Effect Of Rare Earth Alloy Modification On Microstructure And Properties Of High Carbon Equivalent Gray Cast Iron

The high carbon equivalent gray cast iron means the gray cast iron whose carbon equivalent is near the eutectic point. This cast iron gets good wear resistance and good castability. Compared with other metals, it is easier to fill and process, needs simpler production equipments, easier molding process, and lower costs. So, the high carbon equivalent gray cast iron is more and more appreciated in automobile industry. Brake drums and brake discs of the automobile must be made of this high carbon equivalent gray cast iron. This cast iron also can be used for a variety of large thin-walled workpieces, especially for a variety of cylinder blocks and cylinder covers. As the improving of casting technology, the high carbon equivalent cast iron will be more and more widely used. However, the mechanical property of this cast iron is relatively poor, such as lower strength and higher brittleness. So if we can guarantee the castability of high carbon equivalent gray cast iron while make its mechanical properties to meet the needs of industry, the high carbon equivalent gray cast iron will have a better prospect.Rare earth can play the role of modification, not only in the high carbon equivalent cast iron but also in the low carbon equivalent cast iron. Researches have shown that rare earth can make important impact on the structure and property of cast iron. Using rare earth alloy as the modification to change graphite morphology and to produce ductile cast iron and vermicular cast iron has been very mature at home and abroad, but the effect of rare earth alloy modification on high carbon equivalent gray cast iron needs further study. At present, many foreign factories are using rare-earth-based modification, but rare earth alloy modification has not been widely used in China. Therefore, it is important to study the mechanism of rare earth alloy modification on the high carbon equivalent gray cast iron and find the way to improve mechanical properties.In this paper, we analyze the effect of rare earth alloy modification on high carbon equivalent gray cast iron from castability such as liquidity, contractility, etc., mechanical property such as tensile strength, hardness, etc.,as well as microstructure such as graphite, matrix structure, primary austenite, eutectic etc..In the way of castability, the overall liquidity of high carbon gray cast iron equivalent is in the 1020-1400mm before and after adding modification. This explained the high carbon equivalent gray cast iron still remains a good liquidity by rare earth alloy modification; when the proportion of rare earth alloy is 0.10%-0.30% in weight, the chill width is significantly reduce, which is about 2.2mm. This explained rare earth alloy has perfect inoculation within the limited range; after a certain proportion of rare earth alloy modification, the shrinkage tendency of high carbon equivalent gray cast iron is reduced.In the way of mechanical property, when a certain proportion of rare earth alloy is added into high carbon equivalent gray cast iron, the tensile strength of cast iron becomes better. The functional relationship between adding rare earth alloy and tensile strength of cast irons shows a "double peak curve". When the proportion of rare earth alloy is 0.20% in weight, the tensile strength reaches the first peak which is 20% higher than that of untreated high carbon equivalent gray cast iron; increasing the proportion of rare earth alloy to 0.25%, the tensile strength decreases; with further increasing the proportion of rare earth alloy to 0.30%, the tensile strength reaches 188MPa which is 30% higher than that of untreated high carbon equivalent gray cast iron.In the way of microstructure, the addition of rare earth alloy increases the number of primary austenite dendrites, reduces secondary dendritic arm spacings, and changes the eutectic size and quantity. As the primary austenite dendrites increasing, the space for growth of graphite is reducing, the graphite nucleation becomes denser, and the growth of graphite is hindered by skeleton. Finally we could get type D graphite which is much smaller than the type A graphite. Therefore, at a characteristic addition of rare earth is not only to refine the dendrite of primary austenite, to refine graphite, but also to reduce the fragmentation of graphite. When rare earth alloy is added into gray cast iron, the morphology and quantity of graphite play a major role on the improvement of tensile strength. The results showed that the adding right amount of rare earth alloy modification with the molten iron current can not only improve the castabilities of high carbon equivalent gray cast iron, but also meet the need for its mechanical properties. From saving metals and reducing the cost, this cast iron will get considerable commercial and social benefits.