Martensite

Martensite is a hard, metastable crystal structure formed when steel is rapidly cooled from high temperature. Its formation involves a diffusionless, coordinated rearrangement of atoms.

Austenite: A face-centered cubic phase of iron that can dissolve substantially more carbon than ferrite. Martensite forms when austenite transforms during rapid cooling.
Iron–carbon phase diagram: A map of equilibrium phases in iron–carbon alloys as a function of temperature and composition. It shows the high-temperature austenite region from which steel is quenched.
Quenching: Rapid cooling from an elevated temperature to alter a material’s structure and properties. Quenching steel can suppress diffusional transformations and produce martensite.
Vickers hardness test: A hardness test that measures the indentation produced by a diamond pyramid under a specified load. It quantifies the high indentation resistance often produced by martensitic transformation.
Martensitic transformation: A diffusionless transformation in which a parent crystal changes structure through coordinated atomic motion. This transformation converts austenite into martensite without long-range carbon diffusion.
Ferrite: A body-centered cubic phase of iron that holds little carbon in solid solution. Ferrite is a lower-carbon iron phase often present alongside martensite in steel.
Tempering: Heating quenched steel below its critical transformation temperature to adjust its properties. Tempering reduces brittleness and relieves stresses in as-quenched martensitic steel.
Fracture toughness: A material’s resistance to crack growth under applied stress. Un-tempered martensitic steel can have low resistance to crack propagation.
Diffusionless transformation: A phase transformation in which atoms move cooperatively without long-range diffusion. Martensite is a defining example of this transformation mechanism.
Pearlite: A layered mixture of ferrite and cementite formed by slow cooling of austenite. Unlike martensite, pearlite forms through diffusion during relatively slow cooling.

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