Spheroidisation

Spheroidisation is a heat treatment process applied to ferrous alloys, primarily steels, to alter the morphology of carbide precipitates. Instead of lamellar or network structures, the goal is to create spherical or globular carbide particles dispersed within the ferrite matrix. This transformation is achieved by heating the steel to a temperature slightly below the lower critical transformation temperature (Ac1) and holding it for an extended period, followed by slow cooling. Alternatively, it can be achieved through repeated heating and cooling cycles around the Ac1 temperature.

The primary purpose of spheroidisation is to improve the machinability of steels. The rounded carbide particles offer less resistance to cutting tools compared to sharp, elongated structures. This leads to reduced tool wear, lower cutting forces, and a smoother machined surface finish. Spheroidisation also enhances the cold formability of steels, making them more pliable for operations like drawing and stamping without fracturing. While it softens the material, it can slightly reduce tensile strength and hardness. The specific temperature, time, and cooling rate depend on the steel's composition and desired outcome.