Heat treating of gears is performed to achieve the necessary wear resistance and bending strength. A hardened outer layer on the gear flanks resists wear and scuffing as the mating gears slide in and out of mesh with one another. Additionally, surface hardness delays the occurrence of initial pitting and slows or minimizes the progression rate of pitting. Bending strength is dramatically improved when the root and flank area of the gear teeth are hardened. Most gear designs require root hardening, because heavy loads are generally being transferred (Fig. 1). Residual compressive stresses at the surface correlate to bending strength and fatigue life. Root bending crack propagation and other mechanical failures can be avoided with proper gear heat treatment.
Treatment using induction heating
Induction is a noncontact electromagnetic process where metal pieces to be heated are passed through a magnetic field that emanates about a copper induction coil. Current at specified frequency and voltage is passed through the coil to cause the heating, most of which is due to currents induced into the part, and some due to hysteresis. The induction heating method requires no warm-up; is fast, predictable and clean; and like resistance heating, is an in-line process, eliminating the need to store inventory between operations. Noncontact induction heating also can be readily used with atmospheres to eliminate or reduce scale.
The importance of frequency
Frequency selection is very important to obtain specified results (Fig. 2). Too low a frequency will leave the tooth tips unheated. Too high a frequency will not heat the root area, but will overheat the tips. There is a direct correlation between diametrical pitch and frequency.
There have been a number of significant developments in induction heating technology of gears. Many of these developments, focusing on materials and processes, have only come about within about the past 2 years. New processes have resulted from developments in induction power supply technology. Leading this front is SDF (simultaneous dual frequency) and VF (variable frequency).
In the case of SDF, two independent supply currents are simultaneously fed to the induction heating coil. Generally, a low and high frequency are applied. The low frequency is effective for establishing heat in the root area of the gear, while the higher frequency (determined by diametrical pitch and case depth requirements) is used to heat the outer surface layer of the gear. SDF technology is excellent because of its ability to obtain a consistent temperature profile on the heated outer layer. However, obtaining consistent temperature profile prior to quenching is only half the battle.
A consistent temperature profile must be maintained because the part is rapidly cooled. SDF, which is capable of precisely heating the root area, provides a consistent rate of heat extraction, or cooling of the outer layer or tooth surface. Rapid spray quench cooling, with a consistent temperature profile during rapid heat extraction, is key to reduction of gear distortion. This provides the opportunity for less post-heat treat processing. The SDF process can accomplish a number of patterns by independently setting the frequency and magnitude of power from each of the two power sources. Also, the process is compatible with inert gas via quench spray holes to minimize the formation of scale during heat treatment.
Variable frequency induction power units are ideally suited for manufacturers who process a broad range of gears having different diametrical pitches in both small and large quantities. In the past, gear makers would often purchase a second machine or keep a second machine on hand to eliminate the need to change frequency so often. This was due primarily because of difficulties associated with fast and reliable frequency change. Skilled set-up operators were needed to change capacitance values and transformer "turns ratio" to accommodate a different frequency.
By comparison, variable frequency power units feature internally adjustable devices that automatically function to obtain the desired level. Just as power level was treated in the past, frequency can become an element of the part recipe. For example, you can process a 4 in. (100 mm) gear with a diametrical pitch that requires 70 kHz, and then process a 4 in. gear having a finer tooth requiring a high frequency (say 110 kHz) without changeover other than selection of the correct part recipe on the equipment-operator interface. Together with the ease of frequency adjustment comes the ability to get the right frequency and the ideal pattern for each gear without compromising the routine (Fig. 3). The VF process is also compatible with inert gas via quench spray holes to minimize the formation of scale during heat treat.
When To Consider Induction
Gear design and material selection influence the decision making process on heat treatment. Very large gears that need to be tooth-by-tooth hardened will likely be made of a material having a hardenabilty well suited for flame or induction hardening one tooth at a time. Small, lightly loaded gears require wear resistance more than strength and may be best suited for nitriding. Gears processed in batches that consist of a broad variability in product features might be good candidates for induction hardening using a variable frequency power unit, or case carburizing. Most gears must be heat treated to maximize strength and minimize wear. Inexpensive, medium carbon steels are easily machined and hardened and are compatible with induction heat treat processes. (NOTE that all cutting, forming and hobbing should be done prior to induction hardening.) Whenever possible, heat-treating should be the final step.
Two methods used to heat treat gears are furnace heat treatment and induction heating. Why is induction heating often the method of choice?
- Furnaces, because of their size and heat output, are often located in remote dedicated areas of the plant away from the production line. By comparison, induction lends itself to synchronous, in-line, cell-type manufacturing. The inductor can be mounted on the line (in several places if necessary) for uninterrupted flow.
- Fewer processes are needed with induction. Induction eliminates the need for stop-off paints and copper-plating operations because selective areas of the gear can be hardened.
- Furnaces must heat treat batches of product. By comparison, induction can process one gear at a time. Individual parts are accepted or rejected based on the correct process parameters being applied.
- Induction equipment does not produce emissions.
- Induction equipment is more efficient than furnace equipment because only the area of the part to be hardened is heated (as opposed to the entire part). Also, induction does not heat ambient air, and the induction unit is idle about 75% of the time, so much less energy is consumed.
- Induction heating equipment requires no part preheating.
- Induction heating causes minimal distortion of gears.
A supplier of induction equipment needs to know some specific production information to ensure supplying the appropriate equipment including:
- Your potential short-, medium-, and long-term applications. Mention all of these to your supplier even if they are not confirmed. Induction equipment can process different types of gears (with retooling); however, the supplier must know your needs beforehand.
- Dimentions of the gear(s) to be heat treated. Drawings and process sheets are very helpful. The induction equipment supplier will use the customer's gear blueprints and specifications to determine process parameters for the application.
- Production rates. Determine how many parts need to be produced (by day, week or month). Specify the number of shifts the plant runs per day. From this data, the induction supplier can determine how many pieces need to be processed per machine cycle and calculate the power supply needed to meet this rate. The best information should include the total gross hourly production rate required of the equipment.
- Part material. Carbon content, alloying elements and prior microstructure will affect the amount of power and time required to heat the gear. Gear material having lower carbon content (below 0.2%) can be carburized prior to induction heating to speed up the heating rate in applications involving hardening. Figure 4 shows approximate heat-up times for various types of metal. Actual times will vary based on the diameter, thickness and area of the part to be heated, and prior microstructure.
- Dedicated high-volume applications are good candidates for conventional single-frequency hardening. Medium- to high-volume applications consisting of a broad product mix (i.e., different diametrical pitches) are good candidates for variable-frequency technology to facilitate fast change over.
Frequency determination starts with carefully evaluating the desired or specified pattern. Two questions that need to be answered are: (1) Is through hardening of the tooth tip desired, or is the pattern profiled along the face of the gear?; and (2) Is hardness required in the root, and if so, to what depth?
The hardness after heating and quenching (generally specified by the gear end user) will be dependent on material. Carbon content, alloying elements and prior microstructure all affect hardness. Induction processing time is measured in seconds rather than minutes, or even hours, as with most other heat treat processes, and, therefore, it is important to have a consistent prior microstucture before induction processing (Fig. 5). IH