Significant disparity between the diameter and width/height of a gear ring can cause problems during heat treatment, leading to deviations in inner hole circularity, end face flatness, and taper distortion.
Heat treatment-related distortion is a prevalent problem in the heat treatment process of gear rings.
The gear ring machining process is complex and results in a low distortion qualification rate, large machining tolerances, high rates of defective products, high costs and low accuracy, and high noise levels, all of which significantly impact their life. useful.
To improve the qualification rate and processing accuracy of heat treatment distortion in gear rings, it is necessary to coordinate cold and hot processing, optimize processes, improve clamping and furnace methods, and adopt advanced treatment processes and equipment thermal. This will reduce the cost of processing products and minimize the number of defective products.
Improve and optimize heat treatment process to control gear ring distortion
1. Adopt pretreatment process to reduce the distortion of large carbureted gear ring
A large gear ring with dimensions of φ2180mm (outer diameter), φ1750mm (inner diameter) and 550mm (width), made of 17CrNiMo6 steel, has strict requirements for heat treatment distortion. However, after undergoing carburizing and hardening, it is often observed that the height of the top of the tooth increases by 4 to 5 mm and in some cases by 6 to 7 mm.
To this end, the following control measures are adopted:
(1) Pretreatment process preparation
The gear ring is subjected to a quenching and tempering process, which involves heating it to 860°C for quenching (20 to 30°C above the final quenching temperature) and then tempering at 650°C. The ideal result is to control the increase in the internal diameter of the hole between 8 and 10 mm.
After undergoing carburizing and cooling, air cooling, temperature equalization at (820 ± 10)°C, nitrate bath quenching at 170°C for cooling and two tempering cycles at 210°C, the diameter of the upper circle of the tooth is only about 2mm larger than before carburizing and hardening, meeting the expected increase. In addition, the upper and lower circularity and taper of the gear ring meet the requirements.
(2) Process key points
It is crucial to maintain strict control over the quenching temperature during the quenching and tempering process. If the temperature is too low, it will not effectively reduce the large distortion. On the other hand, if the temperature is too high, the size of the tooth tip circle after carburizing and quenching may decrease, requiring further testing.
2. Improve heat treatment process to reduce heat treatment distortion of tricycle driven gear ring
The driven gear ring in a tricycle transmission (see Figure 1) is made of 20CrMnTi steel and has strict technical requirements for heat treatment. The deep layer of Carbonitriding should be 0.6 to 1.0mm, with a tooth surface hardness of 58 to 64HRC and a core hardness of 35 to 48HRC. The positional tolerance of the threaded hole and the single chain groove should be 0.05mm.
Before loading, 10 M8 screw holes are sealed with an anti-seepage coating. After undergoing carbonitriding at 850 to 860°C, the gear ring is directly quenched and tempered.
Upon inspection, it was found that the position of the threaded hole and single chain groove was out of tolerance and the anti-seepage coating was not easily removable.
The improved process and effect are as follows:
(1) Improved process
Gear manufacturing and forming → slow cooling after carbonitriding → turning (cutting) the infiltrated layer, broaching (cutting) the keyway, drilling and threading → heating and quenching at 850 ~ 860 ℃ → low temperature tempering → capping (protecting the threaded hole) shot blasting → grinding (cutting) of the spoke plate → inspection.
(2) Improvement effect
Through inspection, the qualified heat treatment distortion rate of the driven gear crown is more than 95%.
Fig. 1 Schematic diagram of the driven gear ring
3. Adopt new heat treatment process and new tools to control heat treatment distortion of large gear rings
The gear ring of a mine rolling mill reducer has an overall dimension of φ1631 mm (outer diameter), φ1364 mm (inner diameter) and 300 mm (width), with a single-piece weight of 1434 kg and normal modulus of 20 mm . It has 78 teeth and is made from 20CrNi2MoA steel, requiring carburizing and tempering.
(1) Technical requirements for modified gear ring
To control and minimize distortion from the heat treatment of the ring gear, some technical requirements have been revised. The revised technical requirements for the ring gear are listed in Table 1.
Before undergoing carburizing, the gear ring undergoes quenching and tempering treatment, with quenching and tempering hardness of 217 to 255HBW. The effective hardened layer should be 3.90 to 5.10 mm.
(2) New technology
The normalizing process after forging has been changed to a combination of normalizing and high-temperature tempering. Furthermore, a spheroidizing annealing process was added after carburizing to ensure that the size of the carbide particles in the carburizing layer does not exceed 1 μm.
The spheroidization annealing process is depicted in Figure 2, and the results of Process A and Process B in Figure 2 are similar.
(a) Two-stage isothermal spheroidizing annealing
(b) One-stage isothermal spheroidizing annealing
Fig. 2 Spheroidization annealing process of 20CrNi2MoA steel gear
New process route: forging → normalizing + high temperature tempering → rough turning → flaw detection → quenching and tempering → fine turning and gear milling → carburizing → spheroidizing annealing → quenching and tempering → shot blasting → fine turning of the inner hole and two planes → inner hole grinding and two planes → gear grinding → keyway → flaw detection → product.
Tooling design: design the upper and lower cover plates to control gear ring distortion. See Fig. 3 for details.
Fig. 3 gear ring heat treatment device
1. Gear Ring
2. Top cover plate
3. Bottom cover plate
(3) Inspection results
The surface hardness, core hardness and effective depth of the hardened layer of the gear ring meet the technical requirements. The surface of the gear ring has a carbon concentration of 0.76% (by mass), and the carbide particle size reached 0.5 μm.
The metallographic structure is composed of retained martensite and austenite (grade 2), carbide (grade 1) and ferrite core (grade 2). All mechanical property indices meet the technical requirements described in the drawings.
Inspection of gear ring distortion showed that the roundness of the inner hole is 0.90mm, the variation of tooth tip diameter is in the range of +3.1 to +4.0mm, and the normal variation is between + 0.2 to +0.6mm, meeting technical requirements.
Table 1 Technical requirements of modified gear ring
| Effective depth of hardened layer / mm | Tooth surface hardness (HRC) | Heart hardness (HRC) | Surface carbon concentration of the carburized layer (%) | Mechanical properties of the heart after heat treatment | ||||
| σb /MPa | σ is /MPa | δ5 （%） | ψ（%） | α k /J·cm -2 | ||||
| 3.90~5.10 | 58~62 | 30~45 | 0.75~0.95 | ≥1100 | ≥800 | ≥8 | ≥35 | ≥60 |
4. Improve heat treatment process and adopt mold correction method to control carburizing and quenching distortion of ultra-thin large gear rings
The ultra-thin internal gear ring in a large gearbox has overall dimensions of φ1120 mm (outer diameter), φ944 mm (inner diameter) and 260 mm (width). It is made from 17CrNiMo6 steel, weighs 550Kg and requires heat treatment to meet the following technical requirements:
- The effective depth of the hardened layer of carburizing quenching should be 2.40 to 2.90 mm.
- The tooth surface hardness should be 58 to 62HRC and the core hardness should be 30 to 38HRC.
- The metallographic structure must comply with the enterprise standard, and the grain size must not be less than grade 6.
Post-heating distortion requirements: taper ≤ 1.35mm, roundness ≤ 1.35mm, common normal line variation ≤ 0.7‰ and top circular tooth shrinkage ≤ 1.5‰.
(1) Original processing route, process and gear ring distortion
Original processing route: rough turning of forged material → drilling → gear shaping → carburizing and quenching → shot peening → fine turning → gear grinding → finished product.
The original route of the carburizing process is: preheating 650 ℃ × 1h → carburizing (930 ± 10) ℃ × 50h → temperature reduction, thermal insulation 830 ℃ before discharge × 2h → outlet air cooling → high quenching temperature (680 ℃) × 4h → quenching and heating (820 ± 10) ℃ × 2.5h → isothermal salt quenching (160 ± 10) ℃ → low temperature quenching (210 ± 20) ℃ × 10h → air cooling exit.
After cleaning the crown gear, apply an anti-seepage coating to 5 pieces per oven.
The carburizing atmosphere is a gas enriched with methanol and isopropanol.
After inspection, it was found that although all other items meet the technical requirements, the ring gear has significant distortion.
(2) Improved process
High temperature normalizing should be carried out after rough machining and before gear ring shaping.
To reduce the residual stress and thermal stress and decrease the carburizing temperature, the number of stepwise heating stages is increased in the initial stage.
After these improvements, the isothermal temperatures of 400°C and 850°C were increased and the quenching temperature was appropriately reduced.
Inspection revealed that although ring gear distortion has improved one degree, there are still occasional cases where it is out of tolerance. See Figure 4 for the carburizing process.
The carburizing correction quenching process and gear ring quenching quenching use the die correction method. See Figure 4 for the gear ring carburizing correction quenching process curve.
The die correction quenching method cools the die and the gear ring together, allowing the gear ring to be corrected by the die during the cooling process.
In the subsequent long-term tempering process, the stress generated during quenching and cooling is eliminated, the size of the gear ring is stabilized, and distortion rebound is prevented.
Distortion test results are ideal.
Fig. 4 Curve of the large gear ring carburizing correction quenching process
5. Reduce gear ring distortion by adjusting heat treatment process
The JT001 gear ring in TY320, TY220, D65 and other gearboxes has an outer gear diameter of 318.1mm, an inner gear diameter of 251.7mm, and a width of 51.5mm.
The material used is 42CrMo steel, which has a quenching and tempering hardness of 262 to 302HBW.
After nitriding treatment, a ΔM less than or equal to 0.10 mm is required.
(1) Original heat treatment and distortion process
Normalization of the raw part (880 ℃× 3h) + tempering after rough turning (salt bath 820 ℃× 0.5h, quenching in oil + tempering) + correction + aging treatment after finishing turning (300 ℃× 5h) + gear shaping + ionic nitriding (520 ℃) and then cooling with the oven.
Through inspection, the value and amount of variation are out of tolerance, and the qualified distortion rate of the gear ring is only about 70%.
(2) Improved process and effect
The original process of “quenching and tempering before correction and nitriding” was changed to “correction and tempering before nitriding”.
The distortion rate of the gear ring that meets the qualifications has increased to more than 98%, and the variation in the ΔM value of the gear ring has been reduced from a maximum of 0.46mm before adjustment to 0.10mm after quenching and tempered.
The aging temperature has been increased from 300°C to 560°C, which not only ensures complete release of machining stresses, but is also 30 to 50°C higher than the nitriding temperature, thus minimizing distortion of the gear ring nitrided. The maximum value of ΔM was reduced from 0.22mm before adjustment to 0.08mm after adjustment.
6. Solve the distortion and deviation problem of high frequency hardened 40Cr steel gear ring through process flow adjustment
The crown gear (see Fig. 5) is made of 40Cr steel.
The technical requirements are: the quenching and tempering hardness is 28~32HRC, the high-frequency quenching hardness of the gear part is 48~52HRC, and the gear ring runout is <0.048mm.
(1) Gear ring distortion and high frequency quenching technology
The electrical parameters for the high-frequency quenching process are as follows: the current frequency is 250kHz, the anode and grid currents are 7 to 7.5A and 1.4 to 1.7A respectively, the time Heating time is 30 to 40 seconds and cooling water pressure must be greater than or equal to 0.2 MPa.
During high-frequency quenching, the cooling speed of the part near the 30mm hole is fast, while the cooling speed of the part far from the hole is slow.
This uneven cooling rate is the cause of the gear ring running out of tolerance.
Fig. 5 Schematic diagram of 40Cr steel gear ring
(2) Improve process flow and effect
Improved process flow: forging → rough turning → quenching and tempering → fine turning of the outer circle and inner hole → gear milling and deburring → gear cutting and deburring → cleaning → high-frequency quenching of the tooth part → fine turning of empty tools at both ends → drilling and facing → drilling and tapping → single key extraction → deburring and sandblasting → cleaning and storage.
Improvement effect: After adjusting the process flow, the deviation of the gear ring after high-frequency quenching is within the tolerance range.
7. Carbonitriding process is adopted to reduce the quenching distortion of the internal gear ring
The internal gear ring in the steering mechanism of a heavy-duty dump truck has an outer diameter of φ444 mm and an inner diameter of φ372.88 mm, with a tooth width of 140 mm. It is made of 20CrMnTi steel.
The technical requirements for heat treatment are as follows: the surface carbon concentration should be 0.8% to 1.0% (mass fraction), the depth of the carburizing layer should be 1.1 to 1.5 mm , the surface hardness should be 58 to 65HRC, and the core hardness should be 30 to 45HRC. The circularity must be less than or equal to 0.5 mm.
The metallographic structure must be martensite, with retained austenite no greater than grade 4 and carbonitride no greater than grade 5.
(1) Internal gear ring processing flow
Blanking → forging → rough machining → heat pretreatment (normalization) → machining → carbonitriding → quality inspection → sand blasting → product inspection → storage.
(2) Heat treatment carbonitriding process
The Aixielin sealed box multipurpose furnace was used for gas carbonitriding and direct quenching at reduced temperature. The process is shown in Figure 6.
The process consists of:
- Carbonitriding at 870°C for 6 hours with strong carburizing potential of 0.95%
- Diffusion at 870°C for 2 hours with 0.65% carbon potential
- Cooling to 840°C for 0.5 hours with a combination of rapid and slow cooling.
Fig. 6 Gas carbonitriding process of inner gear ring
(3) Heat treatment secondary heating quenching process
After carbonitriding, the gear ring is slowly cooled and then reheated. It is then quenched by pressure by internal support. The process is shown in Figure 7.
The quenching temperature is 830°C for 40 minutes.
A quenching cooling medium with a mass fraction of 10% to 15% PM is used for quenching. The transfer time is 20 seconds, the cooling time is 10 seconds, and the outlet temperature is controlled between 150 and 180°C, which is approximately pressure quenching.
Special tools are used to temper the inner hole of the inner bracket.
Fig. 7 Quenching and tempering process of internal gear ring secondary heating
(4) Loading method of tools and furnaces
The ratio between the inner and outer diameter of the inner gear ring is significantly greater than 1/2, and the wall is thin.
This can result in circularity distortion during quenching.
To reduce the impact of gravity, overlapping stacking should be avoided.
A reasonable spacing between gear rings must be maintained to ensure uniform circumferential cooling.
(5) Extinguishing distortion and control
A substantial amount of tolerance must be reserved based on heat treatment to ensure dimensional accuracy.
The secondary heating quenching process is chosen.
Quenching is carried out using a quenching press.
Parts with out-of-tolerance roundness after quenching must be reshaped and tempered using special tools.
(6) Inspection results
The surface hardness is 60~65HRC, the core hardness is 38~40HRC, the martensite and residual austenite are grade 1, the carbon nitrogen compound is grade 1, and the outer diameter roundness is 0, 13 ~ 0.30mm. Everyone is qualified.
8. Medium frequency quenching heat treatment of 50Mn2 steel gear ring
The gear ring size is 322mm (outer diameter), 281mm (inner diameter) and 77mm (width).
The material used is 50Mn2 steel. The technical requirements for medium frequency hardening are: the surface hardness must be between 50 and 55HRC, and the hardened layer at the tooth root must be 1 to 4mm deep and equivalent to 40HRC.
The accumulated pitch error of the gear ring should be less than 0.10mm, the tooth direction error should be less than 0.055mm, and the tooth shape error should be less than 0.035mm.
(1) Machine tool and medium frequency quenching inductor
The rated power of the medium frequency quenching machine tool should not be less than 400KW.
The inductor is made of 14mm x 14mm square copper tube with 5 turns. The height of the inductor is “a” and a gap of “b” is reserved between the ring gear and the inductor.
Figure 8 is a schematic diagram of the inductor.
Fig. 8 Schematic diagram of inductor for gear ring
1. Conductive magnet
2. Gear Ring
3. Sensor
(2) Specification for medium frequency heating and quenching
A gap of “b + 2 mm” is reserved between the ring gear and the inductor to take into account the influence of the magnet on the magnetic field distribution. The height of the inductor has also been increased to “a + 3mm”.
The specifications of electric heating are: the maximum output voltage is 540V, the maximum output current is 430A, and the frequency is 8000Hz.
After 22 seconds of heating, the heated area turns bright red, indicating that the temperature required for quenching has been reached.
The heating mode is simultaneous heating and quenching.
Common specific power is between 0.8 and 1.5 kW/cm 2 .
The quenching and cooling medium used is Houghton's AQ251 quenching and cooling medium, with a controlled concentration ratio of 9% to 13% (mass fraction).
(3) Inspection results
Metallographic structure inspection and dimensional inspection fully meet the technical requirements.
The depth of the hardened layer of the tooth root is 2.5 ~ 4.0 mm.
Tooth direction eccentricity < 0.05 mm, tooth shape eccentricity < 0.04 mm and circumferential cumulative error < 0.1 mm.
Adopt advanced technology and equipment to control gear ring distortion
1. Adopt new gear ring molding induction hardening and tempering technology to control distortion of high-precision gear ring
(1) The latest induction heating die press tempering technology
German EMA's latest press hardening process combines the advantages of induction hardening and pressure hardening processes.
Its main advantages are:
- Heat is generated directly inside the gear, without loss of heat conduction, short heating time and energy saving;
- Fast heating and quenching, easy process control and good repeatability;
- Easy to form production line, high efficiency;
- No pollution;
- The heat treatment distortion is small and the final dimension accuracy of the gear is high;
- The hardened layer is evenly distributed.
This technology has been successfully implemented in the automotive parts industry. It is suitable for both direct pressure quenching of medium carbon steel gears and pressure quenching of carbureted gears, including high-precision ring components such as gear rings, bevel gears and synchronous rings.
(2) Technological process
Figure 9 illustrates the flowchart of heat treatment distortion correction process for gear ring mold, pressing and induction hardening.
Step 1: Attach the distorted gear ring to the non-magnetic centering and clamping device. The clamping device consists of a solid lower die and an upper die.
Step 2: Heat the ring gear to approximately 900°C using electromagnetic induction. The heating temperature is determined based on the material and can be monitored and controlled using an infrared thermometer.
Step 3: After a certain waiting time, the ring gear reaches a uniform temperature. The upper and lower dies are then pressurized.
Step 4: Immediately quench the gear ring by spraying it with a cooling medium.
Step 5: Move the inductor to the combined position of the crown and correction core die.
Step 6: Season and heat the gear ring.
Step 7: As the temperature increases, the ring gear expands slightly, creating a small gap.
Step 8: Remove the gear ring from the other end of the die.
The correction core matrix, made of stainless steel, effectively prevents the contraction of the gear ring.
Fig. 9 Flowchart of gear ring distortion, pressing and induction hardening
(3) Process parameters and results
See Table 2 for process parameters and results.
It can be seen in table 2 that the gear distortion is small: concentricity < 0.03 mm, roundness < 0.03 mm, flatness < 0.05 mm.
Table 2 Pressure quenching process parameters and inspection results of 16MnCrS5 steel gear
| Process parameters | Power/kw | 250 |
| Frequency/kHz | 10 | |
| Process time/minute | 4 | |
| Hardness and hardened layer | Surface hardness HV30 | 680~780 |
| Hardened layer depth/mm | 0.8~1.2 | |
| Core hardness HV30 | 350~480 | |
| Precision | Concentricity / mm | <0.03 |
| Roundness (inner diameter) / mm | <0.03 | |
| Flatness (bottom surface) / mm | <0.05 |
2. Chemical heat treatment distortion control method of thin-walled asymmetric inner gear ring
The dimension of thin wall gear ring is φ162mm (outer diameter), φ111.4mm (inner diameter) and 48mm (thickness). It is made of 20CrMnTi steel.
The technical requirements for the ring gear are as follows:
- The depth of the carbonitriding layer should be between 0.6 mm and 1.0 mm.
- The surface hardness should be between 58 and 64HRC and the core hardness should be between 35 and 48HRC.
- The circularity of the inner hole must be less than 0.10 mm.
(1) Distortion of original equipment, process and gear ring
The original heat treatment adopts a continuous carburizing furnace, and the carbonitriding process is: strong carburizing 880 ℃ → diffusion 860 ℃ → 840 ℃ cooling and quenching.
Due to the complex structure, thin wall and asymmetry of the gear ring, the roundness of the inner hole after carbonitriding quenching is greater than or equal to 0.12mm, causing the distortion to fall outside the tolerance range.
(2) Improved heat treatment equipment and processes
The production line was changed to a 2-1-1 multi-purpose furnace, which allows for uniform and precise control of temperature and carbon potential.
By controlling the depth of the carburized layer, it was found that a shallower layer leads to lower carbon potential and reduced distortion. Therefore, the carburized layer is precisely controlled to a depth of 0.6 to 0.7mm, and the metallographic structure is maintained at level 1 to 2.
Co-infiltration process: intensive infiltration 860 ℃ → diffusion 860 ℃ → 830 ℃ quenching quenching (quenching adopts Jinyu Y35 – Ⅰ quenching oil with isothermal classification).
Through inspection, the circularity of the inner hole is controlled within 0.10mm, which is qualified.
3. 40Cr steel high-frequency quenching method is adopted instead of 20CrMnTi carburizing quenching to reduce internal gear distortion
The harvester internal gear size is φ315mm (outer diameter), φ268.2mm (inner diameter) and 36mm (width). The material used is 20CrMnTi steel with a module of 4mm.
The technical requirements for the internal gear are as follows:
- The flatness of the large end face after heat treatment should be less than 0.2mm.
- The circularity of the inner hole must be less than or equal to 0.3 mm.
- The depth of the carburizing layer must be between 0.9 mm and 1.3 mm.
- The hardness of the gear component must be between 59 and 63HRC.
(1) Original process and gear ring distortion
Original process: rough machining → carburizing → finishing machining → secondary heating and quenching.
After heat treatment, the distortion of the inner hole is large, and the roundness of the inner hole is between 0.35 and 0.80 mm, and the distortion is out of tolerance.
(2) Improved process and inspection results
An improved gear material was selected with the approval of the main engine manufacturer. It was decided to adopt 40Cr steel.
The gear material will undergo general quenching and tempering treatment to ensure the hardness (strength) of the gear core, followed by high-frequency quenching of the teeth to prevent the distortion from falling outside the tolerance range.
The new technical requirements for gear material are as follows:
- The hardness of the gear blank after quenching and tempering should be between 269 and 289HBW.
- The hardened layer of the gear after high-frequency heating and quenching should be between 1mm and 1.5mm.
- The tooth surface hardness should be between 50 and 54HRC.
New technological process: rough machining of the gear blank → quenching and tempering → finishing machining → high frequency quenching.
Inspection results: through several corresponding cold and hot machining tests, the m parameter value of the internal gear before heat treatment was adjusted, and the technical requirements were met after high-frequency quenching.
Using 40Cr steel for high frequency quenching instead of 20CrMnTi carburizing quenching also reduces the cost and the effect is good after loading.
Distorted gear ring correction method
The correction method to form an ellipse after quenching the gear ring is as follows:
(1) Ellipse hot spot correction method
The hot spot correction method is based on the principle of immediate heating and cooling of the convex part of the outer circle of the ring gear (at the elliptical point of the long axis), which reduces it through the cold contraction effect. The hot spot can be fixed by heating it with an acetylene oxygen or propane oxygen flame.
(2) Specific operation
Specifically, there are two symmetrically located hot spots on the outer diameter of the long axis of the ellipse and one hot spot on both ends of the symmetrical inner diameter of the short axis. Hot spots must be cooled immediately after heating, for example using water cooling.
If the ovality is excessive, the heating counterattack method can be used for preliminary correction, followed by the hot spot correction method for further correction, resulting in a satisfactory result.
Finally, low temperature tempering should be carried out at a temperature between 170-200 ℃ for 1 hour, followed by rechecking.
