Which structure of the following is hardest structure of steel?

Explanation:

Martensite: 

• It is a metastable phase of steel formed by the transformation of austenite below 320°C.
• Martensite is an interstitial supersaturated solid solution of carbon in α – Iron and has a body-centered tetragonal lattice.
• It has a carbon content of up to 2% and is extremely hard and brittle.
• It is a product of rapid cooling (quenching) and possesses an acicular or needle-like structure.
• The rate of cooling and the amount of carbon percentage in steel are directly proportional to the amount of hardness achieved in martensitic transformation.

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.