When the thermal treatment known as austempering is applied to spheroidal graphite cast iron, it produces a structure with a very interesting combination of mechanical properties.

This treatment is preceded by a complete austenitizing of the part at a temperature of around 900°C. It is then rapidly subjected to isothermal austempering, with the following results:

• Upper bainite, if the treatment temperature is between Bs and 350°C.
• Lower bainite, if the austempering is done between 350°C and Ms.

The reaction occurs in two stages:

• Stage 1:  γ parent (α)  +  (γ)
• Stage 2:  (α) + (γ)  α  +  silico-carbides

It is essential to prevent the second stage from beginning, and the end of the first stage is therefore precisely determined, being the point at which the apparent end of the TTT curve is reached, but not the real end, which is determined by the impossibility of any further martensite formation, as a residual austenite is produced, rich in carbon and stable at operating temperature.

Fundiciones Urbina has its own technology for the precise determination of the real end of the first stage, guaranteeing both the structural stability of the cast iron and the predicted operational behaviour.