Significance of Transformations Lines

Line ABCD

The line ABCD tells that above this line melting has been completed during heating the iron. The molten metal is purely in the liquidus form. Below this line and above line AHJECF the metal is partially solid and partially liquid. The solid metal is known as austenite. Thus the line ABCD represents temperatures at which melting is considered as completed. Beyond this line metal is totally in molten state. It is not a horizontal line the melting temperature will vary with carbon content.

Line AHJECF

This line tells us that metal starts melting at this temperature. This line is not horizontal and hence the melting temperatures will change with carbon content. Below this line and above line GSEC, the metal is in solid form and having austenite structure.

Line PSK

This line occurs near 723°C and is a horizontal line and is known as lower critical temperature line because transformation of steels starts at, this line. Carbon % has not effect on it that means steel having different % of carbon will transforms at the same temperature. The range above the line up to GSE is known as transformation range. This line tells us the steel having carbon up to 0.8% up to 0.8% will starts transforming from ferrite and pearlite to austenite during heating.

Line ECF

It is a line at temperature 1130°C which tells that for cast iron having % of C from 2% to 4.3%. Below this line and above line SK, Cast iron will have austenite + ledeburite and cementite + ledeburite.

Critical Temperatures

The temperatures at which changes in structure takes place is known as critical temperatures, these are as follows:

1. The temperature along GSE is known as upper critical temperature. The temperature along GS during heating as (upper critical temperature) where austenite + alpha iron changes into austenite and vice versa.

2. The temperature along GS during cooling as A3 where austenite changes into austenite + alpha iron and vice versa during heating.

3. The temperature along line SE during heating as Acm changes into austenite from austenite + cementite and vice versa.

4. The temperature along PSK is known as lower critical temperature when pearlite changes into austenite on heating as denoted, by A₁.