Soft annealing (also called full annealing) involves heating the steel to above the upper critical temperature (Ac3, ~800°C for 42CrMo), holding, and then slowly cooling in the furnace.
Microstructural Change: It transforms any existing martensite or bainite into a stable, soft mixture of coarse ferrite and coarse spheroidized carbides (cementite). The goal is a spheroidized structure, which is the softest possible state for medium-carbon steels.
Property Change:
Hardness drops to its minimum (~180-220 HBW).
Tensile and Yield Strength decrease significantly.
Ductility (A, Z) increases.
Machinability improves dramatically because the soft ferrite matrix and rounded carbides are easier to cut, reducing tool wear.
This condition (+A) is specified when the component requires extensive machining prior to final hardening and tempering.