Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 2 - First specimen
1.34% carbon steel at 200X etched
 The first specimen in this lesson, which has 1.34% carbon, and which was slowly cooled from the all austenite region of the iron-cementite phase diagram, consists of pearlite delineated by a network of cementite.  There are also Widmanstaetten cementite platelets crossing the interiors of the larger prior austenite grains.

The first photomicrograph at left was taken at 200X; the one below it was made at 500X.  A Nital etch was used.
1.34% carbon steel at 500X etched
 How is the prior austenite grain size indicated, why is it not uniform, and why does the Widmanstaaetten cementite form mainly in the larger austenite grains ?




Think about your answers for a while and then take a look at the explanation.



























Explanation: This is a hypereutectoid steel.  During the (incorrect) slow cool from the all austenite region, cementite precipitated, mainly at the austenite grain boundaries, decorating them during passage through the austenite plus cementite two phase field of the iron - cementite phase diagram.  The austenite grains were of nonuniform size because of discontinuous grain growth; small impurity particles stopped normal grain growth by pinning the austenite grain boundaries until excessive heating permited some of the particles to dissolve.  Thereafter, a few austenite grain boundaries broke free of the remaining particles, consuming most of the remaining austenite grains.  Carbon could not diffuse out of the centers of the larger austenite grains to join the cementite forming at the grain boundaries during the subsequent cooling, so cementite platelets grew inwards from the grain boundaries in order to consume the excess carbon in the grain interiors.  Proeutectoid cementite like that seen here can be distinguished from proeutectoid ferrite because the cementite at the prior austenite grain boundaries is continuous with the minority phase (also cementite) in the pearlite.  Cementite is a different color than ferrite, a faint lemon yellow instead of off white grey in the case of ferrite.  For a hypoeutectoid steel, the ferrite at the prior austenite grain boundaries would be continuous with the majority phase in the pearlite.                                                                                 
Proceed to Specimen 2.