Complete annealing takes a long time, especially for alloy steel with stable undercooling austenitization. If the austenitized steel is quickly cooled to slightly lower than AR1 temperature, a is transformed into P, and then air cooled to room temperature, the annealing time can be greatly shortened. This annealing method is called isothermal annealing.
Process: Heat the steel to a temperature higher than AC3 (or AC1), hold it for an appropriate time, quickly cool it to a certain temperature in the pearlite area, keep it isothermal, transform austenite into pearlite, and then air cool it to room temperature.
Objective: As with complete annealing, the transformation is easy to control.
It is applicable to a relatively stable steel: high carbon steel (WC>0.6%), alloy tool steel and high alloy steel (total amount of alloy elements>10%). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. However, it is not suitable for large cross-section steel parts and large quantities of furnace charge, because isothermal annealing is not easy to make the interior or batch of workpieces reach isothermal temperature.
The heat treatment process of heating the metal or alloy to an appropriate temperature, holding it for a certain period of time, and then slowly cooling (usually cooling with the furnace) is called annealing.