Proof Load Yield Strength Tensile Strength
(psi) Min (psi) Min (psi)
85,000 92,000 120,000
120,000 130,000 150,000
8.8 is on top grade 8 is on bottom they are close enough for your needs.
Do a little more reading. Look specifically for what operating temp those PSI ratings are for. Hint: It's a max of 500F for SAE Grade 8 hardware. So if you use Metric 8.8 material, get lots of spare bolts. You will need them. But don't just take my word for it. Do a little reading:
http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf
"The initial heat-treating process is relatively the same for all three products. The parts are heat treated until fully austenitized. The parts are then quenched and tempered in a liquid (oil). This final tempering temperature is what will dictate our final product. The following are the minimum tempering temperatures for each specification:
• ASTM A193 B7: 1150°F
• SAE J429 Grade 8: 800°F
• ASTM A574: 650°F
As can be seen by the results, a lower tempering temperature will produce a harder and higher tensile strength part for these alloy steels. However, the lower tempering temperatures will also mean lower service conditions, ductility, impact strength and possible fatigue life. For example a B7 has a high temperature limitation of approximately 750-800°F. The socket head cap screws and grade 8’s have a limitation of approximately 400-500°F."
Also note that those materials get extremely brittle when cycled through 1600 F to air cooling repeatedly. Which means they break easily.
My local Ace Hdwr store has a good stainless (304) metric selection. That's the MINIMUM kind of material to consider for 1600+F usage.
