Question

Quenching is an effective means to harden carbon iron steels. If not carefully, the steels can easily break due to the rapid crystal structural change during quenching. An atomic packing factor is an important parameter to explain this phenomenon. When FCC iron changes to BCC iron, the volume will:

A.Decrease

B.Increase

Answer 1

The answer is B, the volume will increase as the FCC iron changes to BCC iron. As we know, the transformation occur below 1180K, that is iron crystallizes in BCC pattern upto 1180K, beyond that phase transition to FCC pattern occurs, which results in increase in density by approximately 9% (shrinking of iron particles is observed). As the density of FCC pattern is more than BCC pattern and they are inversely proportional to volume. Thus decrease in density of BCC pattern results in increase of volume.