Sub-critical annealing / intercritical annealing

Sub-critical annealing (or sub-critical treatment) is annealing carried out slightly below the eutectoid temperature (Ac1 point = eutectoid transformation (723°C for carbon-steels)). Sub-critical annealing does not involve the formation of austenite, while intercritical annealing involves the formation of ferrite and austenite (< 0.8%C carbon-steels).

Benefits of Sub-critical annealing

The aim of the soft annealing process is to form an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher. Normally, increasing the size of the spheroids will increase the steel’s machinability.

Application & materials

Spheroidised structure (globular carbides in a ferrite matrix) on carbon steels improves the cold formability of steels and permits severe deformation like cold drawing or deep drawing. But, a spheroidised structure of carbon-steels (< 0.4%C) is not always adapted for certain machining operations because the material is too soft and can stick on the cutting tools.

Sub-critical annealing Process details

Sub-critical and intercritical annealing are different methods of producing spheroidal carbides.

• Sub-critical annealing
  • Spheroidal carbides can be obtained by either heating to a lower temperature for a longer period of time or using a higher temperature for a shorter time. Sub-critical annealing is based on the former (low temperature for a longer time), using temperatures as close as possible to, but below, the Ac1 temperature. This is typically 680°C, therefore sub-critical annealing will take place in the temperature range 500-650°C. Annealing times may be very long, commonly between 5 and 20 hours. The resulting microstructure and properties depend on the original structure and composition of the steel.
• Intercritical annealing
  • Intercritical annealing treatments involve heating to, and holding at, a temperature between the Ac1 and Ac3 temperatures to obtain partial austenitisation. This is followed by slow cooling or holding at a temperature below the critical temperature, leading to the final microstructure of spheroidal carbides distributed evenly in a ferrite matrix.