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1) Martensite is the hardest and most brittle microstructure obtainable in a given steel.
2) Martensite hardness of the steel is a function of the carbon content in that steel.
3) Martensite results from cooling from austenitic temperatures rapidly by pulling the heat out using a liquid quenchant before pearlite can form.
4) As quenched Martensitic structures are too brittle for economic use-they must be tempered.
5) Reheating as quenched Martensite to a temperature just below the AC1 results in the best combinations of strength and toughness.

This is what you get when you cool faster than the critical cooling (pearlite transition) rate- Martensite

 
Hardness of martensite is a function of carbon content

 
Softening of martensite in 0.35%C, 0.8% C, and 1.2% C carbon steels by tempering at the indicated temperature for 1 hour.

Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result  from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite.
Formation of Martensite involves a transformation from a body-centered cubic structure to  body-centered tetragonal structure. The large increase in volume that results  creates a highly stressed structure. This is why Martensite has a higher hardness than Austenite for the exact same chemistry…
Photo  and Graphs Credit: Cold Finished Steel Bar Handbook

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