Tempering after quenching is commonly used to increase the stiffness and toughness of steel, eliminate internal stress, improve dimensional stability and uniformity, but has little effect on the hardness of steel quenching.
When hot steel is chilled, its microstructure changes into hard and brittle martensite, a quenched martensite that is too brittle to be used directly and has high internal stresses. After quenching, the internal stress can be reduced or relaxed and the tempered martensite structure can be obtained. The tempering temperature is always below the phase transition temperature (A1).
The traditional tempering method of induction hardening parts is carried out in the oven, gas furnace, or infrared furnace. These equipment are usually installed in other places of the workshop, resulting in a large amount of manpower, material resources, and time waste in the process of parts transportation and stacking. In addition, tempering in the furnace often takes 2-3h to complete. Short-term induction tempering can overcome these shortcomings.
The basic method of induction tempering
In a short – time induction tempering, heating time, and temperature are two key parameters. Induction tempering at higher temperatures can achieve the same effect as conventional tempering at a lower temperature. There are several time-temperature relationships between short-time induced tempering at high temperature and long-time conventional tempering at low temperatures, such as Hollomon- Jaffe equation and Grance-Baughman tempering.
The induction tempering temperature range is usually 120~600℃, if the induction tempering temperature of carbon steel is less than 100℃, the tissue will not change. The low temperature tempering of carbon steel (120-300℃) is mainly used to reduce the internal stress, while the hardness reduction generally does not exceed 1~2HRC. If carbon steel is tempered above 600℃, the microstructure changes significantly, resulting in a decrease in the hardness of the large interval, which can exceed 15HRC, and the maximum hardness decreases to 36~44HRC. For alloy steels, tempering above 600℃ may not result in a significant reduction in hardness.
Temper always both hardness and internal stress, and toughness, such as people on both sides of the conflict, due to the elimination of internal stress is an important goal of tempering, therefore, must first understand how internal stress is produced when induction hardening, the formation mechanism of residual stress at this time and some other heat treatment process such as the stress of carburizing, nitriding mechanism is different. There are two kinds of stresses in induction heating: thermal stress caused by different temperature values and temperature gradients, and phase transition stress caused by the transformation of tissues, such as austenite, bainite, and Martensite. The total stress is the superposition of these two stresses. The role of each stress in the total stress changes as the heating process progresses.
Tempering induction coil
Induction tempering can be used for parts that cannot be self-tempering. In general, the same induction coil (inductor) cannot be used for both quenching and tempering because:
1) For induction hardening, in order to make the shape of complex workpiece to reach the required hardness distribution pattern, the electromagnetic field must be redistributed to make a local area to obtain more energy. The tempering sensor is typically designed to heat regions much larger than the hardened zone, or even entire workpieces. For this purpose, a weakly coupled multiturn coil can be used.
2) The energy density used for hardening is much higher than that used for tempering. During tempering, the surface must be heated at a very slow rate to form a temperature gradient from the “gentle” surface to the depth of the hardened layer. Too high energy density will result in the workpiece surface temperature exceeding the best tempering temperature, making the workpiece surface hardness too low.
3), Unlike the hardened coil, the tempering coil does not require a magnetic conductor.
4) Lower frequency should be used when tempering, because tempering temperature is always lower than Curie point, at this time the workpiece is in the magnetic state, skin effect is very obvious. When the same heating frequency is applied, the depth of the heating layer (penetration depth) during tempering is much less than that during induction quenching (even in the magnetic stage of quenching). This is because the permeability of steel is 10 times higher when tempered than when hardened. High permeability leads to a decrease in penetration depth, and permeability is related to factors such as frequency, magnetic field strength, temperature, steel composition, and grain size.