Process: Heat the steel to 20~30℃ above AC3 for a period of time and then cool it slowly (with the furnace) to obtain a heat treatment process close to equilibrium structure (complete austenitization).
Complete annealing is mainly used for hypoeutectoid steel (WC=0.3~0.6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings and hot rolled sections, and sometimes for their weldments. The hardness of low carbon steel after complete annealing is low, which is not conducive to machining; When the hypereutectoid steel is heated to the austenite state above ACCM and slowly cooled and annealed, fe3cii will precipitate along the grain boundary in a network, which will significantly reduce the strength, hardness, plasticity and toughness of the steel, leaving hidden dangers for the final heat treatment.
Objective: To refine grain, uniform structure, eliminate internal stress, reduce hardness and improve machinability of steel. The microstructure of hypoeutectoid steel after complete annealing is F+P.
In actual production, in order to improve productivity, annealing cooling to about 500℃ is discharged for air cooling.