- Surface of component heated by high frequency induction, the frequency and power requirements depending on size and geometry of component and depth of hardening required.
- Three main types of induction machines:
vacuum tube oscillator > 500 kHz
spark gap oscillator 10–30 kHz
motor generator 5–10 kHz
- Heat-up times can be controlled to within ±0.1 s and should be less than 20 s to prevent distortion and avoid post-treatment machining or grinding.
- Process can also be used for through hardening of small components.
- Tempering and/or stress relieving can also be done by induction heating, often with the same induction coil.
- Process is easily automated and can give uniform quality and repeatability, although high capital and running costs make it most suitable for long production runs.
- Typical applications include gears, crankshafts and camshafts.
- Heating by means of oxy-fuel burners.
- Quenching jets follow burners, the quench rate being controlled by the distance from the jets to the burners and the volume of quenchant.
- Tempering and/or stress relieving flame often follows the quenching jets.
- Can be used on flat, circular or irregular-shaped components, but is best suited to circular components which can be rotated between centres.
- The steep thermal gradients required necessitate high flame temperatures, with the risk of overheating, burning and distortion, so the process is usually automated using specially shaped burners.
- Typical applications include gear teeth, brake drums, axles, cams and crankshafts.
- Ferrous metals whose composition is such that the required hardness will be obtained on quenching from the austenite region: a minimum of 0.3% C. Most materials have at least 0.4% C, and up to 0.7% C.
- Plain carbon steels, alloy steels and grey, nodular and malleable cast irons; the temperature depends on the material:
750–760° C 0.5% plain carbon
900–800° C chromium–molybdenum alloy steels
950–980°C nodular iron
- Alloy steels, although more expensive, produce greater case depths.
- The higher the carbon content, the greater the risk of cracks and/or distortion.
- Flame hardening is not usually suitable for steels with core strengths of greater than 900 MPa.
- Required core strength must be obtained by appropriate heat treatment before hardening.
- To enable carbides to go into solution during the short heating times, the carbides should be small, hence normalised structures are preferred.
- Fine grains give better control over the case depth and less chance of cracking and/or distortion.
- Very little grain grown occurs, due to short heating times.
- Little or no decarburising or oxidation due to short heating times.
- Surface hardnesses depend on composition, temperature and quenchant, but are usually in the range 350–700 Hv.
- Processes are best for cylindrical parts such as crankshafts, gears, axles and brake drums.
- Large gears can be treated one tooth at a time, and hardness pattern depends on method employed.
- Long thin components may require pre-heating.
- Flame hardening can be done on small selective areas.
- Case depths vary from 0.1 to 10 mm, depending on requirements. Induction hardening is used for small case depths which require smaller tolerances.
See Also: Ion implantation, Nitrotec process, Carbonitriding/carburising, Plasma nitriding/carburising, Chemical vapour deposition (CVD), Physical vapour deposition (PVD), Electroless plating, Electroplating and Toyota diffusion (TD).
This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.