I am making tools etc out of an unknown carbon content steel. They are A 3/4” B 8” C 3’ grade 8 (I think) U Bolts used on fire truck suspension. They are painted and easily grind down to good usable steel. I have made several tools with this steel and quenched in Mineral Oil and Vegtable Oil. I’m confused has to why the Mineral Oil was useless while quenching and the Vegatable Oil seemed to harden the steel properly. Anybody have any clue to the difference in these two oils and the carbon content in these u-bolts? I have tried to research the carbon content online without any luck. Not really that important cause they are making good tools but it would be nice to know what I’m going working with. Thanks guys! 

Viscosity and flashpoint have a lot to do with oils effectiveness as a quenchant. As to the suspension U boots, I would suspect a medium carbon steel in an sneaked state. 

One manufacture says “U-bolts 9/16″ diameter and less are made from 1541 Grade 5 Material.  Ubolts 5/8″ diameter and greater are made from Grade 8 material. “ some grade 8 fasteners are 1035 others 1541. The harness has (heat treat) has more to do with the rating than other things, as grade 8 fastener stretches less and there for be tightened to a higher “/‘# yielding a higher clamping force (and more friction) than a grade 5. Other than head bolts most automotive fasteners are grade 5 as they are usualy the same alloy just not as hard, their for less likely to snap (generally better to have a bit of give when it comes to suspension as a bent/stretched bolt will still hold wile broken ones tend to let things fly).