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Related characteristics of gear material heat treatment
Hardenability
Meaning: refers to the ability of steel to obtain martensite after quenching. Different steel grades have different ability to receive quenching.
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, and it is also detrimental to the fatigue performance of the work.
Key points to consider when designing:
1. 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 for the strength calculation of large-size parts based on the performance data of small size, but the hardenability of steel must be considered.
2. 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.
3. For carbon steel gears, due to the low hardenability of carbon steel, normalizing and quenching and tempering effects are similar when designing large sizes, but normalizing can reduce the cost and does not require quenching and tempering.
4. Due to the limitation of the hardenability of steel, large-modulus and high-quality gears should be quenched and tempered after gear opening.
Hardenability
Meaning: It means that under normal quenching conditions, the martensite structure formed by exceeding the critical cooling rate can reach the highest hardness.
Key points to consider when designing: Hardenability is different from hardenability, and it mainly depends on the carbon content in the steel. The higher the carbon content in the 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, while steel with low hardness may also have high hardenability.
Overheating sensitivity
Meaning: It means that under normal quenching conditions, the martensite structure formed by exceeding the critical cooling rate can reach the highest hardness.
Key points to consider when designing: Hardenability is different from hardenability, and it mainly depends on the carbon content in the steel. The higher the carbon content in the 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, while steel with low hardness may also have high hardenability.
Tempering stability
Meaning: It means that under normal quenching conditions, the martensite structure formed by exceeding the critical cooling rate can reach the highest hardness.
Key points to consider when designing: Hardenability is different from hardenability, and it mainly depends on the carbon content in the steel. The higher the carbon content in the 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, while steel with low hardness may also have high hardenability.
Deformation and cracking tendency
Meaning: It refers to the tendency of the steel to produce thermal stress and structural stress during the heating and cooling process, and its combined effect exceeds the σs or σb of the steel to cause deformation and cracking.
The main points to consider when designing: heating or cooling too fast, uneven heating and cooling will easily cause deformation or even cracking of the workpiece.