Question about quenching in used motor oil

I know a lot of guys swear by quenching in old motor oil for tools, but I tried it on a set of punches I was making and got a weird result. I had this 10W-40 from my truck change, figured I'd give it a shot. Heated the steel to a nice orange, then quenched it. The punches came out with this soft, almost rubbery skin that flaked off, while the core seemed way too hard and brittle. I snapped one testing it on a piece of scrap. I think the additives in modern oil messed with the quench speed, making it uneven. I went back to my simple canola oil for the next batch and they turned out perfect. Has anyone else run into this with newer automotive oils?

3 comments

3 Comments

kevinallen13d ago

Henry's point about the vapor jacket is spot on. I read an article in a metalworking magazine that said modern multi-grade oils have viscosity improvers that can mess with heat transfer if they're cold. They recommended pre-heating any automotive oil, but said straight 30-weight without detergents is more predictable for quenching.

henry_anderson5413d agoMost Upvoted

That "rubbery skin" sounds like the oil wasn't hot enough, it can form a vapor jacket. Modern oil still works, you just gotta pre-heat it to like 130-150F.

wood.jana13d ago

My old shop teacher in Toledo always said to avoid detergent oils for quenching. Those additives can cause that rubbery decarb layer you saw. Pre-heating helps, but a basic mineral oil like Parks 50 or even warmed canola is just more reliable for consistent hardness. Modern multi-grades are engineered for engines, not heat treating.