Overview of Heat Treatment of Steel
Steel is one of the most important structural materials created by humans. Its superior strength, formability, and malleability makes it an essential element in many applications and industries. Steel can be hardened through a process known as heat treatment wherein various properties such as hardenability, machinability, strength, and ductility are altered. The objective of heat treatment is to change the molecular structure of the material in order to improve or alter its physical or mechanical properties. Heat treatment is a combination of processes such as coal carburising, quenching, normalising, tempering, hardening, and annealing among others. This paper seeks to provide an overview of the basic heat treatment techniques used to treat steel.
Coal carburizing is a surface hardening process wherein a steel part is heated in a controlled atmosphere furnace and carbon is transferred from the atmosphere to the surface of the steel. This process is used for slow hardening where the steel is heated up to a temperature of 900-960°C (1650-1760°F). The steel is then quenched in an oil or water bath to cool it rapidly and provide a uniform hardening effect.
The normalizing process is used to refine the grain structure of steel. The steel part is heated up to a temperature higher than that of coal carburising and quenched in air. This method is often used to reduce internal stresses, improve machinability and ductility, and reduce metal fatigue.
Tempering is a heat treatment technique used to reduce the hardness of steel while still retaining its strength. The process consists of heating the steel at a lower temperature than that used in quenching and then cooling it at ambient temperature or subcritical temperature. This process is used to improve the toughness and ductility of the steel.
Hardening is the process of increasing the hardness of steel by heating it in a controlled atmosphere and then quenching it in oil or water. During hardening, small amounts of carbides are formed in the steel which increase the hardness of steel.
Annealing is used to soften the steel and improve its machinability. The steel is heated to a temperature higher than tempering temperature and cooled in air or sometimes in water or oil. After this process is complete, the steel is then allowed to cool gradually until it has reached ambient temperature.
These are some of the basic heat treatment techniques used for steel. Heat treatment of steel can be a complex process and requires a lot of knowledge and experience. Knowledge of the right temperature and the right amount of time is necessary in order to obtain the desired results. As such, companies specializing in heat treating steel often employ highly qualified metallurgists and technicians to ensure that the process is conducted in a safe and accurate manner.
