Full annealing is accomplished by heating a hypoeutectoid steel to a temperature above the UCT (Upper Critical Temperature). In practice, the steel is heated to about 100 oF above the UCT. It is then cooled in the furnace very slowly to room temperature. The formation of austenite destroys all structures that have existed before heating. Slow cooling yields the original phases of ferrite and pearlite.
Figure 1. Annealing and Spheroidizing Temperatures
Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied. This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes the machining easy. Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite. This structure is desirable when minimum hardness, maximum ductility and maximum machinability are required.
Figure 2. Spheroidizing process applied at a temperature below the LCT.
Figure 3. Spheroidizing process applied at a temperature below and above the LCT.
Low carbon steels are seldom spheroidized for machining, because they are excessively soft and gummy in the spheoridized conditions. The cutting tool will tend to push the material rather than cut it, causing excessive heat and wear on the cutting tip.
Figure 4. Spheroidized cementite in a ferrite matrix.
Stress-Relief Annealing is sometimes called subcritical annealing, is useful in removing residual stresses due to heavy machining or other cold-working processes. It is usually carried out at temperatures below the LCT, which is usually selected around 1000oF.
The benefits of annealing are:
Removal of residual stresses that result from cold-working or machining
Full annealing consists of (1) recovery (stress-relief ), (2) recrystallization, (3) grain growth stages. Annealing reduces the hardness, yield strength and tensile strength of the steel.