Heating and cooling process of steel

. structural changes of plain carbon steel while heating

. the structural changes of plain carbon steel in normal cooling

Distinguish between the lower and upper critical temparature during heating and cooling.

Heating

While heating uniformly a piece of plain carbon steel in a furnace, the temparature will increase uniformly and it will be halted or will fall slightly for a short time although the heat is supplied to the steel.

At this point, the heat, is being used to  cause a rearrangement of the iron atoms which, in turn,, causes the formation of a solid solution called austenite.

The temparature at which austenite is formed is called the lower critical point (heating ) and is at 723°c.

If the steel contains less than 0.83% carbon there will be some ferrite present after the above change also. And if it contains more than 0.83% carbon, there will be some cementite present after this change.

If the heating is further continued, the ferrite or cementite present will be gradually tranformed into austenite. Less heat is required for this page.

The  temparature at which the formation of austenite is total called the upper critical point(heating ).

The temparature of upper critical point(heating )  depends upon the percentage of carbon in steel.

Cooling

While cooling, the temparature of the piece will fall uniformly untill a certain temparature is reached at which stage it starts to cool less rapidly.

The temparature at which the above is happening is called the upper critical point (cooling ) and is 30° c below upper critical point (heating ).

At this stage the austenite starts to break down to produce either ferrite or cementite according to the percentage of carbon in the steel.

The total breaking of the austenite to ferrite or cementite will continue untill the lower critical point (cooling )is reached at 693° c.

Then there will be a halt in the decrease of temparature. This halt in temparature willl be utilised for the breakdown of austenite caused by the rearrangement of the iorn atoms.

This rearrangement reduces heat which causes the changes in the rate of fall of the temparature of the steel.

The changes observed during this heating and cooling of steel are important in the study of its heat treatment. The changes indicated are those that occur when the steel is COOLED SLOWLY and the diagram is accordingly called a thermal equilibrium diagram. It is usual to indicate the temparature at which the changes occur during cooling, the changes also occur during heating, but the temparature at which they  occure are all 30° c higher than those associated with cooling.