bolts at 450¦F

[dvd]

I work in an industry where we routinely bolt items on conveyor chain that runs through an enclosure at 450°F. A very small percentage of the bolts (really screws) occasionally break. We are using 1/2" Holo-Krome flat head socket cap screws and some grade 8 hex head cap screws with prevailing-torque steel hex nuts. My questions are:

1.) Will exposure to these temperatures over an extended period of time (1000's of hours) cause changes to the grain structure of the fasteners?
2.) Would it be better to use a lower carbon steel than a medium carbon steel in this application.
3.) Would a fastener that had not been heat treated be better for this application?

We won't be changing our fastener spec's, but I'm curious about what is happening.

Thank you.

 

[MJCronin]

dvd

At 450F you shouldn't have any permanent grain or proerties changes to mild steel fasteners.

For flanges on power piping systems up to about 700F the "workhorse" material for the past 50 years has been A193 grade B7 (studs or machine bolts) with A194 heavy hex nuts.

Some low pressure/temperature piping systems use A307 material. But, most large projects find that it is too much bother to distingush between them and simply use A193/A194 all over the site.

If you can get this (relatively) cheap material in the form that you desire, you've got it made.....

Good Luck....... Let us know how you made out

MJC

 

[butelja]

One thing to beware of when using SAE graded fasteners, is that there are no elevated temperature requirements in the specification for each grade. All properties are at ambient temperature only. This doesn't mean that they can't be used in non-code elevated temperature environments, but that you may have to do testing to see how they behave.

Using ASME fasteners such as A193-B7 that MJC mentioned will ensure predictable performance at elevated temperatures, as the minimum yield and tensile strengths are spelled out as a function of temperature.