Principles of Heat Treating of Steels

Thermal treatment principle of steel Steel heat treatment principle Steel is generally defined as an alloy of iron and carbon in an amount between hundreds of percent and about 2 weight percent. Other alloying elements may be about 5 wt.% In low alloy steels and higher in higher alloy steels such as tool steel and stainless steel. Depending on the composition and phase and trace constituents present, the steel can show various properties, which in turn depend on heat treatment.

There are several heat treatments commonly used for processing steel. There are three basic types of steel heat treatment. It is annealing, quenching and tempering. Annealing is a heat treatment used to make the steel softer and more ductile. It is also used to remove material stress and reduce particle size. Quenching is the process of quenching metal parts. Quenching treatment is used to increase steel hardness to make steel wear resistant. Hardened steel is also used for manufacturing cutting tools. Tempering is used to reduce the brittleness of hardened steel, thereby increasing its ductility. The tempering process involves reheating the hardened steel to a temperature below the critical temperature and then performing any desired cooling rate.

After heat treatment, the microstructure of the steel changes. The microstructure of carbon steel is generally called cementite, ferrite, pearlite, austenite and martensite. When the steel part is hardened, it is heated to a high temperature to transform the entire structure to the austenite phase. Austenite is a single phase structure of iron and carbon that is stable at high temperature. When steel slowly cools, austenite transforms into pearlite. This is the equilibrium phase at room temperature. The pearlite structure is a relatively soft annealing structure and has low physical properties. When steel is quenched, a very hard and strong structure called martensite is formed as a metastable phase of carbon dissolved in iron. Producing harder structures with lower hardness can be tempered. According to the cooling rate, eutectoid carbon steel mainly produces martensite or pearlite (Totten G E, 2006)