Annealing Process

Annealing is a general heat treatment process based on heating steel to a specified temperature followed by controlled cooling. Different annealing processes can be applied depending on the temperature, cooling method, and furnace atmosphere.

Annealing processes applied to steels can be classified according to the process temperature as subcritical annealing, intercritical annealing, and full annealing. Subcritical annealing is generally carried out in the range of approximately 538–649 °C, where no significant phase transformation occurs in the crystal structure of the steel. In intercritical annealing, the temperature is typically increased to approximately 649–760 °C, where partial formation of austenite may occur. In full annealing, the steel is heated to a temperature sufficient to obtain a fully austenitic structure, depending on its chemical composition. In most applications, this temperature is approximately 816–927 °C.

Annealing in an uncontrolled atmosphere can lead to two types of surface defects. Scale formation may deteriorate surface quality and result in material loss. Decarburization reduces the carbon content in the surface region, which can lead to a decrease in surface hardness and changes in the surface properties of the material.

The atmosphere used during annealing is selected according to the required surface properties of the component and the process requirements. Vacuum or reducing atmospheres may be preferred for applications where oxidation and scale formation on the surface must be minimized. Annealing can be performed in an air atmosphere where surface appearance is not critical. In processes where surface carbon loss, decarburization, must be controlled, endothermic or neutral atmospheres may be used.

Annealing is not limited to steels; it can also be applied to copper, aluminum, brass, and other non ferrous metals to improve their properties.

    PURPOSES OF ANNEALING
•  To reduce the hardness of the steel,
• To improve machinability,
• To increase ductility and formability,
• To relieve internal stresses generated during manufacturing processes such as cold forming, welding, and machining,
• To promote a more homogeneous microstructure,
• To assist in obtaining the desired microstructure for the intended application,
• To provide a suitable starting microstructure for subsequent heat treatment processes,
• To improve dimensional stability.