The most recent #Sachgeschichte (about chains) claims that the cooling bath for hardening is water (it's surely oil: water would visibly boil and Leidenfrost effect would make cooling insufficient).
I've used WDR's contact form to tell them that and we'll see what happens.
@robryk Clearly you'd need to conduct an experiment. ;)
Or maybe I'm wrong, Leidenfrost causes it not to violently boil, and the colling is fast enough for their purposes?