After quenching and tempering, the thick wall alloy pipe reaches the same hardness, the tensile strength is basically the same, and the relationship between hardness and tensile strength is approximately linear. When the thick wall alloy tubes with different components are quenched and tempered to different hardness values, the hardness value is 400HB (tensile strength is about 1400MPa), and the yield ratio is large, about 0.9. The quenched structure has a great influence on the yield ratio.

By adjusting the content of alloy elements, the thick wall alloy pipe can obtain the same hardenability, tensile strength and yield strength. Therefore, when selecting alloy elements, it is best to select elements with high hardenability and low price, such as boron, manganese, chromium, etc. The tempering temperature is also different due to different alloy elements and quenching and tempering hardness in thick wall alloy pipe, that is, the tempering resistance of different steels is also different.

When thick wall alloy pipes with the same hardenability are adjusted to the same hardness, although the tensile strength and yield strength are basically the same, there are great differences in brittle failure tendency, especially in low temperature impact test. The relationship between hardness and fatigue limit of thick wall alloy tubes with different components is different after quenching and tempering. Below 35HRC, the fatigue limit has a linear relationship with hardness, and the fluctuation range of fatigue limit is 130Mpa. When the hardness exceeds 35HRC, the fluctuation range of fatigue limit becomes larger; For example, when the hardness is 55HRC, the fluctuation range of fatigue limit can reach 380MPa.