Flame hardening

          Flame hardening is a surface-hardening method that uses an oxyacetylene flame to heat treatment the surface of the metal. Flame hardening can be performed on only medium or high carbon steel or cast iron. When flame hardening is applied to steels with over 70 points of carbon, extra care must be used in order to prevent surface cracking of the high-carbon steel.

          The process is based on the rapid heating of the outer surface of a ferrous metal to or above its transformation temperature.The minimum distance that the oxyacetylene flame must be held from the base metal is approximately 9/16 inches. If the oxyacetylene flame is closer than 9/16 inches., the base metal will be deformed by the flame. The metal is moved rapidly under the flame, allowing the flame to heat the base metal only on the surface. This surface heating creates two heat-affected zones, a primary heat-affected zone, where the transformation of the metal has taken place, and a secondary heat affected zone, where grain growth has been developed or where the grains have been enlarged or decreased by the application of heat to the base metal. Immediately after it is heated, the metal is subjected to a quenching spray, generally water, that hardens the metal area that has undergone transformation.

          The depth of hardness depends entirely on the hardenability of the material being treated since no other elements are being added or diffused, as in case hardening. With proper control, the interior of the metal will not be affected by this process. Often an average application of this process involves heat treating a complete piece to a certain specified softness or toughness. The exterior then may be flame hardened so that the finished piece resembles an item that has been case hardened. It is quick and the hardening is restricted to parts which are affected by wear.

          Fig shows, a flame hardening of gear teeth. A flame from an oxy-acetylene or similar burner is played on to the teeth so as to arise temperature rapidly above the hardening temperature. Hardening results when the austenised surface is quenched by spray that follows spray.