martensiitiksi

Martensiitiksi is a Finnish term used in metallurgy to describe the transformation of austenite into martensite in iron–carbon alloys, most commonly steel. The expression can refer to the process itself or to the resulting martensitic microstructure.

Mechanism and conditions: Martensiitiksi formation occurs by a diffusionless, displacive phase transformation that takes place when austenite is cooled rapidly, typically by quenching. The transformation is largely athermal, proceeding as the lattice rearranges without long-range atomic diffusion. The carbon atoms are largely retained in solution, producing a supersaturated, hard and brittle martensitic phase. The temperatures at which martensite first forms (Ms) and finishes (Mf) depend on carbon content and alloying elements; higher carbon generally lowers Ms, and elements such as nickel, chromium, manganese and others shift the transformation temperatures.

Microstructure and alternatives: Martensite in steel appears as needles or plates, often described as lath or plate martensite depending on composition and cooling rate. The morphology influences mechanical properties. Tempering after quenching converts martensite into tempered martensite, reducing hardness while increasing toughness.

Properties and applications: Martensiitiksi or martensitic steel typically offers very high hardness and strength, but limited toughness if left untempered. Tempering or alloying can tailor properties for specific applications. Martensitic transformations are central to heat treatments used for cutting tools, gears, springs and certain structural components where high surface hardness is advantageous.

Historical note: The phenomenon is named after the study of martensite in steels in the late 19th to early 20th century, with Adolf Martens contributing to the foundational understanding of such diffusionless transformations.