Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem?

Let’s stop talking about the big situation and go straight to the question:

Fig. 1 shows the parallel gear of a megawatt model of a company.

The material is 18CrNiMo7-6 steel, the gear tooth modulus is 10mm, and carburizing quenching is required.

The gear has an outer diameter of 1680mm, a tooth width of 180mm, and an inner hole diameter of 500mm.

It is designed with a thin web.

See Table 1 for technical requirements of heat treatment.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 1

Fig. 1 Overall Dimension of Gear

Table 1 Technical Requirements for Heat Treatment of 18CrNiMo7-6 Steel Gear

Effective hardened case depth/mmSurface hardness HRCCore hardness HRCcarbideMartensiteRetained austenite (%)Cardiac tissueIGO/mm 
2.9~3.958~64≥30ISO 6336:5MQgradeFine needlelike≤30No massive ferrite≤0.05

1. Process route

The process flow of gear processing is forging → normalizing → rough turning → hobbing → chamfering → carburizing and quenching → shot peening → semi finishing turning → finishing turning → keyway → assembly → boring → gear grinding → warehousing.

During trial production, after carburizing, high temperature tempering, quenching, low temperature tempering and shot peening, the gear was found to have a large distortion during gear grinding.

After trial grinding, the common normal of the part was less than the required value.

At the same time, there were grinding steps at the root of the gear, and the part was scrapped.

2. Trial production process and deformation mechanism analysis

At the initial stage of trial production, considering that the diameter width ratio of the gear is 9.3, the web plate is thin, the weight reduction hole is large, and the parts are prone to warping deformation, the parts with serial number H1 are selected for trial production based on the actual situation on site.

The trial production heat treatment process is as shown in Fig. 2.

The process mode of “carburizing – high recovery – quenching – low recovery” is adopted.

The step heating is adopted when the temperature is raised, and the salt bath quenching is adopted.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 2

Fig. 2 Heat Treatment Process of H1 Parts (Original Process)

The parts are installed flatly with the tooling of a 2m well type carburizing furnace.

To facilitate operation, the chassis tooling with 8 intervals is selected, 4 fan-shaped honeycomb plates are placed at intervals, as shown in Fig. 3.

After high temperature tempering, the quenching charging is changed to hanging, and the hanging position is the inner hole of the part, as shown in Fig. 4.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 3

Fig. 3 H1 Parts Carburizing Charging

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 4

 Fig. 4 H1 Parts Quenching Charging

After the heat treatment of the parts, all physical and chemical indicators are tested to be qualified.

In the gear grinding process, it is reported that the tooth distortion is large.

After the trial grinding, the common normal of the parts is 604.74mm, which is lower than the lower limit of the required value 605.014mm.

Some gear roots have grinding steps, so the parts are scrapped.

In order to determine the cause of part deformation, the alignment data of H1 part during gear grinding was collected and analyzed.

1) Check the grinding gear alignment allowance report of the tooth section, multiple tooth profiles cross in tooth direction, and the whole tooth direction is deformed greatly.

2) Summarize the difference between the high point and the low point of the grinding tooth alignment on the left and right tooth surfaces, and use the radar chart for analysis, as shown in Fig. 5.

It is found that the maximum deformation occurs at the positions of 57~82 teeth, and the deformation at other positions is acceptable.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 5

Fig. 5 Deformation of left and right tooth surfaces of H1 part

3) By comparing the grinding allowance distribution of the left and right tooth surfaces, no significant eccentricity was found during gear grinding, and the requirements were met when turning the inner hole and end face datum.

4) Average the grinding tooth alignment data of the left and right tooth surfaces, as shown in Fig. 6.

It is found that there is an obvious ellipse in the pitch circle of the part, the ellipse trend is pear shaped, and the ellipse amount is about 0.18mm.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 6

Fig. 6 Average Wear of Left and Right Tooth Surfaces of H1 Parts

According to the analysis of the grinding and centering data of H1 parts, it is temporarily impossible to determine that the tooth deformation of parts is caused by end face warping or tooth distortion.

The elliptical trend of the parts is pear shaped.

Although it is impossible to determine whether the specific position is related to the hanging, it can be inferred from the single point hanging that the position of the pear handle (near the 148 tooth) should be the position where the tooling contacts, and the bulge here is the largest;

The position of the maximum deformation of the left and right tooth surfaces has no significant rule, but the deformation trend is the largest near the elliptical pear shaped tail (i.e., the lower part of the suspension).

According to the above analysis, the main causes of workpiece deformation are:

1) The random distribution of part tooth deformation is related to carburizing process, such as heating rate, carburizing temperature, etc.

2) During carburizing, only 4 honeycomb discs are placed at intervals.

Creep occurs during carburizing, resulting in warping deformation of the end face, which causes tooth direction crossing.

3) During hang up quenching, creep during quenching heating causes deformation, which is mainly manifested as ellipse caused by hang up during quenching.

4) When parts are quenched in salt bath, the first contact position shows a greater deformation trend.

This position first contacts the salt bath, and this position is closer to the bottom agitator, so the relative flow speed of fluid is faster.

3. Process improvement

According to the analysis of H1 part, we can not find the key factor that causes the deformation of the part.

Based on the analysis, the heat treatment process was improved at first, and the deformation of the gear after carburizing was tracked to determine whether a large heat treatment deformation had occurred in the carburizing stage.

The serial number of the test part is H2, and the carburizing charging method is the same as H1.

3.1 Improvement of carburizing process

In order to reduce the thermal stress generated by the parts during carburizing and reduce the distortion generated during heating, the carburizing process is updated as shown in Fig. 7.

The process reduces the temperature of parts entering the furnace, prolongs the isothermal time of 650 ℃ and 880 ℃, increases the isothermal section of 770 ℃, and reduces the carburizing temperature of the strong carburizing section.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 7

Fig. 7 Heat Treatment Process of H2 Parts (Improved Process)

3.2 Carburizing charging improvement

In order to analyze the end face warpage of the gear during carburizing and the influence of the end face warpage on the tooth profile data of the subsequent gear grinding alignment, H2 parts repeat the charging method of H1 gear during the first production, make marks after high-temperature tempering, turn out the end face datum in advance, and grind the gear alignment on the gear grinding machine.

When turning the benchmark, it is found that the end face has serious runout, and the specific data is shown in Fig. 8.

At the position where the honeycomb panel is supported, all positions are high points.

At the position where the honeycomb panel is not padded, all positions are low points.

The difference between high points is about 0.2mm, and the difference between low points is about 0.25mm.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 8

Fig. 8 Axial circular runout of H2 parts after carburizing

The grinding teeth alignment data shows that the tooth direction of the part has obviously crossed, and the part does not have obvious ellipses.

The maximum difference between the high and low points of the grinding teeth alignment on the left and right tooth surfaces is the position where the honeycomb plate is not padded.

Through the exploration of carburizing charging mode of H2 parts, it can be determined that excessive axial circular runout caused by creep during carburizing is one of the main reasons for part deformation.

To ensure that the axial circular runout of the gear after carburizing is as small as possible, the number of bottom supporting honeycomb plates is increased from 4 to 8 during carburizing, as shown in Fig. 9.

The axial circular runout of the parts after carburizing and quenching can be controlled below 0.52mm.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 9

Fig. 9 Carburizing charging after improvement

3.3 Quenching charging improvement

To sum up, the fast cooling speed at about 1/4 of the lower part of the gear is one of the factors affecting the gear deformation, so the charging form of quenching should be adjusted.

The carburized H2 parts are used for the test, and the mesh damping tooling is added on the bottom tray to reduce the relative flow speed of the fluid to the lower end of the gear during quenching.

Carry tooth shaped samples of the same specification with the furnace to verify whether the relevant physical and chemical indicators are affected.

See Table 2 for the test results of the tooth shaped sample carried with the furnace after quenching, and the test results are acceptable.

The deformation of H2 parts after quenching is reduced to a certain extent compared with H1, and the common normal of the parts after gear grinding is 0.03 mm less than the lower limit of the standard value, so the parts can be used under concession.

Table 2 Heat Treatment Results of H2 Parts

Project

Requirement

Measurement

Effective hardened case depth/mm

2.9~3.9

3.39

3.46

Surface hardness HRC

58~64

60.26

59.62

Core hardness HRC

≥30

38

Carbide

ISO 6336:5

MQ grade

 

Diffusion

Martensite

Fine needlelike

Fine needlelike

Retained austenite (%)

≤30

15

Cardiac tissue

No massive ferrite

No massive ferrite

IGO

/mm

 

≤0.05

0.025

3.4 Optimization verification

During the production of parts with serial numbers of H3 and H4, the carburizing process shown in Fig. 8 and the flat charging method of fully laying honeycomb plates at the bottom during carburizing (see Fig. 9) are used at the same time.

During the hanging quenching charging, mesh damping tooling is added to the chassis.

After carburizing and quenching, the deformation of parts is greatly improved.

Fig. 10 and Fig. 11 show the radar chart of the grinding tooth alignment data of H3 parts.

The position of No. 109 tooth is the fulcrum position when hanging.

It can be seen from the figure that the ellipse is basically consistent with H1 parts, and the overall deformation and the deformation of the first contact area with the liquid level during quenching are significantly reduced.

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 10

Fig. 10 Left and right tooth surfaces of H3 parts deformed

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 11

Fig. 11 Average Wear of Left and Right Tooth Surfaces of H3 Parts

Table 3 and Fig. 12 show the summary of deformation of parts after heat treatment with different carburizing processes and charging methods.

It can be seen from the comparison that the tooth distortion of the part is reduced by about 40% after the improved carburizing process, optimized carburizing and quenching charging mode are adopted.

Table 3 Influence of Different Charging and Heat Treatment Processes on Gear Deformation

Part number

H1

H2

H3

H4

Carburizing charging

Flat-mounted 4 honeycomb panels

Flat mounted 8 honeycomb panels

Quenching charging

Hanging and placing undamped tooling

Hanging damping tooling

Hanging damping tooling

Carburizing and quenching process

the original process

Improve process

Improve process

Ellipse/mm

0.18

0.14

0.14

0.15

Axial circular runout/mm

1.06

0.52

0.25

Size of common normal after grinding/mm

604.74

604.98

605.04

605.06

Tooth Distortion During Gear Carburizing and Quenching: How to Deal With the Problem? 12

Fig. 12 Box and line diagram of different charging methods and heat treatment process deformation

3.5 Mass production

Based on the experience summarized in the prototype stage, the tooling for quenching and hanging has been re optimized from the original single point support to the two-point support, and the ellipse of the part has been reduced from the original 0.14~0.18mm to 0.05~0.10mm.

For the cold and hot processing fit, the common normal of the part shrinks after carburizing and quenching, about 0.25mm, and 0.25mm hobbing common normal allowance shall be compensated before heat treatment.

After the above improvements, all 30 gears produced in small batch are qualified.

4. Conclusion

1) For flat gears, ensure that all points on the end face are evenly supported during carburizing. When the gears are horizontally mounted and carburized, the spacing of the original four honeycomb discs is changed to the full placement of eight honeycomb plates, which can reduce the end face warping deformation caused by creep.

2) The hanging tooling is used for quenching.

After the mesh damping tooling is added at the bottom of the quenching tray, the tooth deformation in the lower area of the hanging tooling is greatly reduced due to the reduction of the relative flow rate of the quenching cooling medium and parts.

3) For flat gears, the stress and high temperature creep during heat treatment can be reduced and the deformation can be improved by reducing the carburizing temperature, increasing the temperature rise step, extending the isothermal time, and reducing the carburizing temperature.

4) Through heat treatment charging and process improvement, the warp deformation of gear end face is reduced from 1.06mm to 0.52mm, the pitch circle runout is reduced from 0.18mm to 0.1mm, the tooth distortion is reduced by about 40%, and the qualification rate of small batch production is 100%.

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