Spheroidization

Is an annealing process used for high carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. This is done by one of the following ways:

Heat the  part to a  temperature  just below  the  Ferrite-Austenite  line, line  A₁  or below  the Austenite-Cementite line, essentially below the 727 ºC (1340 ºF) line. Hold the temperature for a prolonged time and follow by fairly slow cooling. Or Cycle multiple times between temperatures slightly above and slightly below the 727 ºC (1340

ºF) line, say for example between 700 and 750 ºC (1292 - 1382 ºF), and slow cool. Or

For tool and alloy steels heat to 750 to 800 ºC (1382-1472 ºF) and hold for several hours followed by slow cooling.

All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids)  dispersed throughout  the  ferrite matrix. This structure allows for  improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.