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.
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
Tag: Why Martensite Is Harder Than Austenite