Effect of hardenability of steel on distortion in gear manufacturing

The metallurgical factors of steel, such as grain size, non-metallic inclusions, banded structure, hardenability, etc., have an impact on the distortion of gears, but the hardenability of steel is the most important one. Therefore, in the development of gear steel, especially for gear carburizing steel, ensuring hardenability has always been an important technical requirement for the metallurgical quality of steel. This paper focuses on the effect of hardenability on distortion.

The higher the hardenability of steel is, the larger the volume involved in the structural transformation is. When the whole workpiece is martensite after quenching, the volume difference before and after quenching reaches the maximum, and the volume change of 1% carbon content is about 1%; if only half of the quenching, that is, half of the volume is quenched into martensite, and the other half is pearlite structure, the volume difference before and after quenching will be twice smaller than the former. Therefore, the smaller the hardenability is, the smaller the quenching distortion will be.

The figure shows the relationship between hardenability and distortion.

From the test data, it can be seen that the higher the hardenability of steel, the greater the distortion of gear heat treatment. Therefore, in many carburized gears, in order to solve the distortion problem, the method of reducing the core hardness (below 30 HRC) is often used. However, considering the strength of gear, the core hardness cannot be too low. In fact, in many gear fatigue failure analysis, it is found that one of the important reasons is the low hardness of the core, so this has become a major contradiction in gear production.

In order to solve the contradiction between gear strength and heat treatment distortion, the best choice is to limit the hardenability of steel.

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