The increase of austenitizing temperature (t γ) and the prolongation of austenitizing holding time (t γ) can increase the solid solution amount of carbon in high temperature austenite, and then improve the stability of high carbon austenite after isothermal transformation.
The range of t γ is 870 ~ 950 ℃. When t γ is too low, the as cast microstructure cannot be completely transformed into austenite; when t γ is too high, the grain size of austenite at high temperature is coarse, which leads to the coarse matrix structure of ADI after isothermal transformation.
In addition, the selection of t γ also depends on the w (SI) content in nodular iron, the amount of carbides in as cast structure and the performance requirements of materials.
The selection of t γ is mainly affected by t γ, heating rate and casting wall thickness. If t γ is high, the heating rate is fast and the wall thickness of casting is small, t γ is longer.