This book intended for shop use tries to familiarize the reader with the peculiari ties of a hardening method which due to its many advantages is now in use,many shops. A general knowledge of the principles of hardening and heat treating is presumed. Introduction 1. The name of the process. Flame hardening is a method derived from the old quench hardening and is used for the surface hardening of heat treatable steels. Flame hardening is so named in analogy to flame cutting as the use of a flame is a distinctive feature of this process as opposed to the use of a furnace. 2. Characteristics of flame hardening. In flame hardening the area to be 6 hardened is heated with a burner of large heat capacity (approx. 0.5 · 10 kcaljhr/ meter of flame lengths or 50,000 BTUfhr/inch of flame length) supplied with a mixture of fuel gas and oxygen. The hardening temperature is thus reached in so short a time at the surface that a heat jam is created, that is, more heat is supplied to the surface than can be dissipated to the interior of the workpiece. As the querr ehing takes place immediately after the heating the penetration of the heat to greater depth is prevented and only the outer layer subject to wear is hardened. The core of the workpiece remains unaffected by this heat treatment in cantrast to the other hardening methods where the entire piece is through heated in a furnace.