 m width and 6 m height.

As part of the non destructive tests indicated in the Technical Specifications, it has been approved to perform Vibratory Stress Relief (VSR) of the welding beads during and after the fabrication of the gates at site.

Reading some technical papers regarding with VSR, I have realized that even though this technique has been in use for some decades, it has not been able to substitute the post weld heat treatment.

How reliable (and not harmful) is this test when used instead of the post weld heat treatment?

If this test is not appropriated to be applied for quality control of the welds in the gates, how can we perform heat treatment in such big structures?

As always, your comments are welcome. Thanks.

El que no puede andar, se sienta.

 

[cloa]

Do you have to comply with a particular standard?

 

[dhengr]

21121956:
You’re right, VSR (Vibratory Stress Relief) has been being touted for a long time, and the promoters still haven’t proved that it really works, to the best of my knowledge. But, I haven’t been following its use of late. There is some sparse theoretical merit to its use as a stress relieving means, or weld conditioning means, on a simple enough structural element. It just can’t be made to work, or be proven, on any fabrication of any complexity. VSR is not a non destructive test for QC, so it doesn’t fit in that category. It doesn’t really have anything to do with QC, and doesn’t replace PWHT (post weld heat treatment). Plenty of fabrications like these gates are built without stress relieving, and without any adverse effects.

Who wrote the specs. for these gates? Who determined that a fabrication of that size and type needed PWHT or stress relieving, and why? Is PWHT a common requirement for these types of gates? What is the size of the gates? Why don’t you show some sketches of these gates and how they operate and are guided in use, so that we can get an idea of what you are dealing with? It might be more important that your fabricator do a good job of planning assembly and weld sequencing to control tolerances on this type of fabrication. Much of what they might build into these gates in the way of any out of tolerance conditions will not be fixed with PWHT. There are stress relieving ovens which can handle some pretty large fabrications, but unless there are some very tight post weld machining tolerance requirements, in most cases this PWHT is likely not really required. In fact, you normally go out of your way to design around needing it.

I think you should talk with your client about the real need for this VSR or PWHT, and try to determine if it is really needed. I’m not trying to second guess whoever wrote the spec. for these gates, but many times this kinda stuff gets put in specs. for no good reason, by some inexperience spec. writer. And, the call-out of VSR signals that inexperience or lack of understanding of the process and/or the real needs. And, I would want to understand why it is needed, or how it could a avoided. With a good structural design of the gates, and the tracks they run in and the guidance system, a reasonable set of fabrication tolerances should be able to be made to work without stress relieving. Stress relieving of these gates just isn’t something you do for the fun of it, if it’s not absolutely necessary.

 

[Guest102023]

Vibratory stress relief falls into the category of voodoo engineering. It is not found in any welding code I know of.

It MIGHT be feasible, but for a component with any complexity you would need an expensive research program just to prove it works effectively for just this one part ONLY. Even if it COULD lower the residual stresses of ALL the welds (I'm venturing into metaphysics here), NO metallurgical stress relief will be accomplished.

Setting up a temporary furnace in the field is not at all impossible. Contact the experts at Technical Heat Treatment

http://www.techeat.com

(I receive no consideration for the preceding plug).

 

[21121956]

Hello everybody:

I give thanks to all of you for your comments.

El que no puede andar, se sienta.

 

[EdStainless]

VSR can help redistribute residual stress in parts.
However my experience shows the effect to be limited and requires high excitation (drive) levels of the parts.
We did it to cold finished shafts and found that they did stay straighter in service that non-treated ones.
I would be very reluctant to try it on large parts.
Thermal stress relief delivered much greater benefit and more uniform results.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stanweld]

There have been numerous Threads regarding VSR in these forums. I suggest that you use the Search function provided in the forum to review them.

 

[bklauba]

The following website should help clear the air:

http://www.advancedvsr.com

Several areas of regular and successful application include hydroturbine components, lifting devices / yokes and paper mill machinery.

It is a bit hard to explain why so many companies use the VSR Process, while clearly keeping tabs on their quality, if the process did not have merit.

The most recent test of the VSR Process was done by a builder of hydroturbine equipment. A pair of 40" X 10" X 1" mild steel plates were welded in a capital T shape. One of the plates was secured prior to fillet welding to a thick solid steel block. If not stress relieved, the plates distort very consistently by 0.12" (3 mm) when released from the block.

If the assembly is thermal stress relieved first, the distortion is 0.040" (1 mm).

If the assembly is VSR Processed first, the distortion is 0.012"

[/URL] Uploaded with ImageShack.us]

In the pic, the red arrow points to the vibrator, green to an accelerometer, and blue to one of 3 urethane load cushions. Braces shown in top pic were removed immediately after welding.

Unfortunately, some builders of vibration equipment over-claim / do not recognize the limits of the technology, such as the need to achieve resonance, monitor the change in resonance pattern that results from effective treatment, or recognize that the material must have ductility for the process to be effective.

 

[geesamand]

My company makes gearboxes from cast or welded low carbon steel housings. We used a Metallax VSR machine including the resonance readout with good success. While it was indeed voodoo engineering, the machining tolerances that were held on a freshly welded and Metallax'ed housing certainly proved it was successful on some level.

Unfortunately that machine left our building 10 years ago and to my knowledge none of our vendors are using it in a controlled manner. So I can't offer much more.

 

[bklauba]

Geesamand is right: Resonance is the key to making VSR work. Some of the newer touchscreen controlled VSR units make following the best procedures much easier, by clearly displaying workpiece resonances, and how they grow a/o shift (or new peaks develop) with treatment, which then results in a new stable resonance pattern.

Equipment without good instrumentation results in exactly what Geesmand described: Uncontrolled = undocumented = spotty results. Sorta like removing the controls from a furnace. What would the results of a PWHT be then?

The VSR setup remains a critical aspect, especially placing cushions far from the corners or ends of the workpiece, so as to minimize damping, thus promoting resonance and FLEXURE. Mere mass oscillation DOES NOT do the job.

As to the "lamentable voodoo engineering", this can sorted out by reading the independently produced research. Likely the work of Dawson & Moffat, in which they report 90% stress relief, achieved using servo-controlled vibration, is one of the most important works in this area, and a good place to begin.

This work has been published by the ASME, and can be viewed thru the Advanced VSR Technical Library:

http://www.advancedvsr.com/#!technical-library/c12c9

A summary of Dawson & Moffat’s is shown in Fig 15 of this work. An updated color version of Fig 15 is uploaded.

This amount of stress reduction is clearly significant, and likely explains why properly designed tests, such as the one depicted earlier in this thread, result in more predictable, stable parts, often better than what can be achieved thermally.

Alas, VSR remains a method that works well in practice, but not in theory (unless we update our theory.) Reviewing the research and well-written VSR Reports equips us to do so.