Surface Finishes: Heat Treatments

Heat treatment is a type of surface treatment that includes hardening and thermal diffusion. Both use heating to apply the surface treatments to the work piece. There are two types of heat treatments hardening and thermal diffusion. In this post, we will take a peek at hardening. Hardening is a critical surface treatment for durability without easily breaking. Many applications require their components to be strong and resist wear due to their big impact processes. Industries such as automotive and packaging require this special feature to their components. MISUMI offers a variety of hardened automation components such as linear shafting, spur gears and locating pins.

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 have changed into a heterogeneous material. There are two types of solid solutions, Interstitial solid solutions where atoms intrude into the other atomic space, and Substitutional solid solutions where the atoms of each other have been replaced by the other. The interstitial atoms that constitute the interstitial solid solutions must be small and nonmetal 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).

Hardening

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 treatment 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. The table below shows the types and descriptions of the 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. As you can see, there is a significant difference compared to the conventional heating furnace method.

Flame hardening uses open flames from gas burners to rapidly heat steel surface. Normally, a mixture gases of oxygen and acetylene, propane and utility gas are used. 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. Higher induction frequency yields in shallow hardened surface layer, and lower induction frequency results in a deeper hardened surface layer.

Electron beam hardening uses electron beams and is performed in a vacuum. Therefore, limitations due to the size of the vacuum chamber apply. Coolants are not used and the workpieces are self-chilled.
With laser hardening, high density energy such as a 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.

There are a variety of hardening surface treatments that can accommodate various components and applications in the automation industry. Be sure to check out the variety of hardened components on our website!

Cover photo from H&S Heat Treating

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  • Jennifer Thompson

    This article is pretty helpful for all the beginners who are looking for very basic but conceptual knowledge of heat treatments. If you’re in search of further knowledge on metallic and non-metallic diffusion, consult VaporKote, Inc.

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