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 - Seventh specimen
Meehanite casting at 100X etched
 This is a hard spot in a Meehanite (proprietary alloy) cast iron with 3% carbon. 








The photomicrograph at left was made at 100X; the next image describes schematically the location of the hard spot in the original casting.
Diagram of hard spots in a Meehanite casting








The normal structure of this material is pearlitic grey cast iron.  See if you can make sense of the microstructure in the next image (at 200X) before going to the last image on this page.
Meehanite casting at 200X etched
 Of course, the hard spot is white cast iron, which formed because of excessively rapid solidification of the corners of the section between the tabs in the sketch above.




Now look at the 500X photomicrograph
of the specimen at the bottom of the page.



























Meehanite casting's hard spot at 500X etched
Explanation:  The microstructure is quite obvious now at 500X magnification in the image at left. 

Verify the 3% overall carbon composition by estimating the area fraction of each microconstituent and then  multiplying by the individual carbon contents ... 6.67% carbon for cementite and 0.8% carbon for the pearlite.



There has to be about 3/8ths massive cementite and 5/8ths pearlite for the maths to work out OK.
Specimen 8 is an unusual 5% carbon cast iron.