Question

In a metallurgy TTT diagram plain carbon steel, pearlite 50% is reached. Is this eutectoid, hypo, or hyper and why. Also what is its composition and designation of the alloy.

Answer 1

To obtain TTT Diagram for a particular steel, a small sample is heated to austenitising temperature and is quenched in a salt bath, maintained at constant temperature. After required time, the sample is taken out and rapidly quenched in water now the process of investment starts it carried for a number of different times at particular bath temperature the process of transforming Austenite into pearlite starts at a temperature of 675 ℃ the sample quenched at this temperature so that some of the sample changes during this time.

In this process the carbon precipitates out of the solid solution(Austenite). The precipitation of the carbon forms layers of cementite and as a result of absorption of carbon from the surrounding Austenite the ferrite layers will nucleate along the side of cementite. So that the alternate layers of cementite and ferrite is called pearlite.

So, when unstable Austenite means temperature below 723℃ it is unstable starts to transform into pearlite after the 50% is reached at a time 100seconds and temperature of 675℃ where in Austenite phase decomposes to a mixture of ferrite and cementite upon cooling. If there is no alloying element other than carbon,and cooling rate is slow enough so that there is a sufficient time for diffusion transformations to take place, the reaction starts at temperature 723℃ and at a composition of 0.8wt%C, a Fe-C alloy with exactly 0.8Wt%C is called a eutectoid steel.