Why does everyone keep quenching in old motor oil?

I've been noticing a lot of posts on here where folks are using used motor oil to quench their blades and tools. That stuff is full of dirt, metal shavings, and who knows what else from the engine. It makes a huge mess, gives off way worse fumes than fresh oil, and you never really know how it's going to cool your steel. I had a guy bring me a knife he made last month that had a weird soft spot right in the edge, and I'm pretty sure the contaminated quench oil caused it. I've tried a few different oils over the years and I keep going back to canola or a dedicated quenching oil like Parks 50. Is there some trick to using used motor oil that I'm missing, or is it just cheap?

2 comments

2 Comments

spencer_chen63mo ago

Gotta jump in here and disagree a little on the fumes part... used motor oil actually has way less light ends and volatile stuff than fresh oil since it's been cooked in an engine already. The real nasty smoke comes from the detergent additives and the burnt fuel residue burning off, not the oil itself. That said, the dirt and metal particles in there can definitely mess with your quench speed and cause soft spots, you're right about that part. It's mostly just cheap and available for people who don't want to spend money on Parks 50 or canola.

nancy3513mo ago

Wait, are you saying used motor oil actually has less of the nasty fumes than fresh stuff? Because that matches my experience too, I've quenched a few blades in both and the fresh stuff smoked way worse right out of the jug. The real problem I ran into was getting consistent results, the metal particles and sludge from the engine would settle in the bottom of my quench tank and then the next blade would come out with a weird patchy hardness. I started filtering it through a coffee filter and a old t-shirt before using it, and it helped a ton. Still got that burnt smell though, that never goes away no matter how much you filter it. It's a pain to deal with but when you're on a budget it works good enough for practice pieces.