Solidification characteristics of motorcycle piston casting nodular cast iron

Because the piston ring industry in China started to use nodular cast iron as the piston ring material of motorcycles late, and the piston ring for motorcycles is a very thin small part (for example, the rough quality of the piston ring for 90 type motorcycles is only about 129), the cooling speed is fast when casting, the tendency of chilling is large, and free Fe3C is easy to be produced. However, the “paste solidification” characteristic of nodular iron easily leads to various casting defects. Therefore, the casting technology requirements of motorcycle nodular iron piston ring are high, the production is difficult, and the yield is low. Besides the piston rings, the parts of motorcycles are all made in China. However, the casting technology of piston rings for motorcycles can not fully meet the requirements of high speed and high load of motorcycles. Although the properties and heat treatment technology of nodular cast iron materials in China are comparable with those of foreign countries, the mechanical properties of the piston rings of motorcycle nodular cast iron are greatly affected due to the casting defects of macroscopical and microcosmic, thus reducing the competitiveness with foreign products. Therefore, the reasons of several common casting defects of nodular cast iron piston rings for motorcycles, such as subcutaneous porosity, shrinkage porosity, slag inclusion and anti white hole, are explored continuously in this factory, and the technological measures to prevent these casting defects under production conditions are proposed.

The content of carbon in the piston ring of nodular cast iron produced by foundry is 4.5% – 4.8%, which is hypereutectic. It is well known that the spheroidized molten iron with hypereutectic composition first precipitates spheroidal graphite crystal nucleus and grows up under a certain degree of supercooling. Due to the lack of carbon in molten iron near the spheroidal graphite core, a “liquid austenite layer” will form around the graphite core. When the temperature of molten iron drops to the eutectic crystallization stage, the liquid austenite layer around the graphite ball will solidify into a solid shell, and the graphite ball will be surrounded by the austenite shell. The carbon atoms in the molten iron can only diffuse to the graphite through the solid austenite shell, making the graphite ball grow up. At the same time, the iron atoms diffuse from the interface between graphite and austenite, making the austenite shell grow thick. This process is much slower (about 20 times) than the diffusion of carbon atoms in molten iron.

By comparing the cooling curves of gray cast iron and nodular cast iron, the solidification and crystallization of nodular cast iron can be reflected from one side. It can be seen from the figure that the distance between the eutectic start point and the eutectic end point of gray iron is small, and the eutectic solidification is carried out in a narrow temperature range. On the cooling curve, the eutectic platform is long and the rotation angle is obvious, which shows that the eutectic solidification of gray iron is similar to the sequential solidification, while the graphite ball is surrounded by the austenite shell during the eutectic crystallization of nodular iron. In the later stage of eutectic solidification, because the austenite shell around the graphite ball is also growing and thick, it is more and more difficult for carbon atoms to diffuse to the graphite ball. At the same time, the thermal conductivity of nodular iron is 20% – 40% smaller than that of gray iron. Therefore, the heat dissipation is slow, and the power must be obtained by increasing the degree of supercooling to make the graphite ball grow continuously. Therefore, the eutectic crystallization is slow and the eutectic reaction time is long, which is reflected in the cooling curve, The shape of the end of the cooling curve changes slowly, the corner radius is large, and there is no obvious eutectic solidification end temperature. This shows that the distance between the eutectic start point and the eutectic end point of nodular iron is much larger, the temperature range from the beginning of eutectic solidification to the end of eutectic solidification is relatively wide, and the eutectic solidification is approximately carried out in the whole casting at the same time, and there is still liquid on the casting surface at the later stage of solidification, The whole casting is “mushy” without a completely solid shell.

It can be seen from the above that the Eutectic Transformation of nodular iron needs to be carried out under a large degree of supercooling, so the eutectic crystallization characteristics of nodular iron are: very high nucleation rate and low growth rate, the eutectic group of nodular iron is much smaller than that of gray iron, and the number of eutectic groups per unit area is several hundred times more than that of gray iron. Therefore, in the eutectic solidification stage of nodular iron, once the internal molten iron temperature reaches the eutectic solidification temperature, a large number of solidification begins, so that when the Eutectic Transformation on the surface of the casting is not completed, the eutectic transformation begins in the center of the casting. At the end of eutectic solidification, a large number of graphite austenite eutectic clusters will collide with each other, while the molten iron that has not been solidified will be separated by graphite austenite eutectic clusters and become discontinuous. At this time, the molten iron will not flow, thus presenting the coexistence state of solid phase and liquid phase on the whole casting section, which is the “paste solidification” of nodular iron.

To sum up, the solidification characteristics of ductile iron parts are that, except for a very thin layer on the surface of the casting, the whole casting is solidified at the same time. This leads to the situation that the channel of external feeding and gas and inclusion floating out in the solidification process is blocked too early, which is the basic reason of the large defect tendency of ductile iron casting.

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