I finally tried quenching in used motor oil for a tough steel and the results were wild
Working on a big axe head from 5160 steel, my normal canola oil quench just wasn't getting it hard enough. The edge kept rolling after a few test chops. Out of ideas, I filtered some old 10W-30 from my truck and tried it. The quench was way more violent, lots of smoke, but the file just skated off after. I tempered it at 400 for two hours and it's holding an edge like a dream now. But I've heard guys say used oil can cause cracks or inconsistent hardness. Has anyone else run this experiment and found a sweet spot, or is it just asking for trouble?