Originally Posted by bja
What metals are you talking about specifically?
Mild steel like what the OP's oil drum is made of and a few steel alloy's used to make engine parts. It happens faster with rough surfaces.
My boss at the time was making a crank turning tool and couldn't get an old timing gear clean enough to weld on so he warmed (not red hot) the gear up with the cutting torch. The oil caught fire cause he did not remove the heat soon enough. Up to that point I never knew anything like that was possible on a machined surface.