I thought quenching in used motor oil was just a messy old timer trick

My uncle swore by it for leaf springs, so I tried it on a truck spring last week and got way less warping than with my usual fast oil. The smoke was bad, but the piece came out straighter than any I've done before. Do you think the additives in used oil actually help or was I just lucky?

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the_hugo6mo ago

Actually might be the carbon in the old oil giving you a slower quench. More gunk means it doesn't pull heat as fast as clean oil, so it cools more evenly. Less shock to the metal means less warping.

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jessicajohnson6mo ago

Wow, that makes total sense.

4

nora_campbell664mo ago

Hugo, I gotta push back a little on that carbon idea. Old oil is mostly just dirty with metal particles and combustion byproducts, not pure carbon. The slower quench is probably more about the oil being thicker and having a higher boiling point from all that contamination, so it doesn't pull heat away as fast. I've seen younger guys try this with really sludgy oil and actually got cracks because the cooling was too uneven in spots. You got lucky with that leaf spring for sure.

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