Hot take: I think quenching in used motor oil is way overhyped for most home shops

I was reading through an old blacksmithing forum archive from like 2008 and found a guy who tested hardness on different quench mediums with a file and a rockwell tester. Turns out used motor oil actually gave him less consistent results than plain canola oil on mild steel projects. I mean everyone online talks about how motor oil is this magical cheap quenchant but that data made me rethink it. I tried canola oil on a few knives last month and honestly the finish came out way cleaner with no weird smoke or smell. Maybe it's just me but I think we all jumped on the motor oil train without really testing it. Has anyone else run actual comparison tests on quenchants or are we just going off what the old guys at the shop told us?

2 comments

2 Comments

campbell.nora10d ago

Stella, I have to stop and reread that part about the thermocouple test a couple times. You actually put a thermocouple rig on your quench tank and watched the vapor jacket formation in real time? That is a level of testing I never thought anyone in a home shop would bother with. The hot spots from the motor oil additives making uneven vapor jackets makes so much sense now. I always wondered why some blades came out with weird soft spots even when the oil was the same temperature. That burnt sludge buildup too, I just assumed that was normal after a few uses but canola oil staying clean longer sounds way more practical.

stella_lee11d ago

The thing nobody talks about is how used motor oil has all those additives like detergents and friction modifiers that break down at different temps. Ran a test a couple years back with a thermocouple setup and motor oil had hotspots that formed uneven vapor jackets on the steel. Canola oil gave me a nice even film that snapped off clean every time. Plus you can reuse canola oil way longer without getting that burnt sludge motor oil leaves behind in your tank.