The standard for carbon steel is set by JIS between S10C and S75C. High frequency induction hardening is generally performed on carbon steels between S25C and S55C.
Hardness is indexed according to carbon content (e.g. S45C is HRC54 - 60) and hardness peaks above 0.6% carbon content.
Alloying elements to improve hardenability being absent makes deep hardening difficult and the resulting hardening depth is typically around 2 mm.
Cooling must be performed immediately after heating in order for hardening to occur.
Selecting the appropriate material for the application leads to increased productivity, improved quality, and reduced cost.
Steels alloy is carbon steel to which alloying elements other than C, such as Cr, Mn, Ni, Mo and B, have been added to increase mechanical strength and wear resistance. In terms of heat treatment, steel alloys are characterized by a greater hardening depth (better hardenability) than carbon steels and a greater resistance to temper softening, resulting in a tougher steel.
In high frequency induction hardening of steel alloys (SCM435, SCR435, etc.), the alloys are generally cooled more slowly than carbon steel as they have a relatively high risk of cracking due to supercooling. Particular attention should be paid to deep hardened products with uneven surfaces. Supercooling cracking of steel alloys is caused by delayed internal hardening after surface hardening.
See the following article for countermeasures.
There are three main types of stainless steel: ferritic, austenitic, and martensitic. Martensitic type can be treated through high frequency induction hardening, while austenitic type can be partially annealed to reduce hardness and improve the structure for post-processing.
As stainless steel contains high levels of Cr, achieving austenite diffusion is more difficult than in carbon steel or steel alloys. The temperature must be raised to nearly 1000℃ to diffuse the austenite.
Conversely, if the austenite is overly diffused, it will not transform into martensite when quenched and will remain as residual austenite. In high frequency induction hardening, a high level of control technology is necessary for stainless steel.
Cast metals are prone to problems in high frequency induction hardening, such as hardness variation and cracking. In high frequency induction hardening, in which the material is rapidly heated to its transformation point, the material hardness and structure distribution have a significant effect on hardening quality. Ductile cast iron (FCD) in particular, for which hardness is vital, requires a base material pearlite content of 80% or more.
In order to achieve hardness, the workpiece base material must have a balanced carbon distribution.
Uneven hardness and cracking may occur in materials with segregation from the casting process that undergo high frequency induction hardening.
While the fiber flow lines that form when materials are forged imbue forged products with more strength than those that are cut, they may become a primary factor in hardness dispersion irregularities and hardness insufficiency in the high frequency induction hardening process.
The process must be considered from start to finish, as material defects as well as residual stress and metallographic structure arising from machining can affect the high frequency induction hardening quality near the end of the process.
Thermally refined steel is steel whose mechanical properties are changed through hardening to a martensitic structure and subsequent tempering. Residual stress and grain dispersion generated in steel during rolling and forging can be adjusted. In thermal refining, the steel is hardened in the furnace and then tempered at temperatures above 550℃ to achieve a sorbite structure. Thermal refining is carried out on steel alloys with greater hardness (strength) than carbon steel, such as chrome molybdenum steel, chrome steel, and manganese steel. Thermal refining also makes the high frequency induction hardening quality more stable.
Most pre-heat treatments for carbon steels such as S45C, which are frequently used for induction hardening, are called tempering, whereby parts are heated to and held in the austenitic region and then air-cooled outside the furnace.
Spheroidizing heat treatment refers to annealing in a furnace to spheroidize the carbides in steel. It is used to facilitate machining processes (e.g. cutting) of materials such as steel alloys and high-carbon steels.
Spheroidized structures are unfavorable, however, for high frequency induction hardening: the heating time is only a matter of seconds and the spheroidized structure impedes diffusion of austenite.