Principle of Induction Hardening

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Induction hardening of the surface is a type of heat treatment that utilizes electromagnetic induction, the skin effect, eddy currents, and resistive heating to rapidly heat and then cool the surface of a workpiece.

During induction hardening of the surface, the workpiece is placed inside an induction coil made of copper, and when a certain frequency of AC current flows through the coil, the workpiece in the alternating magnetic field induces electric current. Due to the skin effect and eddy currents, high-density AC current generated by the workpiece's surface causes resistive heating, rapidly heating the surface to the quenching temperature. The surface is then quickly cooled by water spray, resulting in a hardened surface.

During induction heating, the distribution of induced current on the workpiece's cross-section is dependent on the current frequency. The higher the frequency, the more pronounced the skin effect, and the thinner the surface layer where the induced current is concentrated. Therefore, by adjusting the current frequency, different depths of hardened layers can be obtained.

Induction Hardening versus Flame Hardening

Surface heat treatment is done by changing the surface structure of the part to obtain high hardness martensite while retaining core toughness and ductility (i.e., surface hardening), or by simultaneously modifying the surface's chemical composition to obtain corrosion resistance, acid and alkali resistance, and a higher surface hardness than the former method (i.e., chemical heat treatment).

Induction hardening: The induction heating speed is extremely fast and takes only a few seconds or a few dozen seconds. The hardened martensite structure on the quenched layer is fine, and mechanical properties are good. The workpiece surface is not easily oxidized and decarburized, with small deformation, and the depth of hardened layer is easy to control, with stable quality and simple operation. It is particularly suitable for large-scale production.

It is commonly used for medium carbon steel or low-alloy steel parts, such as45, 40Cr, 40MnB, and so on. It can also be used for high carbon tool steel or cast iron parts. Generally, a good combination of strength, fatigue resistance, and toughness can be obtained when the depth of hardened layer is about 1/10 of the radius. Induction heating surface hardening is not suitable for complex-shaped parts due to difficulties in making the induction coil.

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Advantages of Induction Hardening

• The surface hardness is 2-3HRC higher than that of ordinary quenching and is less brittle;

• Fatigue strength and impact toughness are both improved, and general parts can be improved by 20%-30%;

• Deformation is small;

• The depth of hardened layer is easy to control;

• No oxidation or decarburization occurs during induction hardening;

• Lower quenching and tempering steels can be used;

• It is easy to realize mechanization and automation during operation, and production efficiency is high;

• The hardness and thickness of the hardened layer can be adjusted by varying the frequency of the induction current.

Induction Hardening / Flame Hardening

Induction Hardening

Induction hardening is a process used for the surface hardening of steel and other alloy components. The parts to be heat treated are placed inside a water cooled copper coil and then heated above their transformation temperature by applying an alternating current to the coil. The alternating current in the coil induces an alternating magnetic field within the work piece, which if made from steel, caused the outer surface of the part to heat to a temperature above the transformation range. Parts are held at that temperature until the appropriate depth of hardening has been achieved, and then quenched in oil, or another media, depending upon the steel type and hardness desired. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined or preheat treated. The hardness of the case can be HRC 37 - 58. Carbon and alloy steels with a carbon content in the range 0.40 - 0.45% are most suitable for this process. In some cases, parts made from alloy steels such as , or , like steel and paper mill rolls, are first carburized to a required case depth and slow cooled, and then induction hardened. This is to realize the benefit of relatively high core mechanical properties, and surface hardness greater than HRC 60, which provides excellent protection.

While induction hardening is most commonly used for steel parts, other alloys such as copper alloys, which are solution treated and tempered, may be induction hardened as well. Applications include hardening bearing races, gears, pinion shafts, crane (and other) wheels and treads, and threaded pipe used for oil patch drilling.

Flame Hardening

Flame hardening is similar to induction hardening, in that it is a surface hardening process. Heat is applied to the part being hardened, using an oxy- acetylene (or similar gas) flame on the surface of the steel being hardened and heating the surface above the upper critical temperature before quenching the steel in a spray of water. The result is a hard surface layer ranging from 0.050" to 0.250" deep. As with induction hardening, the steel component must have sufficient carbon (greater than 0.35%). The composition of the steel is not changed; therefore core mechanical properties are unaffected. Flame hardening produces results similar to conventional hardening processes but with less hardness penetration. Applications for flame hardening are similar to those for induction hardening, although an advantage of flame hardening is the ability to harden flat surfaces. Flat wear plates, and knives can be selectively hardened using this process.

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