Galling of bolts
Galling of bolts
(OP)
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?
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?





RE: Galling of bolts
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
RE: Galling of bolts
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
RE: Galling of bolts
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
RE: Galling of bolts
RE: Galling of bolts
RE: Galling of bolts
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.
RE: Galling of bolts
RE: Galling of bolts
RE: Galling of bolts
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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RE: Galling of bolts
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.
RE: Galling of bolts
RE: Galling of bolts
Google shows the problem and potential solutions are well discussed:
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RE: Galling of bolts
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
RE: Galling of bolts
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
RE: Galling of bolts
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?
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desertfox
RE: Galling of bolts
agent.