Spheroidizing

Spheroidizing

• Depending on the time-temperature exposure of a steel, the cementite carbides in a steel can form different morphologies, such as the lamellar cementite of pearlite, or a network of cementite along prior austenite grain boundaries in a hypereutectoid steel.

• The shape and distribution of the carbides can be modified by heat treatment, and spheroidization treatments are used to produce carbides with a more spherical morphology.

• Because spherical shapes have lower surface energy than angular shapes, the lamellar shape of cementite in pearlite changes morphology to form spheroids. Portions of the lamellae “pinch off” (dissolve) to form a spheroid from the remaining portions of lamellae. This process can occur over a long period of time, or it can be accelerated by heat treatment. Depending on the steel, spheroidized carbides can lead to improved machinability, toughness, and formability.

• In hypoeutectoid steels, prolonged holding at a temperature just below Ae1 accelerates the process of spheroidization. The process may take several hours. To improve on kinetics for full spheroidization, some treatments involve heating and cooling alternately between temperatures that are just above Ac1 and just below Ar1.

• In low-carbon steels, the typical purpose of spheroidizing is to improve the cold formability of steels. The formability of steel is altered significantly depending on whether carbides are spheroids or present as lamellae in pearlite. Low-carbon steels are seldom spheroidized for machining, because in the spheroidized condition they are excessively soft and “gummy,” cutting with long, tough chips.

• In hypereutectoid steels (C > 0.77%) and tool steels, spheroidization is done to improve machinability and improve toughness. Heating of hypereutectoid steels above Ac1 is done to create dispersed cementite particulates. Heating to dissolve the carbide prevents reformation of a carbide network. If a temperature slightly above Ac1 is to be used, good loading characteristics and accurate temperature controls are required for proper results; otherwise, it is conceivable that Ac1 may not be reached and that austenitization may not occur.


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