FTM 151: Extreme Temperature Limits

Standards aren't always clear about fastener temperature tolerances

Standards aren’t always clear about fastener temperature tolerances

This edition of the Fastener Training Minute with Carmen Vertullo was originally published April 16, 2020 as “The Heat is On” during episode 151 of Fully Threaded Radio.

There are three things that can help inform us about this. The first thing is believe it or not, not the Fastener itself, but the coating that’s on the fastener: the plating, the zinc, the teflon, that is probably is going to be the limiting factor in the service temperature for that fastener. We have to look at the coating performance to know what that is, and we actually have a good reference for that that which we will cover later.

Another thing to consider is the material that the Fastener is made of. For example, an aluminum Fastener obviously is not going to handle the same temperature as a steel Fastener, and a steel fastener is not going to be able to handle the same temperature as a super alloy such as an inconel fastener, and so on. The Fasteners are usually under stress when they’re installed. In some applications the temperature is low and in some applications it is high, and in some applications it is both low and high. So we may have temperature cycling going on. All materials have a property called the coefficient of thermal expansion. What this coefficient means is that as the temperature rises, the material actually grows in size. Not by very much, but enough to matter. Fortunately for most fastener applications, as the Fastener grows, so does the assembly that’s it clamping it together.

The flanges expand along with the Fastener and we can pretty much maintain our clamp load within certain limits as the whole system expands. As matter fact, if that didn’t happen, things like jet engines wouldn’t even work because they depend totally on those fasteners being able to maintain the clamp load on the flanges that they hold together, so that they can contain all those hot gases.

So how do we take our Fastener standards, and use them to relate to the use of Fasteners at elevated temperatures. I just discovered recently as I was reviewing, ISO 898-1  which is where the metric steel and alloy steel fasteners come from. Even though I have been under the impression, and have even taught, that faster standards weren’t very informative as to their temperature limits.

Right in the scope of ISO 898-1 it says the following under Note 1: Fasteners conforming to the requirements of this part of iso 898 that meaning part 1 are used in applications ranging from a minus 50 degrees centigrade to plus 150 degrees Centigrade.

Users are advised to consult an experienced faster metallurgist for temperatures outside the range of -50 centigrade to 150 centigrade and up to a maximum temperature of + 300 degrees Centigrade when determining appropriate choices for a given application. So that doesn’t really give us everything we need but it certainly is very informative and it’s helpful engineering information.

If we go a little bit further into ISO 898, one of the things we’ll find is that there’s an Annex B and it describes the influence of elevated temperatures on the mechanical properties of Fasteners. Part of Annex B says that up to typical service temperatures of 150 degrees centigrade, no detrimental effects due to change a mechanical properties of Fasteners are known.

At temperatures over a hundred 50 degrees centigrade and up to a maximum temperature of 300 degrees Centigrade, the functional performance of the Fastener should be assured by careful examination. This just requires some testing. So that is an authoritative citation. Interestingly enough, ISO 898-1 follows very closely in its application SAE J429, which is an inch faster standard.

At one time I thought there was no help from SAE J429. However, it also has an annex called Appendix A.

Appendix A was put in the standard many years ago. It’s been there at least back to 1999 because of the concern regarding the use of boron steel for grade 8 bolts. Back in the day, these were called grade 8.2. Nowadays, we can make Grade 8 fasteners with boron steel without calling them grade 8.2.

Appendix A says relative to 150,000 PSI (that is Grade 8 bolts), the tensile strength of bolts and screws produced from low-carbon Boron steels designated as grade 8, users should recognize the difference in stress relaxation characteristics of various steels between the temperature range of 650 degrees Fahrenheit or 340 degrees Centigrade. The minimum specified for grade two are Grade 8.2 and 800 degrees Fahrenheit for 25 degrees centigrade minimum specified for grade 8. Now notice that those temperatures change a little bit. That’s because the minimum tempering temperature is the limit at which we can run that bolt and its application without affecting its properties now, that’s a serious limit.

I wouldn’t approach that. I would think we would need some kind of a margin before that have at least a hundred degrees Fahrenheit, if not, maybe 200 degrees Fahrenheit. That’s just a guess. And then it says when considering bolts and screws that may be exposed to such temperature range, when we’re going to use these in high range, what we look at is the tempering temperature. Now, it says this: the data available on elevated temperature properties for grade 8.2 (hat’s the boron steel) indicates that performance testing is desirable in applications where the operating temperature exceeds 500 degrees Fahrenheit or 260 degrees Centigrade. That’s solid information right there. And then it says the same applies to Grade 8 fasteners, and of course, it would. There shouldn’t be any difference. The grade 8 wouldn’t be lower than 8.2. So this was all put together to address the issue of boron steel. So there it is. There’s our grade 8 operating temperature, 500 degrees Fahrenheit.

Now if we’re going to put a coating on that bolt we have to be concerned about what the temperature limit of the coating is. So when it comes to zinc plating, zinc plating will have a top coat on it, which probably has a temperature limit less than the zinc itself. The same applies to cadmium and that’s typically going to be hexavalent chromium or trivalent Chrome or one of these non chromate top coats. So you would have to consult the developer of that coating to know what the top coat limitation is, but we have some other things to be concerned about and in looking for an authoritative Citation for the use of Coatings. Believe it or not, I found this in ASTM A193 and A193M standards. The standards really do not address temperature limits for the materials in the standard, but it turns out it has an appendix X2 that mentions Coatings and application limits.

Appendix X2.1 says that use coated Fasteners at temperatures above approximately one-half the melting point of the coating is not recommended unless consideration is given to the potential for liquid and solid metal embrittlement. That’s not hydrogen embrittlement, that’s another type of embrittlement that we’ll talk about that in a future faster training minute. Then it goes on say: the melting point of elemental zinc is approximately 780 degrees Fahrenheit, therefore the application of zinc coated Fastener should be limited to temperatures less than 390 degrees Fahrenheit. The melting point of cadmium is approximately 600 degrees Fahrenheit, therefore the application of cadmium coated Fasteners should be limited to temperatures less than 300 degrees Fahrenheit. That is a very good authoritative citation interesting leak coming from a standard that does not purport to advise coating their products at all from ASTM A193. Now that’s what’s happening when the temperature is very high.

There are some other standards, ASTM F2281, for example, which addresses superalloys such as inconel. Also, it has some lower temperature stainless Steels in there such as 316 and 304 and it does specify the temperature limits where those Fasteners can be used. There are lots of other examples and I’m not going to cover them all for these high temperature services, but one of the things I would like to do is put together a nice compendium of this type of information and if anyone out there listening would like to help with that or has some information to to point me and some good directions. I’m happy to have the help. I think we need this kind of a document in the industry. So that’s what happens at high temperatures.

What about lower temperatures? When we talk about lower temperatures, on planet Earth, except for some really extreme places, most fastener materials are not going to be severely affected by what the weather. For example, I recently reviewed the document from McMurdo Sound which is in Antarctica, and they were talking about structural steel and the use of ASTM A325 bolting in structural steel. They said absolutely nothing regarding the concern for the temperature of those Fasteners.

Now, I know that in some applications for example in Canada in with transmission towers and things that are going to be used in those very cold places. They need to be concerned that they choose materials that are not going to become brittle because of the cold weather. A standard that does address cold temperatures in the cryogenic ranges is ASTM A320. It’s the partner to ASTM A193. It also does not tell us what the limitations of those materials are, but we do get some testing requirement out of that where we can do for example, an impact test at temperatures close to that of liquid nitrogen for example. So as far as cold is concerned cryogenic temperatures are very low, we don’t see cryogenic temperatures in nature. We only see them in things such as I liquefied gases: liquid hydrogen, liquid nitrogen, liquid natural gas, and turns out that some of our very common materials such as just 304 and 316 stainless steel are very good at withstanding those temperature. So it’s not that hard, you want to be careful, but it’s not that hard to deal with the cryogenic temperatures.

Well, that is about all I want to say cuz this has gone on long enough. But listen when the heat is on, look at those faster standards and you can find an authoritative citation to know what to do with fasteners at high temperatures.

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