Several Heat Treatment Characteristics Of Gear Materials

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1. Hardenability

Definition: Refers to the ability of steel to be quenched to obtain martensite. Different steel grades have different quenching capabilities.

For steels with different hardenability, the depth of the hardened layer obtained after quenching is different, so the metallographic structure and mechanical properties along the cross-section are also different. The depth of the hardened layer refers to the depth from the hardened surface martensite to the 50% martensite layer. All hardened workpieces usually have residual tensile force on the surface, which is prone to deformation and cracking. At the same time, it is also detrimental to the fatigue performance of the work.

※Note:

The larger the size of the part, the larger the internal heat capacity, and the slower the cooling rate of the part during quenching. Therefore, the thinner the hardened layer, the worse the performance. This phenomenon is called “the size effect of steel.” Therefore, it cannot be used to calculate the strength of large-size parts based on small-size performance data, but the hardenability of steel must be considered.

Gears with large cross-sections or complex structures are made of multi-element alloy steel to ensure adequate and appropriate hardenability, to ensure good comprehensive mechanical properties along the entire cross-section, and to reduce deformation and prevent cracking.

For carbon steel gears, due to the low hardenability of carbon steel, the effect of normalizing and quenching and tempering heat treatment is similar when designing large sizes, but normalizing can reduce the cost and does not require quenching and tempering.

Due to the limitation of the hardenability of steel, large-modulus and high-quality gears should be quenched and tempered after gear opening.

2. Hardenability

Definition: It means that the martensite structure formed by the steel at a rate exceeding the critical cooling rate can reach the highest hardness under normal quenching conditions.

※Note:

Hardenability is different from hardenability, and it mainly depends on the carbon content in the steel. The higher the carbon content in steel, the higher the hardness after quenching, which has little to do with alloying elements. Therefore, steel with high quenching hardness does not necessarily have high hardenability, and steel with low hardness may also have high hardenability.

3. Sensitivity to Overheating

4. Tempering Stability

5. Deformation Cracking Tendency

Definition: Refers to the tendency of the steel to produce thermal stress and structural stress during heating and cooling, and its combined effect exceeds the σs or σb of the steel to cause deformation and cracking.

※Note:

If the heating or cooling speed is too fast, the uneven heating and cooling will easily cause deformation or even cracking of the workpiece, so:

When designing gears, the structure should try to avoid sharp corners and sudden changes in thickness.

Use mild quenching medium or quenching method.

6. Dimensional Stability

Definition: Refers to the dimensional stability of parts in long-term storage or use. This is very important for precision gears.

※Note:

The main cause of dimensional changes is the existence of internal stress and the decomposition of retained austenite in the structure. Therefore, when designing precision gears, stabilization treatment should be required, such as cold treatment or low temperature aging after quenching. Stabilize the martensite and reduce the internal stress to make the gear size stable.

7. Tempering Brittleness

Definition: Refers to the impact reduction phenomenon that occurs when steel is tempered in a certain temperature range.

The steel that produces temper brittleness not only has lower impact toughness at room temperature than normal, but also greatly increases the cold brittleness temperature of steel.

※Note:

The impact toughness and fracture toughness of alloy structural steels are reduced when tempered at 250℃~400℃. This phenomenon is generally called the first type of temper brittleness. It cannot be eliminated by heat treatment, and this should be taken into account in the design.

Some alloy structural steels (such as Cr steel, Cr-Ni steel and Cr-Mn steel) will also produce brittleness when tempered slowly at 375°C~575°C. It is generally called the second type of temper brittleness. Fast cooling can To be eliminated. For gears with larger cross-sections, steel containing Mo or W can be used to eliminate or reduce temper brittleness.

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