Schematic room-temperature microstructures for four iron–carbon alloys are as follows. Rank these microstructures (by letter) from the hardest to the softest

(c) A > B > D > C

(d) None of the above

SOLUTION

The correct answer is B: C > D > B > A. Specimen C will be the hardest because it has the highest carbon content inasmuch as proeutectoid Fe3C is present; also the pearlite is fine pearlite. Specimens B and D have approximately the same carbon content since the proportions of proeutectoid ferrite in both are about the same. However, carbon contents for B and D are lower than for specimen C because of the presence of proeutectoid ferrite. Specimen D is harder than B because its pearlite is fine, whereas B's is coarse. Finally, specimen A is the softest since it is composed entirely of α-ferrite and, thus, has the lowest carbon content.