A heat-treatment that transforms the steel into a mixture of lower-bainite (the tough and hard kind) and martensite (which it already was). Since through-hardened triple-tempered 5160 is insanely tough as it is, it was really just a waste of propane and electricity, but it's my waste of propane and electricity and now it's even tougher because its time spent in molten salts. To quote your neighbors: