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 2 - First specimen
High speed steel, 18-4-1 type at 500X etched
 This is another high speed steel of the 18-4-1 type like the previous specimen, but it has been oil quenched from 1315C.  It has a hardness of Rockwell C63.

What has happened to it ?



Pause a little, formulate  an opinion,
and then look at the answer below.



























Explanation:  The specimen has been burned.  It was hot enough to melt partially, followed by catastrophic oxidation of the liquid at the austenite grain boundaries.  The prior austenite grains became very coarse, and virtually all the M6C and VC carbides have been consumed by decarburization, even though sufficient carbon remained in the austenite to allow martensitic hardening anyway. You can't see it well in this photomicrograph, but oxide and an iron carbide eutectic are left in the prior austenite grain boundaries.
Specimen 2 is a stainless steel.