Air vs oil vs water quenching is based upon the "Suddenness" of the quench. Water quenching an oil hardening piece is much more prone to "Cracking" than the other way around. Oil quenching a water hardening part will normally just result in a part not being properly hardened. "Kasenit" along with other carburizing/case-hardening process are designed for steels with a carbon content too low for normal quench & temper processes. This method is designed to put a "Very Hard" skin over a soft core. If you use a through hardening piece of steel & do not tamper it back to correct hardness you have a very brittle part, if you do you just lost the advantage of the case.
