What is Spheroidizing Heat Treatment?
Spheroidizing heat treatment is performed for improving the cold formability of steels. Spherodizing is also performed to improve the machinability of hypereutectoid steels, as well as tool steels. A spheroidized microstructure is desirable for cold forming because it lowers the flow stress of the material. The flow stress is determined by the proportion and distribution of ferrite and carbides. The strength of the ferrite depends on its grain size and the rate of cooling. Whether the carbides are present as lamellae in pearlite or spheroids radically affects the formability of steel.Steels may be spheroidized, that is, heated and cooled to produce a structure of globular carbides in a ferritic matrix.
Spheroidizing can take place by the following methods:
· Prolonged holding at a temperature just below Ae1
· Heating and cooling alternately between temperatures that are just above Ac1 and just below Ar1
· Heating to a temperature just above Ac1, and then either cooling very slowly in the furnace or holding at
a temperature just below Ar1
· Cooling at a suitable rate from the minimum temperature at which all carbide is dissolved to prevent reformation of a carbide network, and then reheating in accordance with the first or second methods above (applicable to hypereutectoid steel containing a carbide network)
It should be noted that it is difficult to establish consistent designations for critical temperatures. In discussions about heating with prolonged holding, the critical temperatures of interest should be the equilibrium temperatures Ae1 and Ae3.
Terminology becomes more arbitrary in discussions of heating and cooling at unspecified rates and for unspecified holding times. The rates of spheroidizing provided by these methods depend somewhat on prior microstructure, being greatest for quenched structures in which the carbide phase is fine and dispersed. Prior cold work also increases the rate of the spheroidizing reaction in a subcritical spheroidizing treatment.