Important heat treatment characteristics of the ho

2022-09-20
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Important heat treatment characteristics of gear materials

hardenability

meaning: refers to the ability of steel to receive quenching and obtain martensite. Different steel grades have different ability to receive quenching

steels with different hardenability have different depth of hardenable layer after quenching, so the metallographic structure and mechanical properties distributed along the section are also different. The depth of hardened layer refers to the depth from martensite on the quenched surface to 50% martensite layer. The fully quenched workpiece usually has residual tension on the surface, which is prone to deformation and cracking, and is also detrimental to the fatigue performance of the work

key points to consider in design:

1 The larger the size of the part is, the larger the internal heat capacity is, and there is no need for additional engineering investment during quenching. The slower the cooling speed of the part is, therefore, the thinner the quenching layer is, the worse the performance is. This phenomenon is called "size effect of steel". Therefore, the strength calculation of large-size parts cannot be based on the performance data of small size, but the hardenability of steel must be considered

2. Gears with large cross-section or complex structure are made of multi-element alloy steel to ensure sufficient and appropriate hardenability and good comprehensive mechanical properties along the whole cross-section. At the same time, deformation is reduced and cracking is prevented

3. For carbon steel gears, due to the low hardenability of carbon steel, when designing large sizes, the effects of normalizing and quenching and tempering are similar, while normalizing can reduce the cost and do not require quenching and tempering

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

hardenability

meaning: under normal quenching conditions, the martensite structure formed by steel at a critical cooling rate can reach the highest hardness. It shows that the demand for coke is still not reduced

key points to consider in design: hardenability is different from hardenability, which mainly depends on the carbon content in steel. The higher the carbon content in steel, the higher the hardness after quenching, which has little to do with alloy 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: under normal quenching conditions, the martensite structure formed by steel at a critical cooling rate can reach the highest hardness

key points to consider in design: hardenability is different from hardenability, which mainly depends on the carbon content in steel. The higher the carbon content in steel, the higher the hardness after quenching, which has little to do with alloy elements. Therefore, the steel with high quenching hardness does not necessarily have hardenability, which makes the domestic mold machinery and 5 gold plastic industry have high technology docking with the same industry worldwide, while the steel with low hardness may also have high hardenability

tempering stability

meaning: under normal quenching conditions, the martensite structure formed by steel at a critical cooling rate can reach the highest hardness

key points to consider in design: hardenability is different from hardenability, which mainly depends on the carbon content in steel. The higher the carbon content in steel, the higher the hardness after quenching, which has little to do with alloy elements. Therefore, steel with high quenching hardness does not necessarily have high hardenability, while steel with low hardness may also have high hardenability

deformation cracking tendency

meaning: refers to the thermal stress and structural stress produced by steel during heating and cooling, and its comprehensive effect exceeds that of steel σ S or σ B, which tends to deform and crack

this kind of tension force shall not be greater than 2% of the corresponding force value of the nominal tensile strength of the experimental wire rod

key points to consider in the design: the heating or cooling speed is too fast, and uneven heating and cooling are easy to cause deformation and even cracking of the workpiece

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