This edition of the Fastener Training Minute with Carmen Vertullo was originally published November, 18th, 2022 as “How to develop good methods for tightening stainless steel structural bolting” during episode 182 of Fully Threaded Radio.
Well hello everyone, this is Carmen Vertullo with your Fastener Training Minute coming to you from beautiful El Cajon California, where on this day in November, it happens to be raining. But it’s still beautiful and I’m here with the Fastener Training Institute.
Today’s topic comes from a composite of two previous topics; one having to do with tightening strategies. As we know, torque is a tightening strategy, angle is a tightening strategy, no strategy at all is a tightening strategy. But when we tried to apply a tightening strategy to stainless steel fasteners, especially larger bolts above a quarter of an inch, it really gets interesting and tricky and very difficult to develop an effective tightening strategy.
When we return I’m going to put those two topics together and help you know how to develop a good tightening strategy for stainless steel bolting.
Well, welcome back everybody this is Carmen Vertullo with the Fastener Training Minute.
I recently had a project (actually I’m still in it), to help develop good methods for tightening stainless steel structural bolting. Now technically, at this point in our fastener technology, there is no such thing as stainless steel structural bolting; there are simply stainless steel bolts which are used in structural applications. We have never developed that technology anywhere near the way we have developed steel structural bolting such as our ASTM A325 and A490 and the tension control bolts that go along with those.
One of the reasons why stainless steel structural bolting can be such a challenge is stainless steel bolting, like steel bolt in comes in different strengths and also it has a propensity to gall, which greatly hampers the torque tension relationship. And like steel structural bolting, the strategy for tightening typically what should not or would not be based on torque or torque alone. As you know if you’re familiar with steel structural bolting, we typically would use a strategy such as snug-tight plus angle or number of turns or number of degrees of turn. And we could also use a strategy such as the tension control bolt. There are no such things at this point as stainless steel tension control bolts, and not likely to be in the future. Or possibly we would use the direct tension indicating washer and I don’t think there are stainless steel direct tension indicating washers at this point either. So we’re kind of stuck with either torque or angle or a combination thereof.
Now what we discovered recently is that because stainless steel has such a propensity to gall. Galling means literally that the metal of one component, (either the nut or the threads of the nut) wipes or smears onto the threads of the bolt, and you’ve literally have those two things cold welded together, and you don’t always know it. You think your torque is going into tension, but all it’s really going into his overcoming that tremendous amount of friction because of galling. The way we combat that is with some kind of a lubricant or anti-galling or anti seize compound, most of which work pretty good, but again not very consistently.
So we have to come up with a better strategy, and one of the issues we also have to deal with in stainless steel bolting is the washer. Stainless steel washers are not that hard. In structural steel applications where are using steel bolting, we have the advantage of using a hard ASTM F436 flat washer that gives us a very nice bearing surface for the nut to tighten against. The stainless steel standard washers are relatively soft, so they tend to embed long before we reach even the low level of tension that we would have on that bolt, and that consumes energy or friction, thereby hampering our torque tension relationship. So what the Hell do we do? How do we know how to get a a decent relationship in our tightening strategy between whatever that is in the final tension on the bolt?
Well I have come to the conclusion that the only reasonable way to do it is with a combination of torque and angle. By the way, this has also been applied recently in the past several years to the steel structural bolting industry with a tension control type bolt called the T&A or Torque & Angle. And the way that product works is an automatic tool tightens the nut to the level of snug tightness based on torque, and then a certain number of degrees of angle, which delivers a specified tension. We can do that in the stainless steel bolting in a manual method, by determining first how much torque it takes to achieve snug-tight. But of course first we need to define snug-tight. And what we would typically define snug-tight as, is about 10% of the final tension. That should always be enough to pull all the plies or whatever you are bolting together into firm contact with one another.
So we would put the bolt in our Skidmore, use our lubrication, whatever it’s going to be (whatever we use in the test we should use in the actual assembly), and snug that bolt down until we get to 10% of our final tension, which for stainless steel bolting off the shelf is not very high. And I’ll just give you an example for a 3/4-10, the final tension is only 11,000 pounds or 11 Kips. That’s based on the very low yield strength of that bolt. So we’re going to tighten it to 1,100 lb or 1.1 Kip, which unfortunately is very hard to see on the Skidmore, because the lowest level you see on the standard structural bolt Skidmore is 2 Kips. So we need a smaller tool or we need a load cell of some sort. I use a load cell. We determine what our snug-tight would be. In this case it turned out that 25 foot-pounds of force gets us to 1,100 pounds or 1.1 Kips, 10% of our final tension.
Then we go ahead and tighten the bolt the rest of the way until we reach our final tightening which is going to be 11 Kips, and we measure that angle. And it turns out that is very consistent within about five or ten degrees at the most, we can achieve the tension we desire even with the washer embedding. The soft washer is still going to embed. The better strategy even might be to use a hardened stainless steel washer which would then have to be made out of a hardenable stainless steel such as 17-7ph or 17-4pH, which is very compatible with our standard 304 stainless You may not want to mix 316 stainless together because you’ll lose the advantage of having 316 stainless steel without the 316 stainless steel washer.
So once that’s done in the structural bolt world, we also want to know or ascertain that the bolt is capable of withstanding a certain amount of over-tightening. So then we go beyond 11 Kips probably another fifteen or twenty percent, maybe twenty-five percent. In this case we basically went up to 16 Kips, and we proved that the bolt could be over tightened by a certain number of degrees, or a certain amount of torque, (even though torque is not our strategy, our strategy would be degrees), without detrimentally hurting the function of the bolt: that is we don’t strip the threads, we don’t induce galling, and we don’t break the bolt. It doesn’t mean we don’t yield the boat, yielding is okay. But we don’t want to end up with a situation where we have over tightened it, and now we’ve either lost tension or we can’t ever get it disassembled or something like that.
So that that strategy is called Torque & Angle or T&A (It means something else in another vernacular), but right now we’ll just stick with Torque & Angle, and that is by far the best tightening strategy that we know of today. As a matter of fact, there are automated tools to do it. In the Aerospace and the Medical industries you can even buy an off-the-shelf torque wrenches these days that will give you snug-tight with a certain torque, and then measure the angle after that. I think that’s the way we’re going to be doing it with stainless steel structural bolting. And as we go forward and we finish our research, we will probably have another report on that or perhaps you might see a magazine article on its own.
Well I hope you learned something how about tightening stainless steel bolting and also about tightening strategies.
This has been Carmen Vertullo with the Fastener Training Minute, thanks for listening.