Induction Hardening – Process, History and Advantages

Induction hardening is a type of heat treatment in which metal parts are heated by electromagnetic induction and then quenched. It is also a type of case hardening and can be used for many steel and steel alloys to improve mechanical properties such as surface hardness and fatigue strength and resistance.

Typical applications of induction hardening is the hardening of steel and cast iron. Especially parts, such as drive shafts, engine components and gears undergo the process.

Induction Hardening Process

Induction Hardening can be split into two steps. The first one is induction heating, in which electrically conducting metals are heated with an electromagnet. The quenching phase follows directly after to alter the surface structure of the material.

Induction Heating

Materials such as steel components are typically placed inside a water cooled copper coil where they are subject to an alternating magnetic field. They undergo electromagnetic induction by means of an electromagnet and an electronic oscillator. This oscillator sends alternating currents through the electromagnet, causing alternating magnetic fields that penetrate the material. The results are eddy currents (loops of electrical current) which heat the object within the coil above the transformation temperature. Induction hardening is a form of surface hardening in which the depth can be up to 8 mm. The deeper the currents penetrate, the higher the frequency of the alternating magnetic fields have to be.

Steels that have a ferromagnetic structure (which is inherited from the iron) can also be heated by magnetic hysteresis losses. Magnetic hysteresis losses produce heat by re-aligning magnetic domains, although it depends on the frequency of the currents, the penetration depth and the properties of the material (size, density, alloys) how much heat can be generated (Ph = (kh )(f)(Bmax)n).

Quenching

Directly after the induction heating process, the object has to be quenched, meaning that it has to be cooled down extremely quickly. To do that, the workpiece is typically placed in a tank of oil or water, although sometimes cold air is used. Quenching ensures that only the surface is hardened and that heat doesn’t spread into the core of the material, avoiding phase transformations from arising. Furthermore, the rapid cooling down creates a martensitic or ferritic-martensitic structure on the surface layer. These structure display higher tensile strength and low initial yielding stress than a purely ferritic structure. Quenching also reduced grain size, which is a key factor to increasing hardness of materials.

History

Induction heating was first developed and introduced in its earliest form in 1831 by Michael Faraday. He could prove that an electromotive force could be created by winding two copper coils around a magnetic core while turning one of the windings on and off, which affected the other one. These currents were created by alternating magnetic fields around the magnetic core. Because neither of the coils touch, the electromotive force is induced into the second coil, the process was called induction heating.

Advantages of induction hardening

  • Deeper Case Depth: Induction hardening can penetrate the surface of up to 0.31 inches (8 mm). This depends on the process of induction hardening and the material’s properties.
  • Finer Grain Size: As mentioned above, induction hardening changes the grain size on the surface of materials. A finer grain size increases hardness because the surface is harder to penetrate.
  • Higher wear and fatigue resistance: Induction hardening improves wear resistance because the structure of the surface layer is altered. Ferritic steels obtain a martensitic structure, which provides improved wear resistance.

Induction hardening is a good alternative to boronizing, which is a type of surface hardening BorTec specializes in. Induction hardening is recommended when mechanical properties, such as hardness, should be improved. If you’re looking for a treatment that can also improve adhesion, resistance against abrasive wear, good stability at high temperatures and resistance against acids, the certified BoroCoat treatment is the better choice. However, it depends on the area of application and other factors which hardening technique is more suitable for your needs. Contact us if you need any help deciding.

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