Hi Temperature bolting query raised in pondering a company engineering specification B8M cl1 1

[robsalv]

Hi Folks.

There seems to be fewer bolt queries in this forum than in the materials forum, but I figure I should ask a bolting question in the right forum first up, so hopefully you can help me out.

In looking through a company engineering spec regarding bolting and flanges, it conspicuously specifies A193 B8M class 1 bolting for service temperatures up to 815degC (1500degF) and flange classes 300# or less. Above this flange class it says you can't use class 1's without detailed calcs, but makes no suggestion about what you'd use in lieu of class 1, presumably class 2? (Interestingly, the specification defaults to class 2's for any SS bolting in cold service - which I would have thought you could have gotten away with the lower yield Class 1 studbolts...)

Despite looking over A193, I can't see what it is about class 1 B8M's studbolts that would make it more suitable for high temperature service than class 2 B8M's... I would have thought that at the higher temperatures you'd want the higher yield strength of class 2's especially if bolt relaxation at temperature is at all a concern - but this class 1 requirement sticks out like the proverbial so I'm thinking it could be some esoteric material thing that's beyond my Mech Eng brain.

I'm hoping one of you learned folks will be able to provide clarity on this issue.

Thanks in advance.

Rob


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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[TVP]

Class 2 is strengthened by cold working the raw material (wire/bar is cold drawn prior to making the head, threads, etc). The effects of cold working (strain hardening due to dislocation entanglement) are undone by only somewhat elevated temperatures.

 

[robsalv]

...well now that you put it like that, that makes a lot of sense!

If the service provides some stress relief, it won't be possible to reliably predict the reduction in strength of each bolt, rendering assumptions and calculations unreliable.

Thanks TVP.

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[XL83NL]

At higher temperatures, other concerns are important too, like gasket selection, bolt/nut creep/relaxation, possible differences in thermal exp. coeff. between the flange and the bolts.
The list is much longer, but just to give you an impression. Just putting the bolts in the flange to a preset torque doesn't (necessarily) give you a proper BFJ.
Design for high temp flanges is quite difficult. A tremendous amount of work on this has been done by Warren Brown, and others, but Brown's work seems to capture most of it, if not all.
I suggest you read some of his work. Take a look at

http://proceedings.asmedigitalcollection.asme.org

for some of his papers, or simple google.

 

[robsalv]

Thanks XL83NL.

I'll give that link a look later on.

We tend to run a retensioning step once a critical hot flange has come up to temperature. It's not a perfect or even ideal approach, but in terms of seat of the pants maintenance engineering, it's held our 24hr production operation in good stead.

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."