Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 4 - Third specimen
Heat treated carburized steel case at 500X etched
 This is another carburized plain carbon steel, originally 0.2% carbon, which has been furnace cooled after carburizing, then reheated to 930C and water quenched.

The photomicrograph at left was made at 500X near the high carbon surface.  It shows martensite plus retained austenite.  The austenitization temperature was high enough to put most of the carbon into solution in the austenite, so the martensite start temperature was above room temperature.
Heat treated carburized steel subsurface at 500X etched
 The subsurface, shown at left at 500X and also etched with Nital, consists of blue unresolvable fine pearlite which formed first (because of the low hardenability of the plain carbon steel) plus lower carbon martensite than that near the surface in the photomicrograph above.
Heat treated carburized steel core at 500X etched
 The core (500X at left) consists of proeutectoid ferrite with a Widmanstaetten morphology due to the rapid rate of phase transformation during cooling plus low carbon martensite.


This is a tough microstructure for the core of such a workpiece - much tougher than a high carbon non case hardened steel would be - hence, the value of carburization.
Specimen 4 is the same steel and carburization treatment, but heat treated differently.