Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 2 - Second specimen
High carbon tap at 200X etched
 This high carbon tap was first roll threaded and then heat treated by oil quenching from 790C.  It is shown here at 200X magnification; a Nital etch was used.

The image is not terribly clear; the less decarburized  metal is to the right of the bright patches of ferrite.  There are more cementite particles at the right than in the middle, next to the ferrite patches.


What is wrong with this cutting tool ?
Here's the answer.



























Explanation: This is a case of extreme decarburization.  There is ferrite at the surface, and then a carbon gradient in the martensite formed from what had been austenite plus cementite.  The decarburization occurred after the threading operation, during the austenitization.  Note that there is a discontinuity in carbon content at the ferrite - prior austenite phase boundary, exactly as dictated by the iron - cementite phase diagram.  The ferrite deformed severely when an attemp was made to use this tool; that is why the usual familiar tooth shape is now quite unclear.
Go on to Specimen 3.