Show harden steel surface one method tor for obtaining the tha re quiren hard metal case is to treat steel in hi a both bath containing m molten olten sodium cyanide cranides cy anides another Is nit nitric rid ing 0 in the latter gaseous ammonia maitla is now lowed 1 ed across steel u under rider controlled conditions Vi nitrogen trogen in the ammonia immon lii combines with the surface layer ol 01 0 metal to form iron nurlie an an extremely hard substance heat and time of processing determine the thickness of 0 this protective case |