Heat treatment of 40Cr round bars can significantly alter their mechanical properties. When 40Cr round bars are quenched, they are rapidly cooled from a high temperature (usually around 850 - 870°C). This rapid cooling causes the formation of martensite, a hard and brittle microstructure. As a result, the hardness of the 40Cr round bars increases substantially. The quenched 40Cr can reach a hardness of around HRC 50 - 55, which makes it suitable for applications where wear resistance is crucial, such as in the manufacturing of gears and shafts that experience high - stress and friction.
However, the high hardness and brittleness of the quenched state may not be ideal for all applications. To improve the toughness and reduce brittleness, tempering is often carried out after quenching. Tempering involves reheating the quenched round bars to a lower temperature, typically in the range of 500 - 650°C, and then cooling them at a controlled rate. During tempering, the martensite transforms into a more stable microstructure, such as tempered martensite. This process reduces the internal stresses generated during quenching and increases the toughness of the 40Cr round bars. The tensile strength of the tempered 40Cr round bars is usually in the range of 800 - 1000 MPa, with an elongation of around 9 - 14%. This combination of strength and toughness makes tempered 40Cr suitable for components that need to withstand both high loads and impact forces, like in automotive suspension systems and some types of machinery parts.