Fuel oil in a metal bucket, you want the oil warm around 120 degrees to begin with and you want the part in motion like stirring paint when you dunk it and a few inches below the surface. You are really only going to shell harden this piece the center will still be a bit soft. Make sure you get the hot part below the surface quickly and you will have no flare up problems. Do it outside. I probably did an interrupted quench on 2 or 3 thousand pavement breaker tool steels this way in about 4 gallons of home heating oil without so much as losing the hairs on my forearms. If you are slick you can draw the temper with heat remaining in the tool after the first few swings in the oil but it takes some practice.