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Carbon steels that have minimum carbon content of 0.4%, or alloy steels with a lower carbon content (hardenable stainless steels with only 0.1% Carbon), can be selectively hardenened in specific regions by applying heat and quench only to those regions. Parts that benefit by flame hardening include gear teeth, bushings etc. These techniques are best suited for medium carbon steels with a carbon content ranging from 0.4 to 0.6%.

Common Selective Hardening Processes

Flame Hardening: A high intensity oxy-acetylene flame is applied to the selective region. The temperature is raised high enough to be in the region of Austenite transformation. The "right" temperature is determined by the operator based on experience by watching the color of the steel. The overall heat transfer is limited by the torch and thus the interior never reaches the high temperature. The heated region is quenched to achieve the desired hardness. Tempering can be done to eliminate brittleness.

The depth of hardening can be increased by increasing the heating time. As much as 6.3 mm (0.25 in) of depth can be achieved. In addition, large parts, which will not normally fit in a furnace, can be heat-treated.

Induction Hardening: In Induction hardening, the steel part is placed inside a electrical coil which has alternating current through it. This energizes the steel part and heats it up. Depending on the frequency and amperage, the rate of heating as well as the depth of heating can be controlled. Hence, this is well suited for surface heat treatment. The details of heat treatment are similar to flame hardening.

Laser Beam Hardening: Laser beam hardening is another variation of flame hardening. A phosphate coating is applied over the steel to facilitate absorption of the laser energy. The selected areas of the part are exposed to laser energy. This causes the selected areas to heat. By varying the power of the laser, the depth of heat absorption can be controlled. The parts are then quenched and tempered. This process is very precise in applying heat selectively to the areas that need to be heat-treated. Further, this process can be run at high speeds, produces very little distortion.

Electron Beam Hardening: Electron Beam Hardening is similar to laser beam hardening. The heat source is a beam of high-energy electrons. The beam is manipulated using electromagnetic coils. The process can be highly automated, but needs to be performed under vacuum conditions since the electron beams dissipate easily in air. As in laser beam hardening, the surface can be hardened very precisely both in depth and in location.