Galling of bolts

[PTorrejon]

I'm using a 1/2" 13 UNC 304 SS hex bolt (SA193-B8) with a Monel nut (SB-164) to tighten a 316L ss 10" sanitary high pressure clamp that is installed on a vessel. We are torquing the bolt assembly to 40 ft-lbs. The 316L SS vessel is heated to 80 deg C, and after reaching ambient temperature the bolts and clamp are removed. About 30% of the time these bolts are galling.

I tried to reproduce the problem at room temperature, but I couldn't make the bolt gall. The nut however sustained damage on the face that contacts the clamp and metal shavings came off relatively easy from the nut this is also happening in the field (see nut damage pics).

The bolt is galling at the threads where the nut stops approximately after torquing.

I'm inclined to think that the thermal expansion of the clamp is making the bolt and nut gall even before is taken apart.

Any thoughts?

 

[redpicker]

Galling is a friction weld. To get galling, you have to have the two surfaces under high load as well as movement between the two surfaces.

Are you using any lubrication on the threads? If so, the heat may be driving off the lubricant so it galls on disassmebly. I doubt the galling is occuring without movement; movement is required to cause the gall.

I think teflon is stable at those temperatures, so teflon tape may solve the problem.

rp

 

[desertfox]

hi PTorrejon

The torque your appling to the bolt ie- 40ft-lbs what level of bolt stress is that supposed to produce?
Reason for asking, if that produces 90% yield stress in the bolt, then when the assembly is heated up and the joint expands the stress in the bolt goes possibly over the yield value.
Are there any calculations for the stresses in the bolt and clamp prior to and post heating.

desertfox

 

[rmw]

Either the photo isn't too good, (which I don't think is the problem) or the threads on that bolt are crap. If you start with wounded threads, galling can occur more rapidly.

Galling is initiated in SS materials when corrosion in the threads begins to slough off as the nut is turned and it dams up in the threads, causing the locking up or welding.

Is this in any type of atmosphere that could be attacking the threads on the bolt?

There are some really anti seize compounds out there, have you tried any?

rmw

 

[IFRs]

The nuts may be being turned too quickly without lubricant. You can easily lock up SS bolts and nuts by spinning them with an impact wrench with the result of them not being tight.

 

[CoryPad]

I agree with rmw - the screw threads have high roughness and nicks, which will have a high tendency to gall during rotation during tightening and loosening.

 

[Tmoose]

I think I'd use nuts with flanges. I also think there is potential for The bolt seat milled into the clamp to induce bending in the bolt when tightening, and tendency for greater bearing pressure on one edge and greater tendency to gall. Not a lot of room for spherical washers.

If I could use hardened washers and anti-seize, I would.

Some sources object to even one washer. But more than once when I've added one or 2 nicely finished hardened washers and a little lube were added to a fastener that normally tightens with a jerky rat-a-tat-tat, the result has been a positively luxurious feel when tightening, and much more consistent angle vs torque. I feel better, too.

 

[IFRs]

For some situations I like the fact that the nut galls on the mating surface - it's like a lock washer without the extra parts.

 

[PTorrejon]

The atmosphere where this equipment is a clean sanitary environment. I can't use any type of lubricant on the bolt because it may contaminate the product when the operators handle the bolt and charge the raw materials. The yield strength on the bolt is 75% approximately at 40 ft-lbs. We have the same bolt configuration at a different location on the vessel (same temperature changes) but it is torqued by the mechanics by feeling (at 15 ft-lbs or so) and have never seen an issue with galling. We had to come up with a 40 ft-lbs torque spec due to a safety recommendation on this clamp. I'm trying to understand what the problem is before making a change.

 

[electricpete]

Since you cannot use threads lubricants, the other tools available to you are thread pitch and material.

Going to a coarser thread pitch should lessen the propensity for galling, shouldn't it? (Are you using fine to begin with).

Also, the way it was told to me, certain material combinations are more susceptible to galling than others. The one who told it to me said stainless on stainless can often cause galling. That's a vague general statement that I'm sure others would disagree, but my point is: try a change in one of the materials (bolt or not). But I would probably investigate the coarser threads first.

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[CoryPad]

electricpete - I don't think anyone would argue that stainless on stainless is the worst for galling. There are some alloys (e.g. Nitronic 60) that are better than others.

PTorrejon,

While you may not be able to use a thick, post-applied anti-seize compound, perhaps you can use a thin, pre-applied dry film lubricant? That would help a lot.

 

[rneill]

What about teflon/xylan coated bolting?

 

[electricpete]

Thanks Cory - makes sense. I see now many other mentioned problems with stainless as well.

Google shows the problem and potential solutions are well discussed:

http://www.estainlesssteel.com/gallingofstainless.html

http://www.ehow.com/how_5240815_prevent-galling-stainless-steel-bolts.html

http://www.fastenal.com/content/feds/pdf/Article - Galling.pdf

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[desertfox]

Hi PTorrejon

So when you subject the bolt to only 15ft-lbs you have no galling issues even after heating?
Well the higher the threads are loaded the worst situation your in for galling to occur and going from 15lbs-ft to 40lbs-ft its bound to get worse.
As others have already stated stainless on stainless is a known problem.
The picture you uploaded showing the damage nut, is that the one tightened at room temperature but not subjected to heating upto 80 degrees C?
Have you got any joints which have been tightened to 15 and 40 lbs-ft and heated,then dismantled, it would be interesting to compare both joints directly.
How is the heating carried out? Is there a possibility that the clamp expands faster than the nut and bolt generating additional stresses into the bolt ie differential thermal expansion?
Finally what is the length of the bolt from the underside of the nut at the clamp face to under the bolt head?

desertfox

 

[rmw]

Whoa, where did this thread get sidetracked? This isn't stainless on stainless. It may be alloy on alloy, but these aren't matching alloys. One of the remedies for stainless on stainless galling is using a different alloy for one of either the bolt or nut.

So, this is Monel (nut) on Stainless (bolt). A Cr-Ni bolt with a Ni-Cu nut. Some anti-seizes are pure Ni based and others are pure Cu based.

What is wrong with this picture?

I think DesertFox may be starting to hone in on some of the reasons if it isn't the condition of the threads out right.

Ptorreon, a question for you? Why the choice of materials? And more to the point, why the choice of Monel for the nut? That must have been hard to find.

rmw

 

[desertfox]

Hi PTorrejon

Just a couple of links about thread galling in addition to those already given.
One possible cause is the increase in fastener torque as that will generate more pressure on the threads, which is why I am interested in comparing two different joints which have been tightened differently.
You also say your having problems in the field with these joints,again is this just joints that are torqued to a higher value?

http://docs.google.com/viewer?a=v&q=cache:gIsB2AV25OcJ:

http://www.jcfasteners.com/Technical

http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf

desertfox

 

[macthenife]

molybdenum dysulphide dryspray is quite a good anti-galling
agent.