KEY POINTS IN SOLIDIFICATION PROCESS OF GRAY CAST IRON AND NODULAR CAST IRON (3)
Second. Primary AUSTENITE DENDRITE IN CAST IRON
Due to the mechanical properties, Gray cast iron is usually hypoeutectic, and there will be primary austenite dendrite in solidification structure. Under the condition of special properties of cast iron (such as high thermal conductivity and good shock absorption), gray cast iron with near Eutectic or hypereutectic composition is also used, but the demand is very small.
In the past, the study on the solidification process of gray cast iron mostly focused on the formation and characteristics of graphite, the number of eutectic cells and eutectoid structure, and paid less attention to the role of primary austenite dendrites. In fact, the effect of Primary Austenite Dendrite on gray cast iron is similar to that of steel bar in concrete.
The majority of nodular cast iron is eutectic or micro-hypereutectic composition, according to the equilibrium phase diagram, there will be no primary austenite, therefore, in the study of nodular cast iron, more emphasis is placed on graphite and Matrix structure The primary austenite is less discussed than that of gray cast iron. However, under the condition of industrial production, the solidification of nodular cast iron is carried out under the non-equilibrium condition, and the primary austenite dendrite precipitates before the eutectic transformation, and its effect can not be ignored.
1. Precipitation of Primary Austenite Dendrite
Even if the carbon equivalent is as high as 4.7 %, there is still a certain amount of primary austenite in the cast iron for industrial use due to the solidification under non-equilibrium conditions.
(1) Hypoeutectic CAST IRON
The low-carbon primary austenite dendrites begin to precipitate when the hypoeutectic liquid iron with sub-Fe equivalent is cooled below the BC line.
After cooling to T1, the carbon content in the liquid phase increased to C1 because of the precipitation of primary Austenite Dendrite.
When the molten iron is cooled to the EUTECTIC temperature TEG, the carbon content in the liquid phase is the EUTECTIC carbon content C, because the molten iron is not in equilibrium and there is no graphite as the support of the EUTECTIC austenite precipitation, the eutectic transformation cannot take place at this temperature.
When cooling to a temperature below TEG, the carbon content in the liquid phase has increased to C2 along the extension line of BC. After graphite precipitation, the carbon equivalent in the liquid phase decreases to near the EUTECTIC composition, and the austenite crystallizes out with graphite as the core, and the EUTECTIC transformation takes place.
（2）HYPEREUTECTIC CAST IRON
The carbon equivalent of F over-eutectic liquid iron is cooled below the CD line and the primary graphite begins to precipitate.
When cooling to 1, the carbon equivalent in the liquid phase is reduced to c 1’because of the precipitation of primary graphite, and the austenite is not precipitated when the carbon equivalent is still higher than the EUTECTIC composition C.
Cooling to the EUTECTIC temperature TEG, the carbon equivalent in the liquid phase is the EUTECTIC carbon content C, because is not in equilibrium, does not precipitate austenite, also can not occur eutectic transformation.
When cooling to T2 below the EUTECTIC temperature TEG, the carbon content in the liquid phase has been reduced to C2’along the extension line of DC, which is hypoeutectic and precipitates the primary austenite dendrite. Due to the precipitation of primary AUSTENITE, the carbon equivalent in the liquid phase returns to the eutectic composition, and the austenite crystallizes with graphite as the core, and the EUTECTIC transformation takes place.
（3）EUTECTIC CAST IRON
In the non-equilibrium state, even if the carbon equivalent of the EUTECTIC composition of liquid iron is cooled to the EUTECTIC temperature Teg, the eutectic transformation cannot take place immediately.
After cooling to below TEG temperature, primary austenite dendrite nucleation and precipitation. Because of unidirectional nucleation in Molten Iron, it is impossible for graphite to depend on austenite precipitation. After the increase of carbon equivalent in the liquid phase, the carbon equivalent in the liquid phase returns to the eutectic composition, and the austenite crystallizes out with graphite as the core, and the EUTECTIC transformation takes place.