For a while now I am trying to harden 4140 for use under the power hammer. I use 3/4 x 3 x 8 blanks that I bought for a blacksmith magician. I forge them to shape, soak them in my propane forge (with a little excess propane to not produce scale) for an hour, quench and then temper. For quenching I tried old motor oil first and for tempering my wife's kitchen range (400 F). The resulting tool did not hold an edge for butchering one tenon and the end mushroomed. I later heard that motor oil is no good and changed to 3 gallons of canola oil: Same result. Then I added 1 third Diesel fuel to my canola oil to reduce viscosity: still not hard. Finally I thought, even if it cracks: lets water quench. It did not crack, but the resulting tool was very soft again (for tempering this time used a knife makers kiln at 400 F. My tool, a feather swage for power hammer use had lost its nice surface after one leaf.