PSA: Old Bob was right about quenching in used oil

I argued with Bob for like an hour last spring at his shop in Tulsa. He said used motor oil works better than new stuff for hardening carbon steel. I called him crazy and stuck with fresh oil. Then I tried his way on a batch of 5 knives last month and the difference was huge. The steel got way harder and more even. Has anyone else found old oil works better or was I just lucky?

3 comments

3 Comments

grant_hart2mo ago

Hold on, is this really that big of a deal? I mean, we're talking about quenching oil, not rocket science. I've used fresh motor oil for years and never had an issue, so Old Bob might just be a bit of a character selling a story. The science says used oil has more carbon and crap in it, sure, but that can't make that much of a difference, can it? Sounds like you might have just gotten lucky with that batch or maybe your heat treat was off before.

grantadams2mo ago

Bob was dead on, I had the same result with a batch of carbon steel chisels and it honestly blew my mind.

kim_west1mo agoTop Commenter

My own tests with fresh 10W30 versus the same oil after 500 miles showed no measurable difference in hardness on 1095 steel. I ran three knives through each and checked them with a file, and both batches felt identical to my hands. It sounds more like Bob is a good storyteller than a metallurgist, and you may have just hit your heat treat right that time.