Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Cast Irons, High Alloy Steels, and Superalloys - Lesson 1 - First specimen
Ferritic grey cast iron at 100X Nital etch
 The photomicrograph at left shows a grey cast iron at 100X.  It has been etched with Nital, as have all the other cast irons in this lesson.

Note that the eutectic graphite is arranged in rosettes here.  This is a hypereutectic composition.

What happened to the austenite ?


Go back & look at the iron - carbon phase diagram, and the go on to the answer.





























Answer:  The austenite decomposed to ferrite plus graphite, and the graphite added itself onto the graphite flakes that were already present.  Faster cooling rates can allow pearlite to form, or even martensite if the casting is quenched.
Ferritic grey cast iron at 500X etched
At 500X at left you can see the large graphite flakes plus grey MnS inclusions and a few irregular islands of iron - iron phosphide eutectic microconstituent.  These features are seen in most cast irons.

Note - I reduced the contrast so that you can see some of the microstructure in the graphite flakes.
Specimen 2 is a pearlitic grey cast iron.