Martensite: The Steel Named After a Man

Outside the mill, martensite means nothing. Most people have never heard the word. Inside metallurgy, it is one of the three main microstructures that iron and carbon form when steel cools, and the hardest and most brittle of them all.

Martensite forms when molten steel cools very fast, so fast that the carbon atoms do not have time to diffuse out of the iron lattice. They get trapped in place. The iron crystal structure shifts from body-centered cubic to body-centered tetragonal, a distorted shape that locks the carbon in and creates internal stress. The result is a hard, needle-shaped structure visible under the microscope as a pattern of fine lines, called laths or plates. This is the microstructure that gives a hardened steel its edge, its ability to hold a sharp point.

The other common microstructures in steel are ferrite, which is soft and ductile, and pearlite, which is a layered mixture of ferrite and cementide (iron carbide) that forms when steel cools more slowly. A blacksmith or heat treater moves between these states by changing the speed of cooling. Quenching in water or oil produces martensite. Slow cooling in air or furnace produces pearlite. Tempering, which is reheating martensite to a lower temperature and cooling again, trades some hardness for toughness by allowing the carbon to move and form cementide particles.

The word comes from Adolf Martens, a German metallurgist and engineer who lived from 1850 to 1914. Martens designed the first metallurgical microscope and was one of the first to observe and describe the needle-like structures in hardened steel. His work laid the foundation for the modern study of steel microstructure. The International Institute for Iron and Steel Research in Berlin, which he helped establish, still bears his name. In 1890, the German metallurgist Georg Dantszen coined the term martensite in his honor, and it has been the standard name in English ever since.