Are you curious about how gears are made and why they sometimes end up distorted during heat treatment? Look no further!
In this article, we delve into the factors that contribute to gear distortion, including the carbon content and alloying elements in the steel, the gear shape and section size, the original structure of the steel, the forging process, and the quenching and cooling operations.
We also explore the impact of residual stress, preliminary heat treatment, and tempering, cold treatment, and aging on gear distortion.
So, if you want to learn more about the fascinating world of gear production and heat treatment, keep reading!
1. Carbon content in steel
The carbon content has the greatest influence on the warpage distortion and volume distortion of gears during quenching.
2. Alloying elements in steel
Alloy elements in steel have a significant impact on gear distortion, with certain elements like C, Mn, Ni, Cr, and Mo increasing hardenability and distortion tendency. Conversely, other elements such as Cr, Mn, Mo, Si, Ni, and Ti can help to reduce distortion.
3. Hardenability of steel
The greater the hardenability of steel, the more pronounced the distortion during quenching; whereas, with lower hardenability, the quenching distortion is less significant.
4. Gear shape and section size
The gear design has poor symmetry in its shape and uneven sections.
The spokes in the gear design lack rigidity.
The positioning of the process hole in the gear design is improper, leading to increased heat treatment distortion of the gear.
5. Original structure of steel
1）The non-uniformity of the microstructure in steel significantly affects heat treatment distortion. This is due to coarse structures, large segregation, and network structures that exacerbate distortion after quenching.
2）Defects like banded structures and segregation have become critical factors leading to gear distortion.
3）Macrosegregation in steel ingots often results in square segregation in the cross-section of steel material, leading to uneven quenching distortion of disc gears.
4）Gear distortion during heat treatment is uniform in gears produced by square continuous casting billets. However, the heat treatment distortion of gears produced by rectangular continuous casting billets has significant directionality that affects gear distortion.
5）The finer the grain size, the less distortion occurs after quenching.
6）Non-uniform normalizing treatment of gear blanks leads to heat treatment distortion in gears.
1）Proper forging techniques can help reduce distortion in metals.
In particular, creating a streamlined metal structure through forging can minimize distortion during heat treatment. Additionally, careful forging can reduce segregation, promote uniformity in the metal structure, improve banding, and further reduce heat treatment distortion.
2）If the mold cavity is not completely filled with metal, the final heat treatment may be inconsistent and lead to distortion.
3）Forging gear blanks can lead to increased distortion during heat treatment due to high-temperature heating, uneven deformation, and high final forging temperatures.
7. Preliminary heat treatment of blank
1）Performing preliminary heat treatment on gear blanks can help to minimize distortion during the final heat treatment process. It has been observed that when using normalizing techniques, the distortion is more pronounced compared to isothermal normalizing.
2）Gear blanks undergo quenching and tempering before the final quenching process. Following quenching, distortion tends to exhibit a predictable pattern, and the overall magnitude of distortion is reduced.
8. Residual stress
The process of machining gears can induce stress, leading to metal distortion.
During the gear heating process, not only does thermal stress contribute to distortion, but the release of internal stress can also cause it.
9. Quenching heating process
1）The maximum stress that may be produced under heating temperature is greater in gears with larger diameter or thickness, resulting in increased distortion.
2）In many production conditions where gears are clamped, there is a significant temperature difference at each part during the initial stage of entering the furnace. The resulting thermal stress is enough to cause plastic deformation at the part that reaches high temperature first, leading to local distortion.
3）The warpage distortion of slender shaft teeth and thin plate gears is greatly influenced by uneven heating, including rapid heating.
4）Generally, when heating is uneven, such as with direct heating elements, the side of the gear with higher internal temperature tends to become concave and produce bending distortion after cooling.
5）The heating speed directly affects the formation of thermal stress during the heating process and can cause gear distortion.
1) Quenching cooling rate and quenching cooling medium
The greater the cooling capacity, the stronger the quenching intensity (H), and the larger the temperature difference between the inside and outside of the gear (or at different thicknesses), the greater the stress generated.
Distortion is related to the type of quenching cooling medium, its cooling performance, and hardenability.
2) Uneven cooling
Distortion can be caused by various factors such as the gear structure, the clamping of the gear, and the characteristics of the quenching cooling medium.
3) Quenching cooling medium temperature
Generally, increasing the temperature of the quenching cooling medium, such as quenching oil, can reduce the distortion of gear heat treatment.
11. Quenching operation
1) Gear clamping and support mode
The distortion of the gear is greatly influenced by the clamping and hanging methods, slings, and their supporting methods when loading the gear into the furnace. This is especially true for thin-walled ring gears with a large inner and outer diameter. In addition to the expansion and contraction of the inner and outer diameters, it can often cause the roundness to be out of tolerance.
Improper charging of the furnace can easily produce large high-temperature creep, which can affect the flow of the quenching cooling medium and the uniformity of gear cooling during quenching. As a result, the uniformity of distortion and distortion is affected.
2) Quenching heating temperature and heating uniformity
The impact of quenching temperature on Warpage distortion is significantly greater than on volume distortion.
In general, raising the quenching temperature results in increased gear distortion.
Distortion is caused by uneven heating.
3) Repeated quenching
If gears have gone out of tolerance due to poor heat treatment quality, repeating the quenching process during repair can result in an increase in distortion due to the additional quenching cycles.
4) Effect of quenching cooling
The gear cools at a high speed, which causes volume expansion simultaneously. If the cooling is not uniform, it can result in greater distortion.
In dual medium quenching or step quenching, the residence time in the first medium is long.
The quenching cooling medium’s high fluidity and impact on the gear have a significant influence on the distortion of the gear during heat treatment.
5) Operational factors
This issue is often linked to the breach of process regulations during operation.
For instance, during the tapping process, the gears may collide with one another. Moreover, the impact between the gear and the furnace, furnace body, furnace door, or other hard objects can result in gear distortion.
When the gear is quenched out of the furnace, an unstable operation and significant shaking can further intensify gear distortion, especially when it comes to slender gear shafts and thin plate gears.
12. Tempering, cold treatment and aging
Changes in the size of tempered gears are primarily caused by transformations in their microstructure.
2) Cold treatment
For gears with high alloy element content or requiring high precision, a cold treatment is often performed at below-zero temperatures to further transform the retained austenite into martensite, which leads to an increased level of distortion.
Moreover, it should be noted that the expansion size tends to increase with higher quenching temperatures.
3) Aging treatment
The main cause of aging distortion in gear quenched structure is the amount of retained austenite.
During natural aging, stress relaxation occurs due to hydrogen escape. This relaxation and stress release may cause a transformation of a small amount of retained austenite.