SPHEROIDIZING | Heat treatment for steels


Spheroidizing is a heat treatment process which result in a structure consisting of globules or spheroids of carbide in a matrix of ferrite. Here cementite of lamellar pearlite for hypoeutectoid and eutectoid steels, both lamellar and free cementite in the case of hypereuctoid steels, coalesce in to tiny spheroids.

● The degree of spheroidization depends on heat treatment temperature and holding time. The process can be completed in short time by increasing the treatment temperature. At high temperature dissolved carbide carbide particle can reapper as lamellae during cooling.
Spheroidizing - heat treatment for steels

Methods of Spheroidizing :

● In spheroidization heating of steel is just below the lower critical temperature, then holding at this temperature for prolonged periods after that cooled slowly.
• Another method involves heating and cooling steel alternately just above and below the lower critical temperature. It can consists of heating of steel to a temperature above the lower critical temperature, followed by slow cooling to a temperature below the lower critical temperature and holding at this temperature for a period till the shape of all carbide particles changes in to spheroids.

● The spheroidizing treatment is also carried out by heating steel above the lower critical temperature. This results in a completely spheroidized structure. The steel is to be heated above the lower critical temperature depends on the chemical composition of steel. Eutectoid steels are heated to about 20 to 30℃ above the lower critical temperature, where as hypereutectoid steels are heated 30 to 50℃ above the lower critical temperature. Medium carbon steels can be spheroidized either by heating just above or below the lower critical temperature.

Purpose of Spheroidizing : 

● High carbon and alloy steels are frequently spheroidized in order to improve machinability and ductility. Low carbon steels are not generally spheroidized. The aim is to make these steels suitable for serve deformation in case they are spheroidized. Low carbon steels, on spheroidization, become very soft and gummy.

● It has been observed that the fine lammellar pearlite coalescess more easily than coarse pearlite. Very fine pearlite spheroidizes still more readily. Quenched structures consisting of fine and well dispersed carbide phase shows greatest spheroidization rates. Cold working of steel prior to the treatment also helps in accelerating the rate of spheroidization.

Q. What is Spheroidizing ?

Ans : Spheroidizing refers to a heat treatment material modification process that is used to convert granular structures of the material into a spheroidal form. The process is performed to improve a metal's cold forming capability.

Short note on Spheroidizing :

Spheroidizing is performed by annealing steels with more than 0.8% carbon. The metal is heated to a temperature of about 1200°F (650°C) and maintained at this temperature for a predetermined amount of time to convert its microstructure. This allows for a cementite steel structure to change from a lamella formation to an alpha ferrite matrix. The alpha ferrite matrix is made up of particles of spheroidal cementite formations.
Spherodizing is industrially performed in an endothermic atmosphere to prevent oxidation and decarburization.
Spheroidizing is used primarily to treat various types of steels, and is used to improve the machinability of hypereutectoid and tool steels. This is accomplished by lowering the metal's steel flow stress.
Spherodizing is typically done on parts that have been work hardened for improved ductility, toughness, strength and reduced hardness.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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