The Role of Frequency in Preventing the Cracking of Gear Ring

1. Preface

As the most widely used and fastest developing new energy generation technology, wind power has been developed and applied on a large scale in the world.

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

The wind power generation capacity was 405.7 billion kw · h, breaking through 400 billion kw · h for the first time, accounting for 5.5% of the total power generation.

Wind power has become an important part of the new power supply in some countries.

With the increasing consensus of the global development of renewable energy, wind power will play a more important role in the future energy power system.

As an important part of the wind power generation device, the speed-up gearbox has an internal gear ring which is an important part of the star transmission mechanism of the gearbox.

In order to ensure the use requirements of the long-life design, for the production and manufacturing process of the internal gear ring, at present, the high-power wind power booster box has basically no longer used the simple quenching and tempering as the final heat treatment.

The surface strengthening process of the gear ring mainly uses the medium frequency induction quenching and nitriding, and a few manufacturers use the carburizing quenching process as the surface strengthening means.

Among them, medium frequency induction hardening has the characteristics of short cycle, high efficiency, no decarburization, small deformation, low production cost and easy to realize automatic operation.

Especially for higher power gearbox, the module of gear ring is above 16.

Since the nitriding layer of nitriding process is too shallow, the nitriding process is no longer suitable, while carburizing means greater heat treatment deformation and greater gear grinding amount for gear ring with larger module.

For the surface strengthening of large modulus gear ring, medium frequency induction quenching has greater advantages than nitriding and carburizing quenching.

However, induction hardening also has its own technical defects, mainly due to the existence of heat affected zone and quenching cracking in induction heat treatment.

The problem of heat affected zone is mainly improved by the hardenability of material and quenching cooling medium, and the quenching crack needs to be solved by a series of parameter combination optimization.

There are many factors affecting induction hardening.

Generally speaking, the process parameters are divided into thermal parameters (including heating temperature, heating time and heating speed) and electrical parameters (including frequency, surface power and current).

This post expounds the function of frequency factor in preventing the cracking of gear ring by medium frequency induction quenching through the combination test of process parameters.

2. Test materials and methods

2.1 Product materials and key parameters

The material of a certain type of wind power gear ring is 42CrMo4 steel, with a modulus of 17, a number of teeth of 96, a helix angle of 8 °, an induction hardening layer depth of 3.2 ~ 4.2mm, and a single tooth induction hardening.

After the induction hardening dissection, the quenching layer depth of the pitch circle is 3.4mm and the surface hardness is 56hrc.

The production process route is: ring rolling and forging → rough turning → quenching and tempering → gear milling → induction hardening → finish machining → gear grinding.

See Table 1 for process parameters of induction hardening.

Table 1 induction hardening parameters

Power / kwFrequency / kHzClearance / mmMoving speed / mms-1
569.61.3260

Because the hardened layer is deep, cracks are easy to occur after induction hardening (see the arrow in Fig. 1), and the cracks are distributed at the tooth top.

From the crack morphology, it can be seen that the opening is large and cannot be removed in subsequent processing. It can only be scrapped, which brings great economic losses.

In order to solve this problem, and considering that the hardening layer depth meets the technical requirements, the inductor is not redesigned, and only the combination optimization test of electrical parameters is carried out.

The Role of Frequency in Preventing the Cracking of Gear Ring 1

Fig. 1 crack morphology of induction hardening tooth top

2.2. Test method

(1) Re measure the accuracy of quenching machine tool

Since the accuracy of the quenching machine tool may affect the clearance between the inductor and the tooth surface of the gear ring, if the accuracy of the quenching machine tool cannot be guaranteed, the clearance between the inductor and the gear ring in some areas will be too small, which will lead to the heating temperature of the induction quenching being too high, the quenching stress being too large, and finally the quenching crack.

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

As for the accuracy of the quenching machine tool, its accuracy is within the controllable range, so it is not the cause of the crack on the tooth top.

(2) Check raw materials and forging process

The smelting method of the steel plant producing raw materials and the re inspection report of the factory were investigated.

The factors that may cause quenching cracks were analyzed from the aspects of chemical composition, inclusion grade, forging process, etc., and no abnormalities were found.

(3) Induction hardening process test

Since no abnormality is found in the accuracy of quenching machine, raw materials and forging, it is more feasible to optimize the induction quenching parameters.

As for the induction quenching parameters, there are mainly thermal parameters and electrical parameters.

The factors affecting the heating include heating power, scanning moving speed and the gap between the inductor and the gear ring;

The electrical parameter is to obtain the desired hardening layer depth and hardness value by adjusting the frequency, current and voltage reasonably.

To avoid quenching cracking, the parameters of induction quenching must be adjusted to reduce the tendency of cracking.

It is obvious that power, moving speed, gap, frequency, etc. can have certain effects on the depth, hardness and cracking of the hardened layer of induction hardening.

In order to verify the influence of various process parameters, we conducted the following optimization tests for process parameter adjustment.

See Table 2 for the parameter comparison of specific adjustment schemes.

The number of teeth tested was 50.

After induction quenching, the steel was tempered at 190℃×5h.

Table 2 test induction quenching parameters

Serial number:Power / kwFrequency / kHzClearance / mmMoving speed / mms-1programme
1569.61.3260Original scheme
2489.61.3260Power reduction
3569.61.5260Increase clearance
4569.61.3300Acceleration
5568.01.3300Down frequency

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

ProgrammePitch circle hardening layer depth / mmCrack conditionScheme evaluation
Original scheme3.42Open crack on tooth topunreliable
Power reduction3.12Open crack on tooth topunreliable
Increase clearance3.08No crackunreliable
Acceleration3.25Open crack on tooth topunreliable
Down frequency3.60No crackfeasible

In order to verify the reliability of the frequency reduction scheme, the quenching process test was carried out on 5 gear rings (96 teeth per gear ring), and the tempering was carried out at 190℃×5h.

After cooling, through magnetic particle testing, the tooth top and tooth surface were free of cracks.

After 5 gear rings were tested, induction quenching was carried out for 20 gear rings, and magnetic particle testing was carried out for each gear ring after quenching and tempering.

It was proved that the crack problem of gear ring top could be solved by reducing the frequency and keeping other process parameters unchanged.

3.2 Result discussion

1) According to the test results, reducing the power will also reduce the hardening layer depth.

This is because reducing the power will directly reduce the quenching temperature, and the reduction of the quenching temperature will affect the hardening depth of the gear ring, so that the hardening layer depth does not meet the technical requirements, and at the same time, the tooth top crack will be generated.

Therefore, the power reduction scheme is not feasible.

2) The relationship between the inductor and the gear ring clearance is: the clearance is large, the layer depth is shallow, the hardness is low, and the tendency of quenching cracking is low;

On the contrary, it is easy to crack.

In order to reduce the risk of cracking, the gap between the inductor and the tooth surface can be increased.

However, according to the test results, the hardened layer of the pitch circle is shallower. Although there is no crack, it does not meet the technical requirements.

3) Increasing the moving speed of the quenching machine tool is equivalent to lowering the quenching temperature, which can reduce the cracking tendency.

However, increasing the moving speed will theoretically reduce the quenching temperature and the hardening layer depth.

From the test results, the hardening layer depth does not drop much, and the process reliability is not enough at the lower limit of the technical requirements.

In addition, after magnetic particle testing, there are still open quenching cracks, so it is not advisable to increase the moving speed of the quenching machine.

4) The quenching frequency was reduced from 9.6kHz to 8.0kHz.

According to the test results, the quenching layer depth was increased from 3.4mm to 3.6mm, and there was no crack on the surface of the gear ring.

From the test results, the technical scheme is feasible, which has been verified in the small batch trial production of 5 gear rings and the medium batch trial production of 20 gear rings.

The scheme is indeed feasible, which not only increases the hardening layer depth of induction hardening, but also avoids the generation of gear ring cracks.

As is known to all, induction hardening is to use alternating current to pass through the inductor, generate induction current between the inductor and the parts and form a closed circuit, that is, eddy current.

Eddy current has four characteristics, namely skin effect, proximity effect, circulation effect and sharp angle effect.

According to the skin effect and proximity effect, the higher the frequency of current, the more significant the skin effect is, that is, the deeper the hardening layer of induction hardening is, the heat will be concentrated on the gear surface.

Meanwhile, the skin effect and proximity effect will produce a superposition effect in the induction heating process, and the heat of induction heating will be concentrated on the workpiece surface.

When the heat is too concentrated on the surface, the temperature of the surface of the workpiece is higher than that of the center.

Because the cooling effect of the surface is better than that of the center, cracks are easily generated on the surface during induction hardening, especially at the chamfering, root transition arc, or there are milling cutter marks on the tooth surface, and the milling teeth are fleshy, which is highly sensitive to the cracks generated by induction hardening.

Therefore, in order to reduce the heat concentrated on the surface, reducing the current frequency is a better option.

However, how to find the appropriate frequency is worth exploring.

If the frequency is too high, it is easy to produce serious skin effect, heat is concentrated on the surface of the workpiece, and cracks are easy to occur;

If the frequency is too low, the surface hardness will be affected.

The Role of Frequency in Preventing the Cracking of Gear Ring 2

Where:

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

However, according to the empirical formula, the heat penetration depth is calculated, and the relationship between the heat penetration depth and the hardening layer depth of induction hardening needs to be determined in combination with the hardenability of the material and the cooling characteristics of the quenching cooling medium of the induction hardening machine tool.

The general experience is that the hardened layer depth is better between 0.25 and 0.6mm of the heat transmission depth.

When the process layer depth meets the minimum layer depth of 3.2mm, the corresponding frequency is 12.8kHz when the frequency is calculated according to the heat transmission coefficient of 0.25.

When the frequency is calculated according to the heat transmission coefficient of 0.6, the corresponding frequency is 5.3KHz.

However, the selection of frequency according to this range is relatively wide, so it is necessary to combine field experience and carry out process test verification.

This process test is to reduce the original frequency from 9.6kHz to 8.0khz.

If the correction is too large, the surface hardness is easy to be insufficient, and the test may fail.

In the combination optimization of process parameters, in addition to theoretical calculation, it is also necessary to adjust the process parameters in combination with the previous process data, and to adjust the single parameter as targeted as possible.

If more than two parameters are adjusted in one test, once the process test results do not meet the requirements, it is easy to lead to misjudgment of analysis.

In particular, large-scale gear rings need to go through inductor design, manufacturing, testing, wire cutting, sample preparation and detection.

There are many processes, and the test cycle often takes about 10 days, and the test cost is huge.

Therefore, theoretical calculation and analysis should be combined with actual accumulated experience to ensure the probability of successful test.

To sum up, in the case of surface cracks in induction hardening, the factors affecting the surface cracks mainly include power, gap, frequency and quenching cooling medium.

In the case that other indicators such as the depth of hardening layer and surface hardness meet the technical conditions, priority can be given to reducing the current frequency.

In this way, the influence on the surface hardness and the depth of hardening layer is small, and the inherent skin effect, proximity effect.

The possibility of cracks caused by circulation effect and sharp angle effect.

4. Conclusion

The heat treatment index of induction hardening of gear ring is restricted and affected by multiple factors.

If induction hardening cracking occurs, comprehensive analysis and judgment are required.

The power, frequency, gap and speed in the process parameters of induction quenching have certain effects on the generation of cracks.

However, in terms of reducing quenching cracks, under the influence of the inherent characteristics of skin effect, proximity effect, circulation effect and sharp angle effect, the frequency factor has a more obvious effect on reducing quenching cracks.

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