Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 4 - Seventh specimen
The present pair of SAE2315 steel specimens ... Specimen 6 and Specimen 7 ... was carburized and heat treated similarly to the earlier, plain carbon steel pair (Specimen 4 and Specimen 5).  However, the present alloy steel is more hardenable.  See if you can correctly interpret the microstructures of the core and case in each one.
All the photomicrographs of these two specimens were made at 500X with a Nital etch.
SAE2315 steel carburized and heat treated core at 500X etched
 This specimen was gas carburized just like Specimen 6, but it was austenitized at only 790C before the oil quench.

Again, the core is shown first.

What is the microstructure of this core ?
OK - proceed.




























Now, the martensite etches tan, even in the core, because its composition is set by the right hand end of the alpha plus gamma tie line ... about 0.7& carbon.  Hence, it is high carbon martensite, and the fraction of low carbon ferrite accounts for the 0.15% average carbon content of the core.
SAE2315 steel carburized and heat treated case at 500X etched
 The case is different now, too.  Why ?


OK - see if you have this worked out right.



























Explanation:  There is no longer any retained austenite in the case because the excess carbon is hed by the proeutectoid cementite in the slightly hypereutectoid case.  Now, the case's martensite has about 0.8% carbon, not much different from the martensite in the core !
You should now go back over this pair (Specimen 6 and Specimen 7)
as well as the previous pair, Specimen 4 and Specimen 5).
Now we switch to a series of three nitrided steels, starting with Specimen 8.