Which description best defines spheroidite?

• A. Microstructure found in steel alloys consisting of spherelike cementite particles within a ferrite matrix; produced by elevated-temperature heat treatment and is relatively soft.
• B. A lamellar alternation of ferrite and cementite layers.
• C. A network of spheroidal ferrite within a cementite matrix.
• D. A single phase of cementite throughout steel.

Answer: (A)

Spheroidite is a microstructure in which cementite exists as small spheroidal particles dispersed in a ferrite matrix. This form develops when steel that has pearlite is held at elevated temperature for a long enough time for the lamellar cementite to break up into spheres, allowing carbon to diffuse and reconfigure into these rounded particles within ferrite. The result is a much softer and more ductile structure than pearlite, because the interfacial area between phases is reduced and the brittle cementite lamellae are no longer present. The other descriptions describe either the lamellar arrangement of pearlite, a cementite-containing network, or a single-phase cementite, none of which match spheroidite’s dispersed spheroidal cementite in ferrite.