Glossary

Martensite is a hard, brittle microstructural phase formed in steel when austenite (the high-temperature FCC phase of iron) is rapidly cooled—or quenched—so quickly that carbon atoms don’t have time to diffuse. Because the transformation happens under diffusionless, high-speed conditions, the crystal structure shifts from face-centered cubic (FCC) to a highly strained body-centered tetragonal (BCT) form rather than transforming into ferrite + cementite (as it would during slow cooling).

This trapped carbon in a distorted lattice creates extremely high internal stress, which gives martensite its defining properties: very high hardness and strength, but low ductility and toughness. That’s why quenched steels are strong but brittle unless further treated.

After martensite forms, steels are typically tempered—heated to a much lower temperature (like 200–600°C)—to relieve stress, reduce brittleness, and allow controlled precipitation of carbides, producing tempered martensite, which offers a better balance of strength and ductility.

Martensite is the foundation of heat-treatable steels used for tools, springs, cutting edges, automotive components, gears, and high-strength fasteners. It is a cornerstone phase in metallurgy because controlling how much martensite forms—and how it is tempered—allows engineers to tune steel performance for extreme mechanical demands.