Designation for the γ-solid solution in the iron-carbon system.

Austenite is only stable in low-alloy or unalloyed cast iron at higher temperatures. However, by using specific alloy elements (austenite formers) the temperature at which the change of the crystal structure from austenite to ferrite  takes place, can be decreased to room temperature or lower. As early as in 1927, research revealed that the addition of
20 % nickel to the cast iron smelt resulted in a purely austenitic matrix at room temperature.

Nickel, being an austenitizing element, decreases the solid state transformation temperature by around 30 K per 1 to 5 % of added nickel. The eutectoid carbon content is decreased by 0.05% per 1% of added nickel. Onset of the transformation is delayed, pearlite transformation is shifted to lower temperatures and the range is leveled off. Between the transformation onset and the bainite range, which is also shifted downwards, there is a zone with stable austenite. Higher nickel contents in combination with molybdenum will lead to cast iron with acicular structure; even higher contents will lead to a martensitic structure. From about 18 to 20 % nickel content, the structure becomes austenitic, with the limit between stable and unstable austenite depending on the wall thickness of the part and on other alloy elements (Cu, Cr, Mn) added. Fig. 1 

provides an overview of the structures to be expected in dependence of the silicon and nickel contents with carbon content between 2 and 3% and a wall thickness of 25 mm. Austenite provides the required preliminary stage for performance of heat treatment, preferably hardening. The preconditions for austenitization must be selected for each type of casting in order to achieve good solution of carbon but prevent grain growth of austenite due to overheating.

Additional references:

Metal matrix of cast iron
Structure formation of cast iron

  • Fig. 1: Structure diagram in dependence of silicon and nickel contents (acc. to E. Morgan)