Theoretical analysis of Nodular Cast Iron Based on magma simulation software

Nodular cast iron belongs to multi-element alloy, and the main influencing elements are Fe, C and Si. Its as cast structure is formed through solidification and solid phase transformation according to the law of ternary alloy phase transformation. The solidification process can be divided into two paths: one is to stabilize the system transformation and finally form the normal ductile iron as cast structure of spherical graphite + matrix; The other is to form the as cast structure of spherical graphite + cementite according to the metastable system. The cooling transformation process of nodular cast iron under the condition of stable system is mainly discussed as follows: with the decrease of temperature, the solubility of carbon in molten iron decreases. Under the action of magnesium and other spheroidizing elements, some spherical graphite is precipitated first, which is also called primary graphite. When it is cooled to 1150 ℃, eutectic transformation occurs and eutectic austenite is precipitated. At the same time, The excess carbon is diffused to the outside world and continues to precipitate in the form of graphite to form a large number of eutectic graphite balls. With the end of Eutectic Transformation, the solidification stage of molten iron is completed, and the cooling continues to enter the solid-state phase transformation process.

In the solid-state phase transformation stage, the temperature continues to decrease, and the supersaturated carbon in the eutectic austenite precipitates with the decrease of temperature. Part of it diffuses to the surface of spherical graphite to grow the graphite ball, and the rest forms secondary graphite at the grain boundary. When the temperature decreases to about 750 ℃ (below the eutectoid temperature), eutectoid transformation will occur, that is, the transformation of austenite to pearlite, The products of eutectoid transformation are affected by the chemical composition and cooling rate. Under the condition of basically fixed chemical composition, different cooling rates of eutectoid transformation will lead to the change of pearlite volume fraction. If the cooling rate is fast, C in austenite will not be able to diffuse, so as to form a mechanical mixture of carburizing and ferrite, that is, pearlite. If the cooling rate is slow enough, C in austenite has enough time to diffuse to form ferrite and graphite.

In general, the volume fraction of pearlite is an important index to determine the mechanical properties of nodular cast iron (except for some solid solution strengthening components). In the actual production process, the volume fraction of pearlite is adjusted by adjusting Ce (usually simplified as C% + 1 / 3Si%) and alloy content (generally Cu and Mn). However, when the chemical composition is fixed (or controlled within a certain range), The cooling rate of molten iron has become the main factor affecting the pearlite volume fraction (hardness) of nodular cast iron.

The built-in module in magma software can simulate and predict the hardness of different areas of parts. Therefore, by simulating the cooling curves at different positions and calculating the cooling rate, and by analyzing the relationship between the cooling rate at different positions and the actual hardness of nodular cast iron, find the law between them, so as to predict the volume fraction and hardness of pearlite at other positions of nodular cast iron, The comparative analysis of hardness as a test item mainly takes into account the convenience of hardness test and appropriate fluctuation sensitivity. In addition, in view of the linear relationship between hardness and pearlite, tensile strength, yield strength and elongation, it has a certain reference value for further predicting the mechanical properties of parts at different positions.