Heat Treatment Distortion of Gear Ring: How to Control It?

Due to the great difference between the diameter of the gear ring and the width (or height) of the gear ring, in the process of heat treatment, there often occur the problem of out of tolerance of the inner hole roundness, end face flatness and taper distortion.

Heat treatment distortion of gear ring is one of the most common defects in its heat treatment process.

Heat Treatment Distortion of Gear Ring: How to Control It? 1

The machining process of the gear ring is complex, the distortion qualification rate is low, the machining allowance is large, the rate of defective products is high, the cost is high, and the product accuracy is low and the noise is large, which seriously affects its service life.

Therefore, through the coordination of cold processing and hot processing, optimization of process, improvement of clamping (furnace) mode, and adoption of advanced heat treatment process and equipment, the qualification rate and processing accuracy of heat treatment distortion of gear ring can be improved, the processing cost of products can be reduced, and the defective products can be reduced.

Improving and optimizing heat treatment process to control gear ring distortion

1. Adopt pretreatment process to reduce the distortion of large carburized gear ring

Large gear ring φ2180mm (outer diameter) × φ1750mm (inner diameter) × 550mm (width), made of 17CrNiMo6 steel, with strict requirements for heat treatment distortion.

However, after carburizing and quenching, the tooth top is usually increased by 4 ~ 5mm, sometimes up to 6 ~ 7mm.

For this, the following control measures are adopted:

(1) Preparation of pretreatment process

The quenching and tempering process is adopted, i.e. 860 ℃ heat quenching (20 ~ 30 ℃ higher than the final quenching temperature) and 650 ℃ high temperature tempering.

It is ideal to control the increase of inner hole diameter within 8 ~ 10mm.

After that, according to the normal process, after carburizing and cooling, air cooling, and then (820 ± 10) ℃ temperature equalizing, quenching into a 170 ℃ nitrate bath for cooling, and then tempering twice at 210 ℃, the diameter of the tooth top circle is only about 2mm larger than that before carburizing and quenching, which meets the expected increase, and the roundness and upper and lower taper of the gear ring meet the requirements.

(2) Process key points

Strictly control the quenching temperature during quenching and tempering.

If the temperature is too low, it can not play a good role in reducing the large distortion;

On the contrary, if the temperature is too high, the size of the tooth tip circle after carburizing and quenching may shrink, so the test is required.

2. Improve heat treatment process to reduce heat treatment distortion of driven gear ring of tricycle

The driven gear ring of tricycle transmission (see Fig. 1) is made of 20CrMnTi steel.

The technical requirements for heat treatment: the deep layer of Carbonitriding is 0.6 ~ 1.0mm, the hardness of tooth surface and core is 58 ~ 64HRC and 35 ~ 48HRC respectively, and the positional tolerance of threaded hole and single chain groove is 0.05mm.

Before charging, 10 M8 screw holes are blocked with anti-seepage coating.

After carbonitriding at 850 ~ 860 ℃, they are directly quenched and tempered.

After inspection, it was found that the position of the threaded hole and the single chain groove was out of tolerance, and the anti-seepage coating was not easy to fall off.

The improved process and effect are as follows:

(1) Improved process

Gear making and forming → slow cooling after carbonitriding → turning (cutting) the infiltrated layer, broaching (cutting) the keyway, drilling and tapping → heating and quenching at 850 ~ 860 ℃ → low temperature tempering → capping (protecting the threaded hole) shot blasting → grinding (cutting) the spoke plate → inspection.

(2) Improvement effect

Through inspection, the qualified rate of heat treatment distortion of driven gear ring is over 95%.

Heat Treatment Distortion of Gear Ring: How to Control It? 2

Fig. 1 Schematic diagram of driven gear ring

3. Adopt new heat treatment process and new tooling to control heat treatment distortion of large gear ring

Gear ring of mine rolling mill reducer, with overall dimension of φ1631mm (outer diameter) × φ1364mm (inner diameter) × 300mm (width), single piece mass 1434kg, normal modulus 20mm, number of teeth 78, material 20CrNi2MoA steel, carburizing and quenching required.

(1) Technical requirements for modified gear ring

In order to control and reduce heat treatment distortion of gear ring, some technical requirements are modified.

See Table 1 for the technical requirements of the modified gear ring;

Quenching and tempering treatment before carburizing, with quenching and tempering hardness of 217 ~ 255HBW;

The effective hardened layer is 3.90 ~ 5.10mm.

(2) New technology

Normalizing after forging is changed to normalizing + high temperature tempering.

Spheroidizing annealing process shall be added after carburizing to ensure that the carbide particle size of carburizing layer is ≤ 1 μ m.

The spheroidizing annealing process is shown in Fig. 2.

The results of process a and process B in Fig. 2 are basically the same.

Heat Treatment Distortion of Gear Ring: How to Control It? 3

(a) Two stage isothermal spheroidizing annealing
(b) One stage isothermal spheroidizing annealing

Fig. 2 spheroidizing 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 inner hole and two planes → grinding of inner hole and two planes → gear grinding → keyway → flaw detection → product.

Tooling design: design the upper and lower cover plates to control the distortion of the gear ring. See Fig. 3 for details.

Heat Treatment Distortion of Gear Ring: How to Control It? 4

Fig. 3 heat treatment fixture of gear ring

1. Gear ring

2. Upper cover plate

3. Lower cover plate

(3) Inspection results

The surface hardness, core hardness and effective hardened layer depth of the gear ring meet the technical requirements;

The carbon concentration on the surface of the gear ring is 0.76% (mass fraction), and the carbide particle size reaches 0.5 μ m;

The metallographic structure of martensite and retained austenite is grade 2, the carbide is grade 1, and the core ferrite is grade 2;

All indexes of mechanical properties meet the technical requirements of drawings;

The gear ring distortion inspection shows that the inner hole roundness is 0.90mm, the tooth tip diameter variation is + 3.1 ~ + 4.0mm, and the common normal variation is + 0.2 ~ + 0.6mm, which all meet the technical requirements.

Table 1 technical requirements of modified gear ring

Effective hardened layer depth / mm

Tooth surface hardness (HRC)

Heart hardness (HRC)

Surface carbon concentration of carburized layer (%)

Mechanical properties of heart after heat treatment

σb/MPa

σs/MPa

δ5(%)

ψ(%)

αk/J·cm-2

3.90~5.10

58~62

30~45

0.75~0.95

≥1100

≥800

≥8

≥35

≥60

4. Improve the heat treatment process and adopt the mold correction method to control the carburizing and quenching distortion of large ultra-thin gear ring

Ultra thin internal gear ring of large transmission gearbox, overall dimension φ1120mm (outer diameter) × φ944mm (inner diameter) × 260mm (width), material 17CrNiMo6 steel, weight 550Kg, heat treatment technical requirements: effective hardened layer depth of carburizing quenching is 2.40 ~ 2.90mm, hardness of tooth surface and core is 58 ~ 62HRC and 30 ~ 38HRC respectively, metallographic structure conforms to enterprise standard, grain size is not less than grade 6.

Post heat distortion requirements: taper ≤ 1.35mm, roundness ≤ 1.35mm, common normal line variation ≤ 0.7 ‰, and tooth top circular shrinkage ≤ 1.5 ‰.

(1) Original processing route, process and distortion of gear ring

Original processing route: rough turning of forging stock → drilling → gear shaping → carburizing and quenching → shot peening → fine turning → gear grinding → finished product.

The original carburizing process route is: preheating 650 ℃ × 1h → carburizing (930 ± 10) ℃ × 50h → temperature reduction, thermal insulation 830 ℃ before discharge × 2h → outgoing air cooling → high temperature tempering (680 ℃) × 4h → quenching and heating (820 ± 10) ℃ × 2.5h → salt isothermal quenching (160 ± 10) ℃ → low temperature tempering (210 ± 20) ℃ × 10h → outlet air cooling.

After the gear ring is cleaned, apply anti-seepage coating, 5 pieces per furnace.

The carburizing atmosphere is methanol + isopropanol enriched gas.

Through inspection, other items meet the technical requirements, but the gear ring distortion is large.

(2) Improved process

High temperature normalizing shall be carried out after rough machining before gear ring gear shaping.

In order to reduce the residual stress and thermal stress and reduce the carburizing temperature, the number of step heating stages is increased in the early stage.

After the improvement, the isothermal temperature of 400 ℃ and 850 ℃ is increased, and the quenching temperature is appropriately reduced.

Through inspection, the distortion of the gear ring has been reduced by one grade, but there are still occasional cases where the distortion is out of tolerance.

See Fig. 4 for carburizing process.

The carburizing correction quenching process and the gear ring quenching cooling adopt the die correction method.

See Fig. 4 for the carburizing correction quenching process curve of gear ring.

The die correction quenching is implemented to cool the die and the gear ring together, so that the gear ring is forcibly corrected by the die during the cooling process.

In the subsequent long-time tempering process, the stress generated during quenching and cooling is eliminated, the size of the gear ring is stabilized, and the rebound of distortion is prevented.

The distortion test result is ideal.

Heat Treatment Distortion of Gear Ring: How to Control It? 5

Fig. 4 carburizing correction quenching process curve of large gear ring

5. Reduce the gear ring distortion by adjusting the 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 is 42CrMo steel with quenching and tempering hardness of 262 ~ 302hbw;

After nitriding treatment Δ M is required to be ≤ 0.10mm.

(1) Original heat treatment process and distortion

Blank normalizing (880 ℃× 3h) + tempering after rough turning (salt bath 820 ℃× 0.5h, oil quenching + tempering) + correction + aging treatment after finishing turning (300 ℃× 5h) + gear shaping + ion nitriding (520 ℃) and then cooling with the furnace.

Through inspection, the m value and variation amount are out of tolerance, and the qualified rate of gear ring distortion is only about 70%.

(2) Improved process and effect

The original “quenching and tempering before correction and nitriding” was changed to “correction and tempering before nitriding”.

The qualified rate of gear ring distortion reached more than 98%, and the variation of ΔM value of gear ring was reduced from the maximum 0.46mm before adjustment to 0.10mm after quenching and tempering;

The aging temperature is increased from 300 ℃ to 560 ℃, which not only ensures the full release of machining stress, but also is 30 ~ 50 ℃ higher than the nitriding temperature, thus minimizing the distortion of the nitriding gear ring, and the maximum value of ΔM value is reduced from 0.22mm before adjustment to 0.08mm after adjustment.

6. Solve the problem of distortion and runout of high-frequency quenched 40Cr steel gear ring through process flow adjustment

The gear ring (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) High frequency quenching technology and distortion of gear ring

Electrical parameters of high-frequency quenching process: current frequency is 250kHz, anode and grid current are 7 ~ 7.5A and 1.4 ~ 1.7A respectively, heating time is 30 ~ 40s, and cooling water pressure is ≥ 0.2MPa.

During high-frequency quenching, the cooling speed of the part close to the φ30mm hole is fast, and the cooling speed of the part away from the φ30mm hole is slow.

It is precisely this uneven cooling speed that causes the gear ring runout to be out of tolerance.

Heat Treatment Distortion of Gear Ring: How to Control It? 6

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 outer circle and inner hole → gear hobbing and deburring → gear shaving and deburring → cleaning → high-frequency quenching of tooth part → fine turning of empty tools at both ends → drilling and spot facing → drilling and tapping → single keyway pulling → deburring and sand blasting → cleaning and warehousing.

Improvement effect: after the adjustment of the process flow, the runout of the gear ring after high-frequency quenching is within the tolerance range.

7. Carbonitriding process is adopted to reduce quenching distortion of internal gear ring

Internal gear ring in steering mechanism of heavy-duty dump truck, outer diameter φ444mm × φ372.88mm (inner diameter) × 140mm (tooth width), made of 20CrMnTi steel.

Technical requirements for heat treatment: the carbon concentration on the surface is 0.8% ~ 1.0% (mass fraction), the depth of the carburizing layer is 1.1 ~ 1.5mm, the hardness of the surface and the core are 58 ~ 65HRC and 30 ~ 45HRC respectively, and the roundness is ≤ 0.5mm.

The metallographic structure is martensite, retained austenite ≤ grade 4 and carbonitride ≤ grade 5.

(1) Processing flow of internal gear ring

Blanking → forging → rough machining → pre heat treatment (normalizing) → machining → carbonitriding → quality inspection → sand blasting → product inspection → warehousing.

(2) Heat treatment carbonitriding process

Aixielin sealed box type multi-purpose furnace was used for gas carbonitriding and direct quenching at reduced temperature.

The process is shown in Fig. 6.

Carbonitriding (870 ℃) × 6h, strong carburizing potential 0.95%, diffusion 870 ℃ × 2h, carbon potential 0.65%, cooling to 840 ℃ × 0.5h fast and slow cooling.

Heat Treatment Distortion of Gear Ring: How to Control It? 7

Fig. 6 gas carbonitriding process of internal gear ring

(3) Heat treatment secondary heating quenching process

After carbonitriding, it is cooled slowly, reheated again, and pressed and quenched by internal support. The process is shown in Fig. 7.

The quenching temperature is 830 ℃ × 40min.

The quenching cooling medium with mass fraction of 10% ~ 15% PM is used for quenching.

The transfer time is 20s, the cooling time is 10s, and the outlet temperature is controlled at 150 ~ 180 ℃, which is approximately pressure quenching.

Use special tooling to temper the inner hole of inner support.

Heat Treatment Distortion of Gear Ring: How to Control It? 8

Fig. 7 secondary heating quenching and tempering process of internal gear ring

(4) Tooling and furnace charging method

The internal / external diameter ratio of the internal gear ring is far greater than 1 / 2, and the wall is thin.

It is easy to produce roundness distortion during quenching.

Overlapping stacking shall be avoided to reduce the influence of gravity.

There should be a reasonable spacing between the gear rings to ensure uniform circumferential cooling.

(5) Quenching distortion and control

Reserve a large amount of allowance according to the heat treatment to ensure the dimensional accuracy;

The secondary heating quenching process is selected;

Quenching with quenching press;

The parts with out of tolerance roundness after quenching shall be shaped and tempered with special tooling.

(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. All are qualified.

8. Medium frequency quenching heat treatment of 50Mn2 steel gear ring

The gear ring size is 322mm (outer diameter) × 281mm (inner diameter) × Gear ring 77mm (width).

The material is 50Mn2 steel. The technical requirements for medium frequency quenching are: the surface hardness is 50 ~ 55HRC, and the hardened layer at the tooth root is 1 ~ 4mm, equivalent to 40HRC.

The accumulated pitch error of the gear ring is less than 0.10mm, the tooth direction error is less than 0.055mm, and the tooth shape error is less than 0.035mm.

(1) Medium frequency quenching machine tool and inductor

The rated power of medium frequency quenching machine tool shall not be less than 400KW.

The inductor is made of 14mm×14mm copper square tube, with 5 turns, inductor height of a, and reserved gap between gear ring and inductor of b.

Fig. 8 is a schematic diagram of the inductor.

Heat Treatment Distortion of Gear Ring: How to Control It? 9

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

Reserved clearance between the gear ring and the inductor: considering the influence of the magnet on the magnetic field distribution, the diameter of the inductor is increased to b + 2mm.

At the same time, increase the height of the inductor to a + 3mm.

Electric heating specification: the maximum output voltage is 540V, the maximum output current is 430A, and the frequency is 8000Hz.

When the gear ring is heated to 22s, the heated area is bright red, and the temperature required for quenching is completely reached.

The heating mode is simultaneous heating and quenching.

The common specific power is 0.8 ~ 1.5kw/cm2.

Quenching and cooling medium: AQ251 quenching and cooling medium of Houghton company is used, and the proportion concentration is controlled at 9% ~ 13% (mass fraction).

(3) Inspection results

The metallographic structure inspection and dimension inspection fully meet the technical requirements.

The depth of hardened layer of tooth root is 2.5 ~ 4.0mm.

Tooth direction runout < 0.05mm, tooth shape runout < 0.04mm, and circumferential cumulative error < 0.1mm.

Adopt advanced technology and equipment to control gear ring distortion

1. Adopt the new technology of gear ring molding induction hardening and tempering to control the distortion of high-precision gear ring

(1) The latest induction heating die press quenching technology

The latest die press quenching process of German EMA combines the advantages of induction quenching and pressure quenching process.

Its main advantages are:

  • Heat is directly generated inside the gear, without heat conduction loss, short heating time and energy saving;
  • Rapid heating and quenching, easy process control and good repeatability;
  • Easy to form production line, high efficiency;
  • No pollution;
  • 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 applied in the auto parts industry.

It is suitable for direct pressure quenching of medium carbon steel gears and also for pressure quenching of carburized gears, including high-precision ring parts such as gears (rings), bevel gears and synchronous rings.

(2) Technological process

Fig. 9 is a flow chart of heat treatment distortion gear ring mold pressing induction hardening.

Fix the distorted gear ring to the non-magnetic centering and clamping device (step 1, the clamping device has a solid bottom die and an upper die);

Heat it to about 900 ° C by electromagnetic induction (step 2, the heating temperature is determined according to the material, and can be monitored and controlled by an infrared thermometer);

After holding for a certain time, the gear ring reaches the same or uniform temperature, and the upper and lower dies are pressurized (step 3);

Immediately spray the gear with quenching cooling medium (step 4);

After quenching in step 4, the pressing die device is not required.

Move the inductor to the combined position of the ring gear and the correction core mold (step 5);

And then tempering and heating the gear ring (step 6);

As the temperature rises, the gear ring expands slightly to produce a small gap (step 7);

Pull out the gear ring from the other end of the core die (step 8).

The calibration core die (made of stainless steel) can effectively prevent the ring gear from contracting.

Heat Treatment Distortion of Gear Ring: How to Control It? 10

Fig. 9 flow chart of distortion gear ring die pressing induction hardening

(3) Process parameters and results

See Table 2 for process parameters and results.

It can be seen from table 2 that the gear distortion is small: concentricity < 0.03mm, roundness < 0.03mm, flatness < 0.05mm.

Table 2 pressure quenching process parameters and inspection results of 16MnCrS5 steel gear

Process parameters

Power / kw

250

Frequency / kHz

10

Process time / min

4

Hardness and hardened layer

Surface hardness HV30

680~780

Depth of hardened layer / mm

0.8~1.2

Core hardness HV30

350~480

Accuracy

Concentricity / mm

<0.03

Roundness (inner diameter) / mm

<0.03

Flatness (bottom surface) / mm

<0.05

2. Control method of chemical heat treatment distortion of asymmetric thin-walled inner gear ring

Dimension of thin-walled gear ring φ 162mm (outer diameter) ×φ 111.4mm (inner diameter) × 48mm (thickness), made of 20CrMnTi steel.

Technical requirements: the depth of carbonitriding layer is 0.6 ~ 1.0mm, the surface and core hardness are 58 ~ 64HRC and 35 ~ 48HRC respectively, and the inner hole roundness is less than 0.10mm.

(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 ≥ 0.12mm, and the distortion is out of tolerance.

(2) Improved heat treatment equipment and process

The equipment is changed to 2-1-1 multi-purpose furnace production line, and the temperature and carbon potential of the equipment are controlled uniformly and accurately.

The shallower the carburized layer, the lower the carbon potential and the smaller the distortion, so that the carburized layer is accurately controlled at 0.6 ~ 0.7mm and the metallographic structure is controlled at level 1 ~ 2.

Co infiltration process: intensive infiltration 860 ℃ → diffusion 860 ℃ → 830 ℃ cooling quenching (quenching adopts Jinyu Y35 – Ⅰ isothermal graded quenching oil).

Through inspection, the roundness 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 the distortion of internal gear

The internal gear size of the combine is: φ 315mm (outer diameter) ×φ 268.2mm (inner diameter) × 36mm (width), the material is 20CrMnTi steel, the modulus is 4mm, and the technical requirements are: the flatness of the large end face after heat treatment is less than 0.2mm;

Inner hole roundness ≤ 0.3mm;

Carburizing layer depth 0.9 ~ 1.3mm;

The hardness of the gear part is 59 ~ 63HRC.

(1) Original process and distortion of gear ring

Original process: rough machining → carburizing → finish 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 0.80mm, and the distortion is out of tolerance.

(2) Improved process and inspection results

Improved gear material: with the approval of the main engine manufacturer, it is decided to adopt 40Cr steel.

First, conduct overall quenching and tempering treatment to ensure the hardness (strength) of the gear core, and then conduct high-frequency quenching of the teeth to ensure that the distortion is not out of tolerance.

New technical requirements:

Quenching and tempering hardness of gear blank: 269 ~ 289hbw;

The hardened layer of gear after high-frequency heating and quenching is 1 ~ 1.5mm;

The tooth surface hardness is 50 ~ 54HRC.

New technological process: rough machining of gear blank → quenching and tempering → finish machining → high frequency quenching.

Inspection results: through multiple matching tests of cold and hot machining, the parameter m value of 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.

Correction method of distorted gear ring

The correction method for forming ellipse after quenching of gear ring is as follows:

(1) Hot spot correction method of ellipse

The hot spot correction method is based on the principle that the convex part of the outer circle of the gear ring (elliptical long axis point) is heated and immediately cooled, and the convex part is reduced by the cold shrinkage effect.

The hot spot can be straightened by heating with acetylene oxygen or propane oxygen flame.

(2) Specific operation

Specifically, there are two hot spots at the symmetric part of the outer diameter of the long axis of the ellipse, and one hot spot at both ends of the symmetric inner diameter of the short axis of the ellipse.

After the hot spot, it should be cooled immediately (such as water cooling).

If the ovality is too large, the heating counterattack method can be used for rough correction first, and then the hot spot correction method can be used for correction, so as to obtain a satisfactory effect.

Finally, conduct low-temperature tempering (170 ~ 200 ℃) × 1h and recheck.

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