Spheroidite

Spheroidite is a distinctive pearlitic transformation product in steel, characterized by cementite (Fe3C) existing as discrete, rounded particles embedded in a ferritic matrix. It forms by prolonged isothermal annealing of pearlitic steel at temperatures below the eutectoid temperature (Ac1), typically in the range of about 550 to 750°C, though exact values depend on composition. Over time, lamellar cementite in pearlite coarsens and becomes spherical to minimize interfacial area, resulting in a microstructure in which ferrite grains contain evenly distributed spherical cementite particles.

This transformation is diffusion-controlled; carbon diffuses from cementite into ferrite, and cementite lamellae disappear in favor of spheroids. The process requires long soak times, from many hours to days, to achieve a uniform spheroidite structure.

Mechanical properties of spheroidite differ from pearlite: the material becomes softer and more ductile, with reduced yield and tensile strength but greater elongation and impact resistance. The improved ductility makes spheroidized steel easier to cold work and machine, and it is commonly used to produce preforms, wires, and components that require high formability after drawing or rolling.

Spheroidite should be distinguished from other low-temperature transformation products such as bainite or tempered martensite. It is typically observed with optical microscopy and electron microscopy as ferrite with dispersed spherical cementite particles.

In summary, spheroidite is a ferrite–cementite composite microstructure formed by long, low-temperature annealing of pearlitic steel, yielding improved ductility at the expense of some strength and hardness.