Induction hardening of steel is similar in many respects to flame hardening, except the heat source is provided by induced eddy currents from a high-frequency magnetic field transmitted by a copper coil. It is a fast and repeatable process, with parts being heated and quenched in a matter of seconds. The process is compact, and readily automated. It is often in-line with the machining process as part of a cell or manufacturing line. A wide variety of parts can be processed using induction hardening, including axles, bearings, camshafts, crankshafts, gears, saw blades, and many types of shafting. Steels used are typically medium carbon steels, either plain carbon, or alloyed, depending on the strength and toughness required.
An induction heating system consists of the induction coil, alternating current power supply, and the workpiece. The coil is connected to the power supply so that a magnetic field is generated within the coil. This is schematically shown in Figure 1. The magnitude of the field depends on the number of turns in the coil and the strength of the current in the coil.
If an electrically conductive object is placed inside a coil of varying current, eddy currents are developed within the part. These eddy currents heat the part rapidly and are a result of Faraday’s laws of induction:
where e is the induced voltage, the magnetic field, φ, and the number of turns in a coil, N. The depth of penetration of the eddy currents falls off exponentially from the surface of the part. This allows the definition of a “penetration depth” where the current density has dropped by 37 percent of the surface value. The penetration depth, d, is given by:
where d is the penetration depth; ρ is the resistivity of the workpiece; μ0 is the magnetic permeability in a vacuum; μ is the relative magnetic permeability of the workpiece; and f is the frequency of the magnetic field. For steels above the Curie temperature, μ = 1. The penetration depth as a function of frequency for a steel above the Curie temperature ( and ρ = 40 x 10-6 ) is shown in Figure 2. The penetration density can be used to estimate the induction heating of the surface of steel.
Surface hardening of steels is the most common application of induction hardening heat treatment. The goal is to provide a martensitic case surrounding a more ductile ferrite-pearlite core [1]. The case depth is usually defined as the location where the microstructure is at least 50 percent martensite. Below this depth the hardness decreases rapidly, as indicated in the discussion of critical and ideal diameter [2]. This point is chosen because it shows very readily the depth of case when sectioned and etched.
Power and frequency are the two most important factors that affect case depth. There is no one choice of power and frequency that will satisfy the desired case depth. Multiple different combinations of power and frequency can achieve the same depth. For instance, a shallow case depth could be achieved by using a lower frequency, and a higher power density, applied for a shorter period to achieve the desired case depth.
If the frequency has been chosen correctly, the layer that is heated above the Curie temperature of the steel will be slightly greater than the required case depth. (Figure 3)
If the frequency is too high, additional heating or dwell time is necessary to achieve the desired case depth. Surface overheating results, and potentially excessive grain growth. Because of the high surface temperatures, decarburization and scaling can occur. Quench cracking can also occur because of the very high temperatures.
If the chosen frequency is too low, heating is deeper than required, and a deep case exceeding specification occurs. A large heat-affected zone (HAZ) occurs, with additional distortion.
For most applications, the current density is selected 1.2–2 times the required case depth. This ratio considers the heat sink effect of the large parts core [3].
The rate of heating in an induction-hardening application is dependent on the field strength, the proximity of the workpiece to the coil and the electro-magnetic properties of the material being heat treated. The heating rate of the part is primarily controlled by the coil voltage, since the field strength is proportional to the coil voltage.
Like flame hardening, induction surface hardening can be accomplished using single shot scanning by rotation, progressive, or tooth by tooth. Combinations of these methods are also common in industrial applications.
Spin hardening of gears or shafts means that small- and medium-size gears (including shafting), are placed within an induction coil and the part is rotated. This accomplishes uniform heating of the shaft and tends to average out non-concentricity within the coil. In single-frequency systems, the part is heated to the austenitizing temperature to the root circle of the gear, without overheating the teeth tips. Typically, short heating times and high specific power is required (Figure 4).
The part is then quenched using a spray or by immersion. This method produces a high surface hardness and contact strength, with high wear resistance. Tooth bending strength is high, and the gear has a favorable distribution of residual stresses (Figure 5).
The size of the gear or module size is another factor in selection of induction hardening. For most applications of spin hardening, the gear diameter is usually limited to less than 250 mm. This method is commonly used for automotive transmission gear applications.
Progressive or scanning induction hardening involves movement of the coil with a fixed part. Rotational induction hardening has a moving part and a stationary coil. Progressive scanning is used when the part is too large to rotate efficiently or safely. There are two different types of progressive scanning: vertical or horizontal. In vertical scanning (Figure 6), the part is held vertically, and the coil moves across the length. In horizontal scanning, the part is held horizontally, and the coils move along the length of the part. Sometimes, the coil is held stationary and the part moves.
The coil can move at different speeds, but usually moves at a speed of 5-25 mm/second [4]. In vertical scanning, the coil usually moves up, with a quench head below the coil. This allows the quenchant to properly cool the part without interfering with the coil. The depth of the hardening can be controlled by adjusting coil power and coil speed.
With a horizontal induction scanner, the part is held horizontally, and the part is held concentric to the coil. The quench head follows the coil. However, with horizontal scanning, the quenchant can interfere with the action of the coil, unless the quench head directs the quenchant flow away from the coil.
Tooth-by-tooth hardening is a specialized process that hardens the flanks of gear teeth individually, one at a time. Tooth hardening requires specialized coil designs and precise positioning of the coil. For individual tooth hardening, the gap between the tooth and the coil is small and must be properly centered to provide proper case depth. Over-heating of the flanks and under-heating of the root may occur. Often, water sprays are incorporated to cool adjacent teeth, or every other tooth is hardened. When every other tooth is hardened, it requires multiple rotations of the gear (Figure 7).
The design of coils is critical to achieve precision case depths. Designing and testing coils is usually the longest lead time when developing an induction heating process. Coils are task-specific and must be designed for specific results on specific materials.
Coils have a variety of different shapes and designs. In the beginning, coils were designed by trial and error, leading to lost production time. Coil designs were manually developed by tediously solving complex mathematical equations. Coil optimization was based on experimental results. Computer simulation and modeling is now an important tool for proper coil design [5].
In many cases, the complex coil designs of today can be readily modeled and tested on the computer at a lower cost and shorter time than a physical coil. With modern software, the coil can be designed, modeled, and sent directly to the machine shop floor. This allows greater coil repeatability of complex coils.
Conclusions
In this article, the basics of induction hardening have been described. As the frequency increases, the depth of hardening decreases. Because the ultimate hardness is based on the carbon content, induction hardening is usually limited to medium carbon steels.
Should you have any questions regarding this article, or suggestions for further articles, please contact the writer or editor.
References
- V. Rudnev, D. Loveless, R. Cook and M. Black, Handbook of Induction Heating, New York, NY: Marcel Dekker, 2003.
- D. S. MacKenzie, “Determining Grossman H-value from cooling curve data,” Thermal Processing, no. February, pp. 21-23, 2020.
- V. I. Rudnev, “A common misassumption in induction heating,” Heat Treating Progress, no. September/October, pp. 23-25, 2004.
- K. Berggren and L. Markegard, “Induction hardening – A quick guide to methods and coils,” Heat Processing, no. February, pp. 97-98, 2014.
- R. Goldstein, W. Stuehr and M. Black, “Design and Fabrication of Inductors for Induction Heat Treating,” in ASM Handbook: Induction Heating and Heat Treatment, vol. 4C, V. Rudnev and G. E. Totten, Eds., Materials Park, OH: ASM International, 2014, pp. 589-606.
