hannahj49•9d agoProlific Poster

Warning: Stop quenching your hot steel in that old motor oil bucket

I see people doing this all the time at hammer-ins and in home shops, and it drives me nuts. That used motor oil you got for free from the garage is full of metal shavings, dirt, and who knows what else. When you quench a hot piece of steel in it, those impurities get baked onto the surface and can cause uneven hardening or even micro-cracks. I learned this the hard way after I made a set of tongs that snapped clean in half after the first real use. Switched to a dedicated canola oil bath three months ago and my heat treats have been way more consistent. Has anyone else run into trouble using recycled oil for quenching?

3 comments

3 Comments

johnson.eva9d ago

My grandfather used to quench in old transmission fluid and swore by it, but his shop always smelled like a burned-up car after. I'll stick with my canola oil though, less stink and fewer surprises.

the_ben8d ago

This whole thing is just the buy-once-cry-once rule but applied to oil.

craig.olivia9d ago

I've been quenching in used motor oil for about six years now and I gotta say my experience has been totally different. The key is to let the oil sit for a few weeks so all the heavy junk settles at the bottom, then pour off the clean stuff from the top into your quench tank. I use a fine mesh filter too when I transfer it, catches most of the shavings. I've made several knives and tools this way and never had a crack yet, so I'm not convinced the oil itself is the problem more so than how people prep it.