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#160 Surface Hardening

Category : Heat treating
May24, 2013

Hardening is a heat treatment where the steel is heated to the transformation temperature (800℃ or more), then enough carbon is dissolved into the steel, and the steel is rapidly chilled in oil or water to migrate to a normal temperature while the dissolved carbon remains supersaturated within the steel. Normally the process is used for mechanical structural steels such as carbon steel and alloy steel.
In conventional hardening, the entire workpiece is heated to 800℃ or more (depending on the type of steel) inside a furnace, then rapidly chilled. The finished hardness would depend on the chilling rate.
On the other hand, surface hardening is a heat treating method where only the surface is hardened by rapidly heating the workpiece surface to above the transformation temperature, and rapidly chilling to harden (turn into Martensitic) only the surface. [Table 1] shows the types and descriptions of the surface hardening.

[Table 1] Types and descriptions of Surface Hardening
TypeHeat sourceHeating methodChilling method
Flame hardeningAcetylene gas+Oxygen, etc.Heating by open flamesWater or water soluble coolant
Induction hardeningAC,1〜500kHzInduction heating by high frequency wavesWater or water soluble coolant
Electron beam hardeningElectron beam in vacuumHeating by electron beamSelf chilling (Coolant not needed)
Laser hardeningCO2 laser, YAG laserHeating by laserSelf chilling

Among the above, high heating speeds approximately are in the order of: [1] Laser heating 105℃ or more /minute, [2] Electron beam heating 104℃ or more/minute, [3] Induction heating 102℃ or more/minute, [4] Heating furnace 1~10℃/minute. There is a world of difference compared to the conventional heating furnace method.

The flame hardening uses open flames from gas burners to rapidly heat steel surface. Normally, mixture gases of oxygen and acetylene, propane,utility gas, and etc. are used and is called flame hardening. This is typically not used for mass production since it is difficult to control the surface temperature.
Induction hardening uses cylindrically wound high frequency coils that are energized to rapidly heat the steel surface to harden, and is used for mass production of shafts and gears, etc. Higher induction frequency yields in shallow hardened surface layer, and lower induction frequency results in deeper hardened surface layer.
Electron beam hardening uses electron beams and is performed in vacuum. Therefore limitations due to the size of the vacuum chamber apply. Coolants are not used and is self chilled.
With laser hardening, high density energy such as CO2 laser can be concentrated locally, making it possible to harden extremely localized surface areas. Since it is possible to be used in the atmosphere, it can be incorporated into production lines of automotive parts and tools for rapid and continuous processes.

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