Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 3 - First specimen
Medium carbon alloy steel bar at 5)x etched
 This medium carbon alloy steel bar caused the failure of a drill bit while being drilled.  In the photomicrograph at left the magnification is 50X.  A Nital etch was used.

The center of the bar is at right here.



The next photomicrograph was made at 500X so you can resolve the individual  microconstituents.
Medium carbon alloy steel bar at 500X etched
 What is the problem with this bar ?




Consider the locations of the hardest microconstituent.

Now look at the explanation.
 



























Explanation: There is ferrite, pearlite, bainite, and martensite in the outer parts of the bar, but all martensite in the center.  The center was therefore more hardenable than the outside of the bar because of macrosegregation; the last metal to freeze, in the center of the original ingot, had more alloying elements than did the first metal to freeze, next to the walls of the ingot mold.
Now see what Specimen 2 has to offer.