ISOTHERMAL ANNEALING

Isothermal annealing

In this process, hypo eutectoid steel is heated above UCT and held at that temperature for some time. This is to enable complete austenite formation and to avoid any possible thermal gradient present within the steel component. The steel is then rapidly cooled to a temperature, which is less than LCT, usually between 600-700° C. fast cooling is achieved by transferring the steel to another furnace maintained at the desired temperature. The required temperature is one at which the super cooled austenite is having minimum stability within lamellar pearlitic region. Steel is held at this temperature till all the austenite is transformed to lamellar pearlite. After this the steel is cooled in air. The term isothermal implies the transformation takes place at constant temperature. The microstructure obtained is similar to that of full annealing.

Major advantage of this process over full annealing is that the time duration of the processes cut down considerably. So the process is cheaper than full annealing. Hyper eutectoid steel are not subjected to this process due to same reasons as in full annealing. Hence this process can be termed a modified form of full annealing. The structure is more homogenous since the transformation takes place at constant temperature through out the steel.

            The process result in improved machinability and better surface finish on machining. Since it is difficult to cool uniformly, to the transformation temperature, heavy components are not subjected to this process. This is mainly for small sized components.

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