| 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.
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This is the core of the first
specimen, gas carburized 8 hours at 895C, furnace cooled, then
reheated to 955C and oil quenched. What do you see ? Formulate your
opinion, then proceed.
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The core consists of low carbon martensite ... there are no proeutectoid microconstituents because of the adequate hardenability of this steel. |
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The case is shown here ... What microconstituents do you see ? OK - now proceed to the answer.
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| Explanation: The case,
which has a much higher carbon content than the core,
has retained austenite (which is not so easy to see in this photomicrograph) plus high carbon martensite, which
etches tan instead of the core's bluish-brown
color. |