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Trying to understand how to read phase transformation diagrams?

  1. May 12, 2013 #1
    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.
     

    Attached Files:

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