Trying to understand how to read phase transformation diagrams?

[jklops686]

I am trying to learn how to use phase transformation diagrams and I don't get it. Any help from someone who knows about this would be greatly appreciated (I have a final tuesday) An example problem is below with the picture attached. I have to be able to find what the microstructure is after certain heat times (austentite, martensite ect.) Here's the statement:

10.18 Using the isothermal transformation diagram for an iron–carbon alloy of eutectoid composition
(Figure 10.22), specify the nature of the final microstructure (in terms of microconstituents present and
approximate percentages of each) of a small specimen that has been subjected to the following time–temperature
treatments. In each case assume that the specimen begins at 760°C (1400°F) and that it has been held at this
temperature long enough to have achieved a complete and homogeneous austenitic structure.

(a) Cool rapidly to 700°C (1290°F), hold for 10^4 s, then quench to room temperature.
Solution
The Figure 10.22 upon which is superimposed the above heat treatment.

Solution: After cooling and holding at 700°C for 10^4 s, approximately 50% of the specimen has transformed to
coarse pearlite. Upon cooling to room temperature, the remaining 50% transforms to martensite. Hence, the final
microstructure consists of about 50% coarse pearlite and 50% martensite.


So, first of all, how do I know that after cooling for 10^4 s makes 50% pearlite? Then, after cooling, how do i know that the remaining went to martensite? It's not straightforward on the diagram.

 

[Navin A S]

The diagram shows the time-temperature conditions of the heat treatment and the corresponding microstructures. In this case, cooling rapidly to 700°C (1290°F) and holding for 10^4 s will cause 50% of the specimen to transform to coarse pearlite. This can be seen on the diagram because the transformation line intersects the vertical line at the intersection point of 10^4 seconds and 700°C (1290°F). The remaining 50% of the specimen will transform to martensite when cooled to room temperature. This can be seen on the diagram because the transformation line extends beyond the vertical line at the intersection point of 10^4 seconds and 700°C (1290°F), indicating that the transformation continues after cooling to room temperature.