Spheroidize Anneal Spheroidize annealing is benefi cial when subsequent machining and/or hardening is required (since the microstructure consists of rounded cementite particles in a ferrite matrix). The spheroidized condition is the true equilibrium state of the steel and is its softest condition. The spheroidized microstructure also possesses good cold-forming characteristics. Generally, the larger the spheroids and the more distance between them, the greater the ability of the steel to be cold formed. The simplest method of spheroidizing is to employ a subcritical anneal (see below). A more common commercial method consists of heating to a temperature of 50˚F (13-26˚C) below Ac1, hold at this temperature, then increase the temperature setpoint between Ac1 and Ac3 and hold again. Following the second soak period, the temperature is decreased slowly. Another common method is to heat to a temperature of 50˚F (5-26˚C) below the Ac3, holding at one temperature and then increase it to slightly above Ac3 followed by slow (controlled) cooling. It is essential in any of these practices that nuclei be present to ensure formation of spheroids. The nuclei may be undissolved cementite, carbon concentration gradients (inhomogeneous austenite) or, in some instances, nonmetallic inclusions. If excessively long annealing times are employed at relatively high temperatures, however, a very coarse and abnormal agglomeration of the cementite particles will result. This condition is extremely undesirable from the standpoint of machinability. Furthermore, cementite particles of this size are extremely difficult to dissolve in austenite and result in poor response in subsequent hardening operations. The importance of prior condition in spheroidize annealing applies to all steels regardless of carbon content, and the presence of coarse pearlite is undesirable because of resistance to spheroidization.
