An induction process consists of an induction power supply that passes a high-frequency alternating current (AC) through an inductor. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flowing through the resistance of the material heat it by Joule heating.
The frequency of current used depends on the object size, material type, coupling between the work coil and the object to be heated, and the penetration depth. Determining the process parameters of an induction hardening process can be very complex and challenging. I will attempt to break this process into steps to better understand what is required to identify the optimal induction process for any given application.
Is the part a good candidate?
Material composition and pre-treatment must be conducive of heat treatment to the required heat-treat specification. The heat-treat specification must be accommodating to the induction heating process. The part geometry needs to provide adequate clearance to present an inductor coil and generate the required induction field to achieve specification.
These parameters must be considered to determine the feasibility of any given application. Figure 1 shows various part samples successfully processed by induction heating. These samples have been cut and etched to reveal the hardened areas.
There are two primary methods of heating parts by induction: scanning and single shot. Scan hardening is commonly used to heat treat long, uniform parts. The part is rotated and passed through the center of the inductor with the power turned on at a controlled rate of travel.
At the inductor, a controlled band of heating is generated into the surface of the part (Fig. 2). As the part continues to travel, it moves into a liquid shower quench. The energy required to scan harden can be significantly less than single-shot hardening because you are only heating a small portion of the part at any given time.
Single-shot hardening heats and quenches the required hardness pattern all at once and is often the only method available for parts with complex geometries (Fig. 3).
There are simple calculations that can be used to estimate the power requirements for a given process. First, it is necessary to calculate the square inches of the surface area to be heat treated. For single-shot hardening, this would include the total square inches of all surfaces to be heat treated. Single-shot hardening typically requires approximately 15 kilowatts (kw) per square inch of surface area for a successful heat treat (e.g., 10 square inches x 15 kw = 150 kw required).
For scan hardening, it would be the surface area of the heating band, which is the largest circumference of the part multiplied by the face width of the inductor coil. Scan hardening typically requires approximately 10 kw per square inch of the heating band surface area (e.g., 2.00-inch-diameter shaft x 0.5-inch inductor face width = 3.14 square inches x 10 kw = 31.4 kw required).
The reason less power is required for scan hardening is because the power is turned on and the part dwells in place for a short period of time before scanning begins. This dwell is necessary to allow time for the heat to build up and penetrate into the part to achieve case-depth requirements at the start position. This dwell also allows thermal conduction to move up the part and acts as a preheat. As the part starts to move from dwell through the inductor, it already has some heat in the surface ahead and requires less energy to achieve the target temperature.
Common frequencies available for selective hardening vary from as low as 1 kHz to as high as 450 kHz. Most of the induction equipment available today falls into one of three different categories.
- Low frequency (1-8 kHz) is typically used for deep hardness specifications of 0.100-0.400 inch (2.5-10.0 mm) case depth.
- Medium frequency (8-100 kHz) is typically used for medium hardness specifications of 0.050-0.100 inch (1.3-2.5 mm) case depth.
- High frequency (100-450 kHz) is typically used for shallow hardness specifications of 0.015-0.050 inch (0.4-1.3 mm) case depth.
Sometimes the equipment available is not ideal for a given application. Figure 5 illustrates a case study run in our induction laboratory that shows two cross sections of a cross roller bearing that have been acid etched to show the heat-treat pattern.
These parts were both heated with the same power at the same scan rate using the same quenching method. The variable that changed was the frequency. The sample on the top was run at 15 kHz, and the sample on the bottom was run at 8 kHz. You can see a significant difference in the case depth at the valley of the groove. Although the difference in the frequency range may not seem substantial, you can see what a difference frequency can make to any given process.
The proper inductor design and size are essential to achieving good repeatable results in any given process. Figure 6 shows various sizes of scanning inductors designed to scan harden the outer diameter of round shafts. The quench shower is machined into the inductor to provide optimal quenching. The sizes vary based on the size of the parts to be processed.
These inductors offer flexibility to run different parts of similar size. However, this does come with limitations. As the gap between the outside of the part and the inside of the inductor increases, the induction field generated by the inductor may not couple to the part efficiently and may suffer side effects. These side effects may include fuzzy or not fully transformed structure.
Spotty hardness can also occur due to the field not wanting to remain coupled with the part. The field may jump and lead to bands of hardness. Figure 7 is an example of a single-shot inductor with machined integral quench to heat treat two independent race areas simultaneously on an automotive wheel-bearing outer-race hub. Single-shot inductors are typically dedicated to one unique part geometry. It is very important to keep the inductors clean and free of debris and buildup because contamination is the leading cause of premature inductor failure.
Once a part is heated to a desired temperature, it needs to be cooled rapidly to achieve proper hardness transformation and structure. This cooling process is typically achieved by liquid quenching with a water-polymer solution. If the quenching is not sufficient, hardness and structure in the part may not reach specification. If the quenching is too aggressive, cracking may occur.
There are variables that may contribute to the quenching effect that must be considered. The physical mass of the part behind the area of the part being heated can help remove some, if not all, of the heat in a given application. If this mass is great enough, it may offer enough heat removal to fully transform the structure without the use of additional liquid quench.
Polymer concentration can also be adjusted to retard or make the quench more aggressive. The position of the quench shower in scan-hardening applications can be positioned farther away from the heating zone to delay the quenching action. In single-shot applications, a dwell can be added between the power off and quench on to delay the quenching action as well.
Cycle time from part to part can vary greatly depending on process, material handling and other factors. Automation can improve cycle times, but it is often common practice to run multiple parts at once in high-volume applications (Fig. 8) if enough power is available.
Once testing parameters for a given process have been established, part samples need to be processed and then submitted for metallurgical evaluation. There are commercial labs offering these development and metallurgical services. A typical development program yields a fully characterized inductor, process parameters, validated ISO 17025-compliant metallurgical report and additional prototype pieces for testing (Fig. 9).
With any complex process, it requires some knowledge and some experimentation to achieve success. To quote Wernher von Braun (1912-1977), rocket engineer and designer: “One test is worth one thousand expert opinions.” I hope this article has provided knowledge to those looking to implement induction heat treating to their process or improve their existing process.
For more information: Contact David Lynch, vice president, engineering for Induction Tooling, Inc., 12510 York-Delta Drive, North Royalton, OH 44133; tel: 440-237-0711 Ext. 14; e-mail: firstname.lastname@example.org; web: www.inductiontooling.com.