This edition of the Fastener Training Minute with Carmen Vertullo was originally published August 16th, 2024 as “The problems of substituting various fastener grades of stainless steel bolting in joints involving flanges and gaskets” during episode 203 of Fully Threaded Radio.
Well hello everyone, this is Carmen Vertullo with your Fastener Training Minute, coming to you from Fastener Training Institute and Carver Labs in beautiful El Cajon California.
Today’s Fastener Training Minute is all about stainless steel, or some things about stainless steel, based on a recent consultation I had regarding the wisdom or lack thereof in substituting various fastener grades of stainless steel or you might want to call conditions and the ups and downs of that. In this particular application, the use of flanges and gaskets were involved, which adds further complexity to that decision. So when I return I’m going to tell you about a couple of different fastener stainless steel fasteners standards, the various products contained within them as it regards to flange bolting, and the implications of using fasteners and gaskets joints.
Well welcome back everybody, this is Carmen Vertullo with the Fastener Training Minute. Today we are talking about stainless steel bolting in flanged joints using gaskets. First I want to start with what I guess is a commonly held misperception that stainless steel is high-strength. Stainless Steel generally is not high strength in comparison to alloy steel. it’s actually lower strength than most steels. We have two major standards from ASTM that cover stainless steel fasteners, and those are ASTM F593 and ASTM A193. Those standards both have companion standards for nuts in the case of ASTM F593 it is F594 and the case of A193 it’s A194. But for this discussion we’re going to stick with the bolts. If you’re in the metric world, your standard would be ISO 3506 and that standard covers stainless steel bolting dash one, ISO 3606-1 and ISO 3506-2 would be for nuts, Within all of those standards we have alloys and we have conditions, the alloy would be the chemical properties or the chemistry of the bolt and called out by specific AISI alloy, the American Institute of Steel and Iron. So in ASTM A193 and ASTM A194 the two primary alloys that were going to be considering are 304 stainless steel and 316 stainless steel.
In ASTM 593 we have alloy groups alloy group one contains 304 stainless steel and a whole bunch of others which are generally referred to as Austenitic or 18/8 Stainless Steels. These include 302, 302HQ, 303 and so on but for the most part we’re going to stick to what would be considered 304 stainless steel and in A193 we also have two different alloy groups for 304 stainless and 316 stainless. In the case of 304 stainless it’s called grade B8 and in the case of 316 stainless that’s called B8M. Now back to F593 for 316 stainless we would call that alloy group 2 and it’s all by itself, no other choices live in that group.
Within all of those alloy groups and grades we also have what’s called condition. Condition determines the strength of the material and it is generally achieved by one of two methods either Heat Treating or Strain Hardening. For the most part, these two alloy groups are not able to be strengthened through Heat Treating, so they must achieve a higher strength through Strain Hardening. In the case of F593 we call that condition SH Strain Hardened , and in A193 we call that condition Class 2. It’s the same thing. In essence what we’re doing when we manufacture the fastener is that before we manufacture it, we cold work the material in such a way as to increase its strength. These two stainless steel alloys 304 stainless and its brothers and sisters (excluding 303) and 316 stainless are miraculous in their ability to be strengthened through strain hardening. As a matter of fact if you don’t know, one of the miracles that we have in these alloys is we can strain harden it very high especially in thin sheets and wire. And that’s what rocket ships are made out of. Elon Musk makes all of his rockets out of strain hardened stainless steel. As a matter of fact, all the rockets that were made back in the fifties and sixties for the Apollo program were made out of that same material as well, and we have not found a way to improve it.
Going forward into fasteners, fasteners generally are not thin sections. There may be starting at a quarter of an inch let’s go up to an inch and a half, and the problem is the strain hardening effect only goes so deep into the material. So the cross section of the fastener, especially when it gets very large, is going to be stronger out on the edges of that round bar, or that hex bar, than it is towards the center. Now the consequence of this is that if we’re going to make strain-hardened fasteners out of let’s say round bar, let’s say it’s a larger-diameter, I’m going to make a one-inch fastener probably out of inch and half round bar I don’t know what that might be. First thing we’re going to do is we’re going to machine the hex maybe, then we’re going to machine a quarter of an inch of material off the outside of that bar, in order to achieve our finished diameter. Essentially we’re peeling off all the good stuff, so now we have an inner part of the fastener. While still strain hardened, it is not as strong as the outer. So that’s a consequence that we must take into consideration when using strain hardened material, either in hex bar or round bar. One of the things in this is an aside; I’ll step out of the topic for a minute, that I would advise or I guess consult clients on, is when you really want to redeem the strength of strain hardened stainless steel bars, use studs and nuts that way you’re not peeling off the good stuff.
Let’s continue on with bolts. Now in this particular consultation, a client was putting some pipelines together in a water treatment system, and the specified fasteners were ASTM F593 condition CW and in this case the CW (Cold Worked) fasteners were adequate for the application. However another part of the application required ASTM F593 condition SH, strain hardened, which are substantially stronger in yield strength and tensile strength. The condition CW or cold worked fasteners, have a relatively low specified yield strength compared to the SH or strain hardened.
So the question was, would we be able to use these CW fasteners which are readily available off-the-shelf, in place of the SH fasteners. Well, we did some calculations based on the pressure in the system, and we determined that there was adequate strength for the CW fasteners to be used. In other words, all the pressure in the pipeline in PSI times the cross-sectional area added up to a certain number of hundreds of thousands of pounds, and we had enough bolts in the glands in order to make it so that pressure would not be exceeded with a reasonable safety margin. However, there is another rub, and that rub is the gasket mount. One of the things we have to do when we put flanges together with a gasket is that we have to ensure that that gasket is adequately pressurized to make the seal, and that pressurization of the gasket is achieved by clamping it between the flanges with the assembled bolting. For most gaskets this is no problem, however we have some gaskets on the market where it is a problem. And in particular this gasket is a fibrous gasket which requires an extensive amount of compression in order to be effective that’s because unlike a soft or even a hard rubber gasket these gaskets are actually porous, they have fibers in them, and in order for them to seal, we must crush the gasket enough to push those fibers together and create the seal. In many cases condition CW bolting or in the case of A193 condition one bolting, is not strong enough to compress the gasket even though it may be strong enough to hold the pressure as supplied by the system. So the solution to this often times is we use a gasket in a joint with a raised face flange. Now if you imagine a gas
get a big ring donut shaped with a bunch of holes in it it has a cross-sectional area which we determined by the formula Pi r squared, the area of the gasket on the outside, minus the area of the gasket on the inside, including all of the holes, and then we take that cross-sectional area and we multiply it by the force that a gasket manufacturer is telling us to compress that gasket with. And our bolting has to add up to at least that force with some margin of safety. And that’s a large area by the way with the typical flat-faced gasket, however if we have a raised face on the flange, now we have a much smaller area to compress because we’re not compressing the whole gasket we’re only compressing the amount of the raised face, and that’s what makes these gaskets work with the lower strength bolting. And sometimes they don’t. So we have to make that calculation: what’s the area of the raised face, what’s the gasket compressibility requirement, and what is our available bolting that we can use in order to achieve that load. The best way to determine whether or not the system is going to work is to do a simple pressure test. Make sure that the gasket is compressed, and another way to make sure that the gasket is compressed is that we can measure the gap. In other words, when these flanges come together there’s going to be a gap, generally these gaskets need to be compressed by somewhere between 10% and 25% of their original thickness, and we can measure to make sure that we have that. We also need to be aware that in the case of soft rubber gaskets these boltings probably are strong enough to damage the gasket. We can crush the gasket in therefore end up losing our seal, because we’ve damaged the gasket.
So at the end of the day, when it comes to flanged joints with gaskets, we have to remember it’s all about the gasket. It’s not about the bolts, it’s not about the pressure in the system, we must be able to compress the gasket adequately to achieve the seal. We have to make sure we have enough oomph in the bolting to do that, and we also have to make sure that if we have a soft gasket, we don’t over tighten. And with some of these larger bolts the torque and the tightening can be very very low. So now we don’t have a very tight joint and that can be an issue as well. Also, when it comes to stainless steel, we have other things to consider in the torque tension relationship, and that means we want to make sure that we use a very good lubricant (molybdenum disulfide or some kind of nickel-based anti-seize is what’s recommended), that will do two things for us. It will prevent galling, and it will give us a predictable torque tension relationship.
Well there is actually a lot more to know about flanges with gaskets when it comes to bolting. I have developed a really cool Excel spreadsheet calculator for calculating gasket bolting requirements, and also I’ve written a book on Waterworks fastening that I’d be happy to share with you if you’re interested.
Well this has been Carmen Vertullo with the Fastener Training Minute thanks for listening.