Hardenability of Steel
Hardenability is an important characteristic of steel that is used in the manufacture of forging and mold steel components. It is defined as the ability of the steel to obtain a hardness greater than that of untreated steels of the same carbon content when quenched from an elevated temperature. Hardenability is affected by both the chemical composition and metallurgical structure of the steel.
Chemical Composition
The chemical composition of the steel plays a vital role in determining its hardenability. Generally, higher carbon content results in increased hardness, but the addition of other alloying elements such as chromium and molybdenum may also increase the hardness of the steel. Other alloying elements, such as manganese and nickel, can act as hardening agents and typically improve hardenability.
Metallurgical Structure
The metallurgical structure of the steel is another key factor in determining its hardenability. The grain size of the steel has a significant influence on the hardness. Generally, smaller grain size will give higher hardness and wear resistance. Grain boundary carbide precipitation can also affect hardenability and is usually avoided during heat treatment to ensure the steel retains its maximum hardness. The presence of retained austenite can also dramatically reduce the hardenability of the steel.
Heat Treatment
Heat treatment is essential for achieving maximum hardenability in steel components. Generally, the steel should be heated to a temperature that is above 1,100°C (2,012°F) and then quenched. The quenching medium should be matched to the steel composition to ensure the desired hardness is achieved. Protective atmospheres are often used during quenching to prevent oxidation and decarburization of the surface.
Testing
Impact testing is generally used to measure the hardenability of the steel. In this test, a sample of the steel is cooled and then fractured in an apparatus that measures the impact energy and hardness at the fracture point. The higher the impact energy, the higher the hardenability of the steel.
Conclusion
Hardenability is an important characteristic of steel that affects the performance of components in forging and mold steel applications. Its hardenability is determined by the chemical composition and metallurgical structure of the steel. Heat treatment is essential to achieve the required hardness, and impact testing is used to measure the hardenability.
