Understanding the Role of Frequency in Preventing Gear Ring Cracking

1. Preface

Wind power is the most widely used and rapidly growing renewable energy technology. It has been adopted on a large scale globally.

In 2019, China installed 25.74 million KW of wind power, accounting for 10.4% of the total installed capacity.

Wind power generation reached a record high of 405.7 billion KW · h, surpassing 400 billion KW · h for the first time and contributing 5.5% to the total power generation.

Wind power has become a crucial part of the energy mix in some countries.

As the global focus shifts towards renewable energy, wind power will play an increasingly important role in the future energy system.

The gearbox, a crucial component of wind power generation, includes an internal gear ring that is a key part of the gearbox’s star transmission mechanism.

To ensure long-term durability, the internal gear ring production and manufacturing process no longer relies solely on simple quenching and tempering as the final heat treatment.

Surface strengthening of the gear ring is primarily achieved through medium frequency induction quenching and nitriding, while some manufacturers use carburizing quenching as a surface strengthening method.

Medium-frequency induction hardening has several advantages, including short cycles, high efficiency, no decarburization, minimal deformation, low production costs, and ease of automation. This method is particularly useful for high-power gearboxes with a gear ring module greater than 16.

However, nitriding is no longer a suitable option as the nitriding layer is too shallow, while carburizing results in greater heat treatment deformation and larger grinding amounts for large module gear rings. In comparison, medium-frequency induction quenching provides better surface strengthening for large modulus gear rings than nitriding and carburizing.

Although induction hardening has some technical limitations, such as the presence of a heat-affected zone and quenching cracking. These issues can be addressed through improvements in the material’s hardenability and the cooling medium used during quenching. Additionally, the optimization of process parameters can help prevent quenching cracks.

There are several factors that affect induction hardening, including thermal parameters (heating temperature, heating time, and speed) and electrical parameters (frequency, surface power, and current).

This article focuses on the role of frequency in preventing cracking in gear rings through medium-frequency induction quenching by examining the impact of various process parameters.

2. Test materials and methods

2.1 Product materials and key parameters

The material used for a specific type of wind power gear ring is 42CrMo4 steel with a modulus of 17, 96 teeth, a helix angle of 8°, and an induction hardening layer depth ranging from 3.2mm to 4.2mm, with a single tooth induction hardening.

After the induction hardening process, the quenching layer depth at the pitch circle was found to be 3.4mm, with a surface hardness of 56 HRC.

The production process for the gear ring involves the following steps: ring rolling and forging, rough turning, quenching and tempering, gear milling, induction hardening, finish machining, and gear grinding.

Please refer to Table 1 for the process parameters of the induction hardening process.

Table 1 induction hardening parameters

Due to the depth of the hardened layer, cracks are prone to occur after induction hardening (as indicated by the arrow in Fig. 1), and they tend to be located at the top of the teeth.

From the appearance of the cracks, it is evident that the openings are wide and cannot be eliminated in further processing. The only solution is to scrap the material, resulting in significant economic losses.

To address this issue, the inductor design is not modified as the hardening layer depth meets the technical requirements. Instead, the optimization of electrical parameters through combination testing is carried out.

Fig. 1 crack morphology of induction hardening tooth top

2.2. Test method

(1) Verify the Accuracy of the Quenching Machine Tool

The accuracy of the quenching machine tool plays a crucial role in determining the clearance between the inductor and the tooth surface of the gear ring. If the accuracy is not up to par, some areas may have insufficient clearance, resulting in high heating temperature during induction quenching, excessive quenching stress, and ultimately, quenching cracks.

After evaluating the perpendicularity, roundness, and flatness of the quenching machine tool beyond the full tooth height range, the results indicate that the perpendicularity, roundness, and flatness are 0.09mm, 0.12mm, and 0.06mm respectively.

However, the accuracy of the quenching machine tool is within the acceptable range, and therefore, cannot be considered as the cause of the cracks on the tooth top.

(2) Examination of Raw Materials and Forging Process

An investigation was conducted into the smelting method used by the steel plant producing the raw materials and the re-inspection report from the factory.

The factors that could lead to quenching cracks, such as chemical composition and inclusion grade, were analyzed, and no anomalies were detected.

(3) Testing of the Induction Hardening Process

Since no issues were found with the accuracy of the quenching machine, raw materials, and forging process, optimizing the induction quenching parameters is a more feasible solution.

The induction quenching parameters mainly consist of thermal parameters and electrical parameters. The factors that affect heating include the heating power, scanning speed, and the gap between the inductor and the gear ring. Meanwhile, the electrical parameters are adjusted through frequency, current, and voltage to achieve the desired hardening layer depth and hardness value.

To prevent quench cracking, the parameters for induction quenching must be adjusted to minimize the risk of cracking.

It is clear that factors such as power, speed of movement, gap, frequency, etc. can have a significant impact on the depth, hardness, and cracking of the hardened layer produced through induction hardening.

In order to assess the effects of different process parameters, we carried out optimization tests and adjusted the process parameters accordingly.

Refer to Table 2 for a comparison of the specific adjustment parameters.

Fifty teeth were tested in total.

After induction quenching, the steel was tempered at a temperature of 190℃ for a period of 5 hours.

Table 2 test induction quenching parameters

3. Results and discussion

3.1 Test results

After induction quenching and tempering, see Table 3 for comparison of results of anatomical testing and magnetic particle testing.

Table 3 test results of induction hardening

To assess the reliability of the frequency reduction scheme, a quenching process test was conducted on 5 gear rings with 96 teeth each. The tempering was performed at a temperature of 190℃ for 5 hours.

The magnetic particle testing revealed that the tooth tops and surfaces were free of cracks after the cooling process.

Following the initial test on 5 gear rings, induction quenching was performed on 20 gear rings. Magnetic particle testing was conducted on each gear ring after the quenching and tempering process.

The results proved that reducing the frequency and maintaining other process parameters can effectively resolve the issue of cracks on the gear ring tops.

3.2 Result discussion

1. According to test results, reducing power will also decrease the depth of the hardening layer. This occurs because reducing power directly reduces the quenching temperature, which in turn affects the hardening depth of the gear ring. As a result, the hardening layer depth fails to meet technical requirements and can lead to tooth top cracks. Thus, the power reduction scheme is not a viable option.
2. The relationship between the inductor and gear ring clearance is that a larger clearance leads to a shallower layer depth, lower hardness, and a lower tendency of quenching cracking. On the other hand, a smaller clearance leads to a higher likelihood of cracking. To reduce the risk of cracking, the gap between the inductor and tooth surface can be increased. However, this results in a shallower hardened layer on the pitch circle, which does not meet technical requirements, even though there are no cracks present.
3. Increasing the speed of the quenching machine tool is equivalent to lowering the quenching temperature, which could reduce cracking tendency. However, the test results indicate that while this theoretically reduces the quenching temperature and hardening layer depth, the process reliability is not sufficient to meet the lower limit of technical requirements. Additionally, magnetic particle testing still reveals open quenching cracks, making it inadvisable to increase the moving speed of the quenching machine.
4. The quenching frequency was decreased from 9.6kHz to 8.0kHz. Test results showed that the quenching layer depth increased from 3.4mm to 3.6mm with no cracks on the surface of the gear ring. This technical scheme has been verified in small batch trials of 5 gear rings and medium batch trials of 20 gear rings, proving it to be feasible. It not only increases the hardening layer depth of induction hardening, but also avoids gear ring cracks.

As is widely acknowledged, induction hardening uses alternating current which flows through the inductor to generate induction current, also known as eddy current, between the inductor and the parts, forming a closed circuit.

Eddy current has four distinct characteristics: the skin effect, proximity effect, circulation effect, and sharp angle effect.

As per the skin and proximity effects, the higher the frequency of the current, the more pronounced the skin effect becomes. This means that the deeper the hardening layer in induction hardening, the more heat is concentrated on the surface of the gear.

Additionally, the skin and proximity effects create a superposition effect during the induction heating process, leading to a concentration of heat on the surface of the workpiece.

When there is excessive heat concentration on the surface, the temperature at the surface of the workpiece exceeds that of the center. Given that the cooling effect at the surface is better than that of the center, cracks can easily form on the surface during induction hardening, particularly at chamfering, root transition arcs, or milling cutter marks on the tooth surface, or when the milling teeth are fleshy, which increases the sensitivity to cracks generated by induction hardening.

Therefore, to minimize heat concentration on the surface, reducing the current frequency is a more suitable option.

However, determining the appropriate frequency remains a topic worth exploring.

If the frequency is too high, it leads to a severe skin effect, causing heat concentration on the surface of the workpiece and increasing the likelihood of cracks. On the other hand, if the frequency is too low, it affects the surface hardness.

Where:

• Δ_H – penetration depth of heat current (mm):
• f – current frequency (Hz).

According to the empirical formula, the heat penetration depth can be calculated, and the relationship between the heat penetration depth and the hardening layer depth in induction hardening must be established. This relationship is determined by the hardenability of the material and the cooling characteristics of the induction hardening machine tool’s quenching medium.

Typically, the hardened layer depth is optimal when it falls between 0.25mm and 0.6mm of the heat penetration depth.

When the process layer depth reaches a minimum of 3.2mm, the corresponding frequency is 12.8kHz when calculated using a heat transmission coefficient of 0.25. With a heat transmission coefficient of 0.6, the corresponding frequency is 5.3kHz.

However, the frequency range is relatively broad, so it’s important to consider field experience and conduct process tests for verification.

In this process test, the original frequency of 9.6kHz will be reduced to 8.0kHz. If the correction is too significant, the surface hardness may be insufficient and the test may not be successful.

When optimizing process parameters, it’s necessary to consider both theoretical calculations and previous process data. It’s important to adjust single parameters as specifically as possible, as adjusting multiple parameters in a single test can result in misjudgments if the test results are not met.

For large-scale gear rings, the process involves inductor design, manufacturing, testing, wire cutting, sample preparation, and detection, which can take up to 10 days and result in high costs.

Therefore, it’s important to combine theoretical calculations and analysis with actual accumulated experience to increase the likelihood of a successful test.

To summarize, the main factors contributing to surface cracks in induction hardening are power, gap, frequency, and the cooling medium used during quenching. If other indicators such as the depth of the hardening layer and surface hardness meet the technical requirements, reducing the current frequency should be prioritized to minimize the impact on the surface hardness and hardening layer depth. This also reduces the risk of cracks due to the skin effect, proximity effect, circulation effect, and sharp angle effect.

4. Conclusion

The induction hardening index of gear ring is limited and influenced by various factors.

If cracking occurs during induction hardening, a thorough investigation and assessment are necessary.

The process parameters of induction quenching, such as power, frequency, gap, and speed, have an impact on the formation of cracks.

However, in terms of minimizing quenching cracks, the frequency factor has a more noticeable effect in reducing them, due to the influence of inherent factors such as the skin effect, proximity effect, circulation effect, and sharp angle effect.