Diagnostic Tool Helps Develop Profile for Induction Hardening Process

July 01, 2007

A high-speed infrared camera with software to calibrate, freeze and record temperatures after each heat cycle is a diagnostic tool developing a process for profile hardening gears. The method can eliminate costly and most often destructive examination.


Combining process monitoring and a diagnostic tool to develop a process for contour (profile) hardening gear teeth or tooth-like objects on parts having gear-like geometry can eliminate costly and most often destructive quality checks. The process tool is an extension of the dual pulse induction hardening (DPIH) process.

A high-value gear requires a hard wear-resistant surface with a soft core. A gear transmits torque, so its teeth are subjected to a combination of cyclic bending, contact stresses, and different degrees of sliding or contact behavior. This makes it critical for a gear to have a proper case/core structure, a condition that can be achieved using a surface hardening process, such as induction hardening, which is discussed in this article.

Contour gear hardening processes are currently performed using one or two power supplies. The process goal is to achieve a uniform case depth both at the root, as well as along the pitch diameter and the tip. This pattern uniformity is achieved by maintaining a definite range of temperatures between the tip and the root area, which depends on the geometry, pitch, and other gear characteristics.

Figure 1 illustrates the dual-pulse induction hardening process for gear profile hardening. Other known methods of profile hardening gears are dual frequency heating, variable frequency, and simultaneous frequency. A common process variable in all of these processes is that the part must be rotated at a high speed for uniform surface heating during the short heating cycle.

The profile nature of the case depths at the root area and the tip area can be adjusted for a given gear geometry by changing process variables; these are preheat (time and power), soak (time), and final heat (power and time). There virtually is no scientific method to alter these variables to predict the K ratio, defined as the ratio of the case depth at the tip of the gear to the case depth at the root (Figure 2). Typically, a process is derived by means of a trial and error method to produce a profile hardness pattern with a K ratio close to 1. Table 1 lists the values for preheat, soak, and final heat (in seconds) and the temperatures achieved for the steps in the development of a typical profile or contour hardening process.

A number of tests were conducted using the various process steps mentioned above. Samples were sectioned and the K ratios were measured. Table 2 shows case depths at two critical areas and their corresponding ratio. Sample number 15 is an ideal profile hardened sample, while number 18 has no case at the root, and sample number 20 has a through hardened gear pattern. Figure 3 hows two examples of gears having different K ratios. The gear at the top has a heavy case at the tip, similar to sample number 18, while the gear at the bottom has a more pronounced case at the tip (also known as through hardened) similar to sample number 20.

Gears are sectioned in all contour hardening processes to examine the hardened profile until achieving a profile pattern as in Figure 2. This destructive process is not only part of process development (Figure 4), but also it is used during a production run, where parts are cut at regular intervals to monitor quality.

Proposed Process Development Method
The tool used for this process is a high-speed infrared camera with software to calibrate, freeze, and record temperatures at the end of preheat, the end of soak, and at the end of final heat cycles (Figure 5). By controlling the settings (power and time), an exact heat cycle can be defined without cutting gear samples to verifying case depths. It is possible to achieve the desired profile hardening pattern in gears by monitoring the surface temperatures at specified regions. Figure 6 Figure 7 and Figure 8 illustrate the different stages of heating and soaking cycle during processing of a gear. Regions of nonuniform heating at the surface of the gear are shown in Figure 8 , which indicate that the process is far from optimum.

The DPIH process or any other gear profile hardening processes can be optimized using the proposed type of diagnostic tool, which will result in a uniform temperature between the root area and tip region. Figure 9 shows such a process developing step.

The DPIH process was used to heat treat gears at various levels of preheat time, soak time and power levels. The high-speed camera measured temperatures at four critical areas of the gear (Figure 10). By comparing the temperatures of the four areas, an optimum process setting was possible for that particular gear. The process was verified by sectioning samples at different heat treat process variables. Figure 11 shows cross sections of gears having different hardened profiles resulting from using different power settings for each gear.

The other characteristics of this process is a closed-loop system that sends a signal to the power supply to alter the process frequency (variable frequency process), or to adjust the ratios of the amount of high to lower frequency (simultaneous frequency process) at different power settings. This allows the process to be altered during the heating cycle. Figure 12 shows a series of photos at different stages during profile hardening of a gear using simultaneous dual frequency. Both the frequencies and amount of power were altered to achieve uniform surface temperatures at critical regions to achieve a uniform contoured hardening pattern.

Summary
The proposed method depends on measuring temperatures of critical areas and comparing them with established temperatures required to obtain a contoured gear pattern. Temperature profiles from these regions send signals through a closed-loop system to the power supply to alter power level, heat time, and also the frequency of the power supplies. By adjusting the frequencies of the power supplies, the power levels, and the heat times, a contoured or a profile gear pattern can be achieved (Figure 13). The process is a nondestructive method.


References:
1. U.S. Patent 4,639,279 (example); Chatterjee, M.S.

2. U.S. Patent 5, 428,208; Chatterjee, M.S., et. al.
3. GPC 2002; Dinwiddie, R. and Chatterjee, M.S.

For more information:
Madhu S. Chatterjee is Director of Special Projects, Inductoheat Inc., 32251 N. Avis Dr., Madison Heights., MI 48071; tel: 248-585-9393; fax: 248-589-
1062; e-mail: mchatterjee@inductoheat.com; Internet: www.inductoheat.com