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Wednesday, 13 November 2013

Case Hardening Process - Flame Hardening and Induction Hardening

Flame Hardening and Induction Hardening
Points : Flame Hardening and Induction Hardening, Definition Flame hardening Flame hardening is a surface-hardening method that uses an oxyacetylene flame to heat treat the surface of the metal. Flame hardening can be performed on only medium or high-carbon steel. 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 in. If the oxyacetylene flame is closer than 9116 in., 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 harden ability 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.

Induction Hardening Induction hardening is a method similar to flame hardening, with the exception that the heat is generated in the metal by an induced electric alternating current. The only metals that can be induction hardened are those that are conductors or semiconductors. The ACHF (alternating current high frequency) obtained form a pulsating magnetic field about a wire produces the heat in induction hardening. The heat results from molecular agitation induced by the electricity. The high-frequency, low-voltage, high —amperage current produce a great number of eddy currents which are primarily responsible for the heating of the metal, although hysteresis is another source of heating.

An inductor block, similar to a primary coil in a transformer, is placed around the part to be hardened. This coil does not touch the metal. A high frequency current is passed through the block or the coil and induces a sympathetic current in the surface of the metal, creating heat, a process called hysteresis. As the temperature of the metal reached the transformation range, the power is turned off, the heat source is removed, and the area is quenched, usually by a spray. The most important aspect of induction heating is its rapid action. For example, it requires only a few seconds to heat steel to a depth of 1/8 in.

The heat produced by induction is the result of both current and frequency. Higher currents produce stronger magnetic fields, while higher frequencies produce more pulsation of the field within a given time. A specific degree of heating can be obtained either by using high current at low frequencies or low current at high frequencies. However, high-frequency induction heating of metals requires a device that can convert 60 hertz (Hz) power to a high frequency of several hundred or more cycles per second. The frequency most used for metal treating applications is 450 kilohertz per second (kHz/s).

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