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
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
- 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.