What are the mechanical properties of tempered steel?

Mechanical properties of quenched and tempered steel;

1, Effect of alloying elements on mechanical properties

When steels with the same hardenability are quenched and tempered to the same hardness, the tensile strength is basically the same, and the relationship between hardness and tensile strength is approximately linear.

When alloy steels with various compositions are quenched and tempered to various hardness values, when the hardness value is 400HB (tensile strength is about 1400 MPa), the yield ratio is the highest, about 0.9. Quenching structure has great influence on yield ratio.

The same hardenability, tensile strength and yield strength can be obtained by adjusting the content of alloying elements that increase the hardenability of steel. Therefore, when selecting alloying elements, priority should be given to those elements that have a significant effect on improving hardenability and are low in price, such as boron, manganese and chromium. However, steels with different alloying elements have different tempering temperatures to achieve the same hardness, that is, the tempering resistance of various steels is different.

When steels with the same hardenability are quenched and tempered to the same hardness, the tensile strength and yield strength are basically the same, but the brittle failure tendency is quite different, especially in low temperature impact test. The relationship between hardness and fatigue limit of steels with different compositions after quenching and tempering is different. When the hardness is lower than 35HRC, there is a linear relationship between fatigue limit and hardness, and the fluctuation range of fatigue limit is 130MPa. When the hardness exceeds 35HRC, the fluctuation range of fatigue limit becomes wider. When the hardness is 55HRC, the fatigue limit fluctuation range reaches 380MPa.

2. Determination of Hardness of Quenched and Tempered Parts

When the hardenability of parts is the same, the hardness after quenching and tempering can reflect the yield strength and tensile strength of parts, so the drawings and technical conditions of parts generally only specify the hardness value. Only very important parts are specified with other mechanical properties.

The determination of hardness of quenched and tempered parts must consider the requirements of manufacturing process and the load conditions in use. Considering the manufacturing process, it is hoped that the parts will be quenched and tempered in the blank state and then cut and assembled. In this way, the deformation and decarbonization caused by heat treatment of parts will be eliminated in the later cutting process. However, the hardness of parts using this manufacturing procedure should not be too high, generally not exceeding 300HB, and individual parts not exceeding 350HB, otherwise it will be unfavorable to cutting. Parts with high hardness requirements (such as some automobile half shafts with hardness of 34 1~4 15HB) can only be cut first and then quenched and tempered. At this time, decarburization and deformation should be prevented when the parts are heated, and sometimes the straightening process should be added after heat treatment. For parts produced in small batches or single pieces, the allowable hardness for cutting can be appropriately increased.

When determining the hardness of quenched and tempered parts, the characteristics of production must also be considered. Different hardness can be selected for small batch products and different parts. Mass production factories hope that the hardness range of most parts is consistent or fixed in several hardness ranges, which is very convenient for tissue heat treatment production.

Considering the use of parts, when determining the hardness of quenched and tempered parts, we should pay attention to the working conditions and shapes of parts. Generally speaking, when the hardness is higher, the tensile strength, yield strength and fatigue strength of smooth samples are higher, but the plasticity index decreases, and the brittleness failure tendency and stress concentration sensitivity increase. Therefore, when there is a notch on the part that plays the role of stress concentration, in order to make the stress distribution uniform and reduce the stress concentration phenomenon, lower hardness can obtain higher fatigue performance.