Powder-metallurgy (PM) technology offers a cost-effective means to produce complex near-net shape automotive parts, such as splined hubs, cams, sprockets and a wide variety of gears. These parts usually require hard, wear-resistant surfaces in certain locations, while maintaining the ductility of the sintered matrix in the core of the part. The method most commonly specified to achieve this combination of material characteristics is induction hardening, which is a cost-effective hardening treatment when high volumes of parts are being produced. The capability to use induction heating enables faster production, consistent uniform hardening, negligible part distortion and significant cost and space reductions.
The method most commonly used to increase abrasion resistance and fatigue strength on gears and sprockets is a modified classic induction heating technique. However, the recently developed simultaneous dual-frequency (SDF) heating method (medium and high frequency power introduced at the same time) can accomplish better results, but requires less energy, less floor space, and permits a very high production rate as a result of the short heating cycles. Simultaneous dual-frequency heating can easily be integrated into virtually any manufacturing process and opens the door for continuous start-to-finish production lines. Furthermore, the equipment used to produce simultaneous dual frequency can be used to temper the part after hardening by subsequently applying only medium frequency power to the part while still placed in the coil. This translates into less initial investment for equipment and material handling to create a more efficient manufacturing process. Additionally, parameters for hardening can be documented for each work piece and integrated into existing quality control systems. Finally, treated part distortion or run out is far smaller when applying the simultaneous dual frequency method due to its extremely short austenitizing times resulting in heating of smaller volumes of the part.
Classic Induction Hardening Methods
Powder-metallurgy sprockets and gears made typically are inductive hardened using high frequency to increase their resistance to wear and to increase their strength in the root area of the tooth. However, in an effort to ensure that the root is properly hardened, the tooth usually ends up being through hardened in the process. Heating times of approximately 5 seconds generally are used at a medium power density of around 1.5 to 2 kW/mm2 in this classic high-frequency method, which results in through hardening of the teeth. In addition, this process can create residual tensile stresses in the tooth-root area. The risk of these tensile stresses increases proportionally with the hardening depth in the root area. Eventually, the tensile stresses can become so great that cracks develop. Methods used to avoid cracking in the sprocket during the hardening process include preheating the part, soft quenching (in oil, for example) and tempering immediately after hardening. In extreme cases, a combination of all of these methods is necessary.
Quest for true contour hardening
Gears and sprockets are induction hardened to increase quality, improve efficiencies and cut costs. What makes the technology attractive to manufacturers is the ability to surface harden the parts in a matter of seconds, realized savings on energy and reduced floor space required. Research efforts to advance the technology led to the understanding that two separate frequencies (a medium frequency of 10 to 25 kHz and high frequency of 200 to 900 kHz) are required to properly and quickly harden a gear. However, the contour shape of the gear or sprocket creates a conflict between the two frequencies when they are used separately to harden it.
For example, if the sprocket is to be hardened using only high frequency, the tooth tip is austenized first, followed by austenitization of the root area. Thus, the resulting depth of hardening of the tooth tip is too great, giving a depth ratio to the root of about 5:1 instead of the desired ratio of 2:1 (Fig. 1).
Conversely, if the same sprocket is hardened using only medium frequency, the root area of the tooth is austenized first, followed by austenitization of the tip of the tooth, resulting in the entire tooth being through hardened (Fig. 2).
These results suggest that the solution to achieving the desired hardening of sprockets and other similarly shaped parts is to take advantage of the attributes of both frequencies to properly harden the contours of the part. Simply put, it is necessary to use high frequency to harden the flange and tip of the tooth and medium frequency to harden the root.
With true-contour hardening (that is, the hardening of only the sprocket surfaces at a prescribed depth without the through hardening of the teeth), it is possible to eliminate the negative tensile stresses and instead, manufacture a stable part having desirable compressive stresses (Fig. 3). However, the following conditions must be satisfied to achieve this objective:
- Use a heating time of less than 0.5 seconds to avoid through hardening of the tooth
- Use a relatively high power density (15 to 20 kW/mm2) for short hardening times
- Simultaneously heat the root area, tip and flanges of the tooth to properly harden the contour of the part
Currently, the only way to achieve all of these conditions is through the use of simultaneous dual frequency hardening, where the benefits of both frequencies are used in concert to produce the required results.
Simultaneous dual frequency heating is a process that uses one common output circuit with one common inductor, efficiently using both medium and high frequencies concurrently. In the process, the medium frequency becomes the ground wave with its amplitude fully controllable. This provides the primary oscillation pattern with the high-frequency oscillation superimposed on it with its own independently controlled amplitude. The medium frequency will operate at around 12 kHz, and the high frequency (depending on the size of the inductor used) will operate between 350 kHz (small inductors) and 200 kHz (large inductors). The medium frequency heats the root area of the tooth and the high frequency heats the flanges and tip of the tooth. The power ratio between the medium and high frequency can be independently set to a power output level ranging from 1:99 to 99:1. In this way, the depth of root hardening can be individually controlled independent of the depth of tip and flange hardening, all without risking through hardening of the tooth itself.
Root hardening is solely dependent on the input of medium-frequency power, and its application is made independent of the high frequency used to harden the flange and tip. Depending on the dimensions and contours of the sprocket being hardened, the typical times for simultaneous dual frequency hardening range between 0.2 and 0.4 seconds.
Furthermore, hardening can be accomplished without quenching when the part has a sufficient density (that is, 7 g/cm3, or 0.253 lb/in.3, min) and good hardening characteristics. Using this mass quenching, the need for tempering often is eliminated. For sprockets requiring additional surface hardness, special polymer quenchants can be used to obtain the required hardening. A benefit of using dual-frequency equipment is that this subsequent tempering can be done immediately after simultaneous dual frequency hardening with the part still positioned on the same spindle. In this mode, the power settings will use exclusively medium frequency. This is a great saving in material handling and equipment needed, thereby reducing overall costs.
Several examples of PM parts processed using the simultaneous dual-frequency hardening method are shown in Figs. 5-7. The key observation is how the hardened zone perfectly conforms to the contours. All parts have been hardened in fractions of seconds.
Initial testing of the simultaneous dual-frequency heating method is extremely encouraging and the areas of application are continually expanding. True-contour hardening of complex shaped components has been achieved for the first time. These developments have paved the way for future savings on a variety of components. SDF has moved induction surface hardening into the next generation, using an optimal frequency mix of medium and high frequency combined with high-power output and short processing times to achieve extremely high production rates with negligible distortion.
We are only at the beginning of what this new technology is capable of accomplishing. While it is ideally suited to powder-metallurgy applications, it also can be applied to stampings, castings (gray and nodular irons) and forgings. Additionally, it works well with numerous grades of carbon and alloy steels. Only our imagination limits our exploration for ways to apply this new technology to existing manufacturing processes. This method will allow manufacturers to consistently produce high quality parts at high production rates and at substantially lower costs, providing true-contour hardening of the tip, flange and root of each tooth.