Stress Relieve Processes?

[ajn22anellie]

Does anyone have any good recommendations on stress relieve processes? I have a part (2' by 2' by 2') with two wings on it and a bore through both wings (like a giant clevis). Currently we weld the part with tack bars in place then post machine the bore. However, we get significant spring back when we pull out the tack bars. I would like to stress relieve the part before machining, but I don’t want to purchase an oven. Does anyone know any other processes I can use. I once herd about vibratory stress relieving but I don’t know anything about it.

 

[TVP]

If you don't want to purchase an oven why don't you use a contract heat treater for stress relieving the part? For more information on vibratory stress relief, check out the Bonal Technologies website:

http://www.meta-lax.com/

 

[dvd]

Setup for welding with the ears out-of-square to accommodate for draw during the welding process. Then machine the faces parallel after welding and before boring the hole. Try posting a question about how to anticipate the amount of draw during the welding process on the Welding, Bonding and Fastener forum. If you could show a sketch or picture indicating the welds and weld procedure, it would be helpful.

 

[metengr]

Also, there is no mention of the materials of construction, size of the welds, weld process, etc. Stress relieving may help but more information is needed to help you.

 

[ajn22anellie]

We are using A36 ½ steel plates to make a box structure. The top and bottom plates are longer than the sides creating tabs. There are holes in these tabs with a ½ wall 4” dia by 2” long slug welded in each hole. Visualize a giant clevis. Currently all the pieces are fixtured in a tool and welded. Tack bars are placed between the tabs to help prevent weld draw. After the piece is welded it is put through a machining operation to bore out the holes, making them coaxial. Once the tack bars are removed the part springs just enough to cause misalignment between the bored holes. If the bars were to be removed after welding the spring back is slight…not enough to cause problems. The tack bars are left in the part during machining to make machining easier (though I’m not convinced they are doing anything except adding stiffness to the structure)

The welding process is manual so I’m not sure how repeatable the draw is. Can parts be fixtured accurately to account for weld draw?

My theory is that the welding process is putting stress into my part and the heat/ vibrations caused by machining cause some stress relief…hence the spring back. Any other ideas?

We are looking outside vendors, but I was hoping there were some tricks of the trade out there that I am not aware of…..like this vibration thing. Has anyone used it?

 

[metengr]

If the bars were to be removed after welding the spring back is slight...not enough to cause problems.

I would follow what you have stated above, use preheat during welding (150 deg F) and avoid stress relief altogether. Look at other ways to fixture for machining.

 

[tbuelna]

ajn22anellie,

The residual strain in your welded assembly is due to localized thermal expansion/contraction while the weld joint itself, plus other portions of the structural weld joint not yet fully fused (ie. still tacked), are plastically or elastically displaced.

The least costly remedies (as others have noted) is to employ pre and post heating. You should also work out a process for welding each part of the joint in a specific order, since balancing the heat inputs to the structure will help to minimize thermal strains.

Finally, yes there is such a thing as a vibratory stress relief process. It is generally employed on weldments that are too large to thermally stress relieve. However, personally I've only seen it used with large welded aluminum fixtures, that needed post weld machining. Aluminum weldments tend to have lots of residual strain, probably due to aluminum's high thermal conductivity and the higher amounts of heat input required to fusion weld it. You are using A36 mild steel, which welds beautifully and probably results in a welded structure with the least amount of weld distortion of any material I can think of.

For vibratory stress relief services, check Meta-Lax.

Good luck.

 

[bklauba]

ajn22anellie,

The solution might be as simple as what you related: Cut the braces, allow the strain energy to balance out, and, if need be, reattach those braces needed for the next procedure.

Stress relief with vibration is more effective if resonant vibration is used; see thread:

http://www.eng-tips.com/viewthread.cfm?qid=216357&page=7

Another approach is to vibrate during welding, which we should avoid calling stress relieving, or vibratory stress relieving, since it is neither. You certainly do not need a vibratory stress relief system to perform VDW (Vibration During Welding); modest-sized air vibrators, which are both far cheaper and more reliable for this app would be a better choice, might be worth pursuing if the first suggestion (cut and reconnect the braces) doesn't do the trick.

BK