Gear Gas Nitriding: Common Problems and Countermeasures

1. Preface

Compared with ion nitriding, gas nitriding has the advantages of simple operation, high process repeatability, simple equipment structure and nitriding principle, and can realize automatic control of the process.

The most prominent feature is better nitriding temperature and atmosphere uniformity.

For gear nitriding, especially for small module gears, the difference between the depth of nitriding layer of tooth root and pitch circle is smaller than that of ion nitriding.

In recent years, the gas nitriding process has developed rapidly, such as the pre oxidation and gas nitriding process and various gas nitriding processes.

These new processes have shortened the production cycle gap between ion nitriding and gas nitriding to varying degrees.

However, the following problems should be paid attention to in the process of controllable atmosphere nitriding of gears.

2. Cleaning before gas nitriding

The gear shall be cleaned before gas nitriding and charging, and there shall be no water and impurities left.

Otherwise, the appearance color of the gear after nitriding is uneven and dark spots appear.

Although there is no obvious difference in nitrided layer depth, nitrided hardness and metallographic structure between the speckle and the normal area, the speckle will seriously affect the appearance quality of the gear.

The macro morphology of the irregular speckled area was analyzed by ultra depth of field stereomicroscope (see Fig. 1).

Gear Gas Nitriding: Common Problems and Countermeasures 1

Fig. 1 macro morphology of gear end face speckle area

The microscopic morphology of the spotted area was observed by scanning electron microscope (see Fig. 2. The spotted area observed by the naked eye was white under the scanning electron microscope).

Gear Gas Nitriding: Common Problems and Countermeasures 2

Fig. 2 micro morphology of gear end face speckle area

The chemical composition of the spotted area with abnormal appearance of the gear end face and the normal area of the gear end face are compared and analyzed.

The results are shown in Table 1.

Table 1 comparison results (mass fraction) (%) of chemical composition determination in micro area of gear end face

Detection location:

C

N

O

Na

Al

Si

P

S

C

K

Ca

Cr

Mn

Fe

Zone M

Normal surface

0.22

0.98

0.26

0.19

0.63

97.72

0.21

0.99

0.24

0.25

0.57

97.73

Zone A

White spot surface

0.43

1.03

2.91

0.22

0.09

0.21

0.19

0.35

94.59

0.38

0.89

2.58

0.07

0.18

0.05

0.11

0.08

0.10

0.25

0.12

0.31

94.86

Peeling flat bottom

0.28

0.73

8.59

0.09

0.56

0.04

0.08

0.10

0.12

0.06

0.33

1.40

89.61

0.39

0.84

11.59

0.15

0.10

0.67

0.13

0.11

0.09

0.12

0.06

0.34

1.71

83.71

Zone B

Dark patches

1.03

0.58

29.53

0.50

0.10

0.51

0.05

0.90

0.39

1.21

0.18

0.26

0.51

64.25

0.80

0.55

28.27

0.38

0.04

0.53

0.07

1.04

0.36

1.33

0.11

0.20

0.50

65.84

White round spot

0.62

0.83

3.29

0.25

0.16

0.39

0.10

0.15

0.20

0.22

0.60

0.29

0.58

92.33

0.96

0.66

5.05

030

0.16

0.60

0.06

0.14

0.23

0.30

0.96

0.32

1.09

89.16

It can be seen from table 1 that the content of O element in the abnormal spotted area is higher than that in the normal area.

In addition to the higher O element, it also contains a small amount of Na, S, Cl, K, Ca, Mg, Al and other elements.

These elements mainly come from the residues of water, cleaning agent and shaving oil.

From the above analysis, it can be seen that the appearance of gas nitriding is caused by unclean cleaning before nitriding.

Therefore, the following points should be paid special attention to in the cleaning of gas nitriding gear.

1) When using water-based cleaning agent for cleaning, the water temperature for final rinsing should not be too high.

When the rinsing water temperature is too high, the water will be directly dried on the surface of the workpiece.

Therefore, it is better to reduce the rinsing temperature properly and use the high-pressure air gun to blow dry.

2) Acetone, gasoline and other substances are used for cleaning, but the flash point of these substances is low.

Attention should be paid to the ventilation and fire safety of the cleaning environment.

3) The workpiece shall be dewatered before cleaning with a hydrocarbon cleaner.

4) Pay attention to the quality of water used in the cleaning process, and use pure water as much as possible.

3. Control of gear nitriding distortion

Gear nitriding distortion is related to gear shape, machining residual stress, gear material, nitriding tooling, nitriding process and gear cooling speed after nitriding.

In daily production, people often pay attention to the first few factors, but ignore the cooling speed of the gear after nitriding, especially the distortion of the inner hole of the thin-walled gear is more sensitive to the cooling speed after nitriding.

Take the gear shown in Fig. 3 as an example

Gear Gas Nitriding: Common Problems and Countermeasures 3

Fig. 3 gear size

Internal hole size before nitriding φ 52-0.02-0.035mm.

See Table 2 for the relationship between gear cooling time and inner hole expansion.

Table 2 Relationship between gear cooling time and inner hole expansion

Cooling time after nitriding / hInternal hole expansion / mmPass rate (%)Remarks
≥60.012~0.02175The inner hole size exceeds the upper tolerance
4~50.008~0.01299The inner hole is basically within the size range
≤40.005~0.0178Inner hole size out of tolerance

It can be seen from the test results that the distortion trend of the gear inner holes at different cooling speeds is the same, which is basically increased, but the distortion variable is different.

When the cooling rate after nitriding is slow, the distortion of the inner hole is large; when the cooling rate after nitriding is fast, the distortion of the inner hole is small.

In order to stabilize the distortion of gear inner hole and improve the qualified rate of gear inner hole after nitriding, the cooling speed of gear gas nitriding must be strictly controlled.

4. Remedy of oxidation caused by gas leakage of gas nitriding furnace

When gas leakage occurs in the gas nitriding furnace due to sealing and other problems, the surface of the workpiece will be oxidized.

This oxidation has no impact on the internal quality of the workpiece, but does not meet the appearance quality requirements.

In this state, the repair process in Table 3 can be used for reduction.

Table 3 repair process of oxidized workpieces

Repair temperature / ℃Repair time / minAmmonia decomposition rate (%)
480~50030~4030~40

The appearance of the gear after gas nitriding and gas leakage oxidation and reduction is shown in Fig. 4.

Gear Gas Nitriding: Common Problems and Countermeasures 4

Fig. 4 appearance of gears before and after gas ammonia leakage reduction

During appearance reduction, it must be noted that the nitriding temperature during reduction shall not exceed the first nitriding temperature.

Generally, the nitriding temperature during reduction shall be about 20 ℃ lower than the last nitriding temperature.

According to the severity of oxidation, the process time can be appropriately extended.

5. Conclusion

From the above three problems and solutions, it can be seen that when there are problems in the heat treatment process, attention should be paid to every detail in the process, and the key can be solved.

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