Induction hardening is the manufacturing process that produces a surface hardening of a conductive material by placing that material within a large, rapidly fluctuating magnetic field. The magnetic field induces a temporary electric current that heats the material but only to a rather shallow depth. The material is then immediately quenched in a bath. The heating and sudden cooling causes crystal formation within the outermost layers of the material, but the core material is unaffected and maintains its original properties. This dual nature is a key characteristic of induction hardening.
The hardening of steels and other metals has long been accomplished by heating the piece in a flame of some origin or in a furnace and then quickly dropping the piece in water or other coolant. A hardened metal does not gouge as easily, slides against other surfaces more easily and resists wear. The piece is also more brittle and could break or shatter more easily when struck or dropped. By heating only the surface, the hardness characteristic is acquired only by the surface. The remainder of the piece retains the strength of the original material.
Heating a metal or other conductive material by conductance or direct heat causes the entire piece to heat because the electrons are excited and become more mobile, quickly flowing from hotter areas to cooler areas. In induction hardening, the outer electrons are "induced" to react to the fluctuating magnetic fields by producing electric eddy currents. These currents flow in tiny circles as the electrons respond to the constantly changing direction of the magnetic field. The heat does not have a means to be conducted deeply into the material.
The type, size and uniformity of the crystals formed during the quenching step of induction hardening determine the ultimate quality of the hardened piece. The material undergoes a phase change from a solid to a crystal called a diffusionless transformation. The atoms move essentially simultaneously a very short distance. In steel, a very hard crystalline structure known as martensite is usually the desired final form of the surface layer. Martensitic crystals are also found in other hardened materials, including ceramics.
Applications that require strong but smooth, hard surfaces are ideal candidates for induction hardening. Drive-train components in automobiles, gears in many applications, tooling requiring close tolerances, molds and high-speed manufacturing operations that trim parts all benefit from the dual nature of induction hardened parts. The process is relatively inexpensive; the largest operating cost is the energy input itself. Induction furnaces run from table-top sizes to capacities that can handle major rocket components. Reproducible, high-quality results are standard in these operations.