Soft Annealing Process

The primary purpose of soft annealing is to relieve internal stresses induced by cold working or prior heat treatments, to transform the hard, lamellar (pearlitic) carbide structure into a spherical/granular carbide morphology, and thereby to improve the machinability and cold formability of the steel.

Unlike full annealing, soft annealing is carried out below the lower critical temperature (A1 ~727°C) of the steel, typically in the range of 650°C to 700°C meaning that austenite transformation does not occur in most applications. The component is held at this temperature for several hours depending on its cross sectional thickness; during this holding period, the lamellar cementite (Fe3C) within the pearlite structure transforms via diffusion into small, spherical carbide particles homogeneously distributed within the ferrite matrix.

Cooling after soft annealing must be performed very slowly inside the furnace. Rapid cooling prevents the formation of the intended soft, spheroidized carbide structure and induces internal stresses.

The specific processing temperature, heating/cooling rates, and soak times for soft annealing depend on the chemical composition of the steel, geometry and dimensions of the component, and the target final properties. This process is particularly preferred in:

High carbon and tool steels prior to cold forming or precision machining.

Applications where reducing hardness is required to extend cutting tool life.

Compared to full annealing, soft annealing is performed at a lower temperature, and its goal is not austenitic transformation, but the direct modification of the carbide morphology.

Soft annealing is not unique to steels. It can also be applied to certain non-ferrous alloys, such as copper and brass, to eliminate work hardening resulting from cold working and to restore material formability.