12 Factors Affecting Distortion of Gear Heat Treatment

1. Carbon content in steel

The carbon content has the greatest influence on the warpage distortion and volume distortion of gear during quenching.

2. Alloying elements in steel

Alloy elements in steel have great influence on gear distortion, such as alloy elements C, Mn, Ni, Cr, Mo, etc. increase hardenability and distortion tendency;

The alloy elements in the steel, such as Cr, Mn, Mo, Si, Ni, Ti, etc., can reduce the distortion.

3. Hardenability of steel

The higher the hardenability of steel, the greater the distortion;

The smaller the hardenability, the smaller the quenching distortion.

4. Gear shape and section size

The shape of the gear design is asymmetrical and the section uniformity is poor;

The rigidity of the spokes of the gear design is poor;

The improper position of the process hole in the gear design will increase the heat treatment distortion of the gear.

5. Original structure of steel

1) The inhomogeneity of microstructure of steel has great influence on the distortion of heat treatment.

In addition, the microstructure in the steel is not uniform, the structure is coarse, there is large segregation, network structure, etc., and the distortion increases after quenching.

2) Due to the existence of defects such as banded structure and segregation, it has become one of the important factors leading to gear distortion.

3) The macrosegregation of steel ingot often leads to square segregation on the cross section of steel material, which often causes uneven quenching distortion of disc gear.

4) The heat treatment distortion of gear produced by square continuous casting billet is uniform;

The heat treatment distortion of gears produced by rectangular continuous casting billet has obvious directionality, which has great influence on the heat treatment distortion of gears.

5) The finer the essential grain size, the smaller the distortion after quenching.

6) If the original structure of gear blank normalizing treatment is not uniform, the heat treatment distortion of gear will be caused.

6. Forge

1) Full forging helps reduce distortion.

In particular, forming a reasonable metal streamline structure after forging can reduce the distortion of heat treatment.

Reasonable forging can also reduce the segregation, uniform the structure, improve the band structure and reduce the distortion of heat treatment.

2) When the metal is not filled in the mold cavity, the final heat treatment distortion will be inconsistent.

3) When forging gear blanks, the distortion of gear heat treatment will increase due to high temperature heating, uneven deformation and high final forging temperature.

7. Preliminary heat treatment of blank

1) Preliminary heat treatment of gear blanks can reduce the distortion of final heat treatment.

When normal normalizing is adopted, the distortion of heat treatment is larger than that of isothermal normalizing.

2) Gear blanks are quenched and tempered before quenching.

After quenching, the distortion has a strong regularity and the absolute distortion will be reduced.

8. Residual stress

The machining of gears can cause stress that is easy to cause metal distortion.

In the process of gear heating, not only the distortion caused by thermal stress will occur, but also the distortion caused by the release of internal stress.

9. Quenching heating process

1) The maximum stress that the gear with larger diameter or thickness may produce under the heating temperature, thus causing larger distortion.

2) For the gear clamped in many production conditions, the temperature difference at each part is large at the initial stage of entering the furnace, and the thermal stress generated is enough to cause plastic deformation at the part that reaches the high temperature first and cause local distortion.

3) The uneven heating (including rapid heating) has a great influence on the warpage distortion of slender shaft teeth and thin plate gears.

4) In general, when the heating is uneven (such as the direct temperature of the heating element), the side with high internal temperature of the gear tends to appear concave and produce bending distortion after cooling.

5) The heating speed will directly affect the formation of thermal stress in the heating process, and thus cause gear distortion.

10. Cooling

1) Quenching cooling rate and quenching cooling medium

The greater the cooling capacity, the stronger the quenching intensity (H), and the greater the temperature difference between the inside and outside of the gear (or at different thicknesses), the greater the stress generated;

The type, cooling performance and hardenability of quenching cooling medium are related to distortion.

2) Uneven cooling

Distortion caused by gear structure, gear clamping and quenching cooling medium characteristics.

3) Quenching cooling medium temperature

In general, when the temperature of quenching cooling medium (such as quenching oil) is properly increased, the distortion of gear heat treatment will be reduced.

11. Quenching operation

1) Gear clamping and support mode

When the gear is loaded into the furnace, the clamping and hanging methods, slings and their supporting methods also have a great influence on the distortion of the gear, especially for the thin-walled ring gear with large inner diameter and outer diameter, in addition to the expansion and contraction of the inner diameter and outer diameter, it often makes the roundness out of tolerance.

If the furnace is not properly charged, it is easy to produce large high-temperature creep.

At the same time, it will affect the flow of quenching cooling medium and the uniformity of gear cooling during quenching, thus affecting the uniformity of distortion and distortion.

2) Quenching heating temperature and heating uniformity

The effect of quenching temperature on Warpage distortion is much more than that on volume distortion.

Increasing the quenching temperature generally increases the gear distortion;

Distortion caused by uneven heating.

3) Repeated quenching

For gears with out of tolerance due to heat treatment quality, if quenching is repeated during repair, the distortion will be superimposed with the increase of quenching times.

4) Effect of quenching cooling

The cooling speed of the gear is high, and the volume expansion occurs at the same time.

If the cooling is not uniform, it will cause greater distortion;

The residence time of dual medium quenching or step quenching in the first medium is long;

The high fluidity of quenching cooling medium and the impact on the gear have certain influence on the distortion of gear heat treatment.

5) Operational factors

This is often related to the violation of process regulations during operation.

For example, the gears collide with each other during the tapping process;

The impact between the gear and the furnace, furnace body, furnace door or other hard objects will cause the gear distortion;

When the gear is quenched out of the furnace, the operation is unstable and the shaking is large, which will increase the distortion of the gear, especially the influence on the slender gear shaft and the thin plate gear.

12. Tempering, cold treatment and aging

1) Temper

The size change of tempering gear is mainly caused by the transformation of microstructure.

2) Cold treatment

For high alloy element content or precision gears, cold treatment at below zero temperature is generally carried out to further transform the retained austenite into martensite, thus showing a distortion trend of increasing size.

The higher the quenching temperature, the larger the size expansion after cold treatment.

3) Aging treatment

The amount of retained austenite in gear quenched structure is the main cause of aging distortion.

Stress relaxation due to hydrogen escape during natural aging;

The relaxation and release of stress during natural aging may cause the transformation of a small amount of retained austenite.

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