May 2013 Archives

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

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.

Surface heat treatment

Heat treatment is a technology that has been developed in metallurgy from ancient times. Here, Surface Heat Treatment which is focused on surfaces among the heat treatments.
Some fixtures and tools do not reach the hardness required by the normal hardening and annealing processes, thus they will have short product lives. In order to solve this problem, surface heat treatments are applied.

Summary of surface heat treatment

Surface heat treatment is defined by JIS as "Heat treatment processes to give required properties to metal product surfaces", and can be classified into Surface Hardening, and Thermal Diffusion Treatment as shown in [Table 1]. Both use heating to apply surface treatments, and inevitably, atomic diffusion phenomena are occurring on the surface of the treated material.

[Table 1] Classification of surface heat treatments

In general, metals are solid solutions, and ceramics are compounds. A solid solution is where multiple elements are blended into a single phase. A compound is where multiple elements do not blend but has changed into a heterogeneous material.
There are two types solid solutions, the Interstitial solid solution where atoms intrude into the other atomic space, and Substitutional solid solution where the atoms of each other have been replaced by the other.
The interstitial atoms that constitute the interstitial solid solutions must be small enough, and are non-metal such as nitrogen and carbon. Both elements of the substitutional solid solutions are metal.
The compounds are formed by non-metals diffusing on metals. For instance, oxygen (O) diffuses on titanium (Ti) forming Titanium oxide (TiO2), Titanium carbide (TiC) if its carbon (C).