Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 4 - Fourth specimen
Heat treated carburized steel case at 500X etched
 This is the same steel and carburization treatment as in Specimen 3 which you just examined, but this time it was reheated to only 790C and water quenched.

The three photomicrographs were all taken at 500X with a Nital etch.  The first one, at left, shows the case.  The second one shows the subsurface; and the third one shows the core.  Be sure you understand the differences in case and core microstructures in these two specimens.

The surface case consists of martensite plus cementite without any retained austenite ... the austenite from which the  martensite formed had a lower carbon content ... about 0.8% ... because the excess carbon was sequestered in the cementite formed just above the eutectoid temperature, even though the overall composition was hypereutectoid at the surface.
Heat treated carburized steel subsurface at 500X etched
 The subsurface consists of proeutectoid ferrite formed during the partial austenitization at 790C.  A very small amount of fine pearlite formed during the water quench, as did an even smaller amount of martensite. 

There is less pearlite here at a hypoeutectoid carbon level than in the specimen austenitized at 930C because of the higher carbon content of the austenite in equilibrium with ferrite.  That is the converse of the hypereutectoid case.  This austenite is now more hardenable because of its higher carbon content.  That is why the patches of martensite could form next to the pearlite ... which did not completely transform from that austenite during the quench; it was this "retained" austenite from which the martensite formed.
Heat treated carburized steel core at 500X etched
 The core has more ferrite than the one austenitized at the higher temperature, because it spent time sitting in the equilibrium ferrite plus austenite field before being taken out of the furnace to be quenched ... not just a brief interlude during the quench as did the specimen heated to 930C in the single phase field.

There is also some fine pearlite as well as a little more associated martensite than in the subsurface because of the faster quench rate of the overall specimen that begins, once the outer surface temperature drops below the boiling point of the water.  The cooling rate during transformation of the subsurface austenite was actually slower because there was a steam envelope insulating the piece at the time that the austenite was decomposing there.
You should now go back to Specimen 3 and restudy these six images until the quantitative and qualitative differences in microstructure are clear to you.
Once you are satisfied, go on to Specimen 5.