Flange bolt stress and torque at 20° C vs. at operation temp of 300° C

[JouHar]

Dear Forum. I have tried searching this issue from the forum. Did not find a clear answer/ the issues covered has been a bit different ones.

My question is as follows;

If eg. a 8.8 grade metric M20 bolt is graded to yield value of 640 N/mm2, and when calculated with 80% max stress one can use value of 512 N/mm2. With 245mm2 area for stress in the bolt, the max. stress in the bolt could be 245*512=125kN and torque about 500 Nm. But the 512 N/mm2 is at 20° C. When the flange joint heats up to 300° C, is that torque/ stresss too high for the bolt?

My concern/ question is, that if the mex. yield value at 300° C is for 8.8 roughly 480 N/mm2, and with 80% loading, it would mean max. stress of 245*(480*0,8)=94 kN. With these values I get roughly 375 Nm of torque to be applied.

Q: Shall the bolt in this case, tightened at 20° C, but operated at 300° C (the bolt and flange will heat up the same, no changes in dimensions that could add stressses) be torqued so, that the max. stress is calculated based on the 300° C values and the torque at 20° C would be the 375 Nm..?

 

[LittleInch]

There is a welding, bonding and fastener forum so the answer may be in there.

You do though appear to mixing up stress and force.

so your calc 245 x 512 is 12kN which is a force, not a stress.

A few things to note
1) you need to check the relative expansion coefficient of the flange material and the bolt material. If they are different there will be a difference in expansion of the two elements
2) there is usually a longer length of bolt material versus flange thickness
3) torque is a poor way to get accurate bolt tension as it depends on many things including friction and lubrication, none of which are accurate or guaranteed. only hydraulic bolt tensioners will get you the accuracy you're talking about.

The usual issue is that the bolts expand more than the flanges, hence a reduction in bolt force. You can use things like bellville washers or similar to reduce this impact.

Search high temperature bolting on this site including the fastener forum and you might get somewhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GD2]

OP,
In an flange joint assembly it is not only the bolt but also the gasket and the flanges are involved.
Yes, you are right, all bolting stress determination is done at the room temperature. To determine in the higher operating temperature (say 300C), another factor called, 'relaxation' of the assembly components will play in.
You need to compare the reduction in yield to the amount of relaxation occurring and if the reduction ratio exceeds the relaxation, the effect should be included. This is an complex process and no one bothers about it.

For more information, read ASME PCC-2 Appendix O.

GDD
Canada