Sementtiittiyhdistelmä

Sementtiittiyhdistelmä refers to cementite, a chemical compound of iron and carbon with the formula Fe3C. It is an important phase in steel and cast iron metallurgy, characterized by its extreme hardness and brittleness. Cementite is the hardest constituent found in steels, contributing significantly to their strength and wear resistance.

The formation of cementite is a crucial aspect of heat treating processes like hardening and tempering in steel. When steel is heated to high temperatures and then rapidly cooled (quenched), carbon atoms are trapped in a metastable phase called martensite, which is a supersaturated solid solution of carbon in iron. Upon subsequent heating to lower temperatures (tempering), carbon atoms diffuse and precipitate out of the martensite, forming fine particles of cementite within a ferrite matrix. The size, distribution, and morphology of these cementite particles greatly influence the mechanical properties of the tempered steel, such as its toughness and ductility.

In cast iron, cementite can exist as a stable phase, particularly in white cast iron, which contains a high percentage of carbon. The presence of cementite in white cast iron makes it very hard and abrasion-resistant, but also brittle, making it difficult to machine. In contrast, gray cast iron contains graphite flakes, which reduce its overall hardness and strength but improve its machinability and damping capacity. Understanding the behavior and formation of cementite is fundamental for controlling the properties of ferrous alloys.