Which of the following is the hardest constituent of steel ?

Question

Question

This question was previously asked in

RRB JE ME CBT 2 Previous Paper 1: Held on 29 Aug 2019

Answer 1

Concept:

Martensite:

It is the hardest constituent of steel. The primary reasons accounting for this could be, the internal strains within BCC iron due to the excess carbon presence and due to the plastic deformation of parent FCC iron (Austenite) surrounding the martensitic plate.
The rate of cooling and the amount of carbon percentage in steel are directly proportional to the amount of hardness achieved in martensitic transformation.

Important Points

Bainite:

It is a plate-like microstructure that forms in steels at a temperature of 125-550 (depending on alloy content).
It forms by the decomposition of austenite at a temperature which is above MS but below that at which fine pearlite forms.

Austenite:

It is also known as Gamma-phase iron is a metallic, non-magnetic allotrope of iron or solid solution of iron, with an alloying element.
In plain carbon steel, austenite exists above the critical eutectoid temperature of 1000 K.
Austenite is of FCC crystal structure.

Ledeburite:

In iron and steel metallurgy, ledeburite is a mixture of 4.3 % Carbon in iron and is a eutectic mixture of austenite and cementite.
Ledeburite is not a type of steel as the carbon level is too high although it may occur as a separate constituent in some high carbon steel.