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 - Eighth specimen
5% carbon cast iron at 100X etched
 In this 5% carbon cast iron, shown here at 100X magnification, what is the primary phase ... the first solid to form during solidification ?
Think about the metastable iron - carbon phase
diagram again ... and then go on to the answer below.



























Answer:  Of course, this has to be a white iron, and so the primary phase is cementite.  It couldn't be austenite ... that would have decomposed and would have developed a recognizable characteristic microstructure by now.  The matrix is ledeburite, but it is too deeply etched to reveal the pearlitic microconstituent.  This alloy might be useful for wear plates in heavy machinery, but it would be extremely brittle.
Specimen 9 is a nodular iron with an unusual microstructure.