Often neglected metallurgical factors affecting shrinkage cavity and porosity of ductile iron

Although the metallurgical quality of molten iron is affected by many factors, foundry workers have found the quantitative index of metallurgical quality, namely undercooling. Undercooling degree is an important quantitative index to judge the metallurgical quality of molten iron, which directly reflects the metallurgical quality of molten iron. It is considered that the spheroidizing effect of ductile iron is the best when the undercooling is controlled at 2 ~ 5 ℃. Thermal analysis technology can be used to detect the undercooling and judge the metallurgical quality on line.

With the help of thermal analysis tools, it is considered that when the recycled iron of ductile iron is used too much, the probability of reverse chill is higher. When there is too much recycled ductile iron with anti chill tendency, some should be sold to avoid vicious circle of anti chill quality problems. When the technicians who have been working for a long time on the front line communicate with each other, they often have the same feeling: for example, if the proportion of recycled ductile iron is too large, the shrinkage of the material will increase even if the chemical composition of the product is controlled unchanged and the spheroidizing inoculation process remains unchanged. In this paper, the author has done a targeted comparative test on the proportion of recycled materials used in ductile iron.

(1) The original plan is a long-term stable production plan, the quality of the selected products is stable, and there is no feedback of shrinkage and leakage for half a year;

(2) The main differences between the test scheme I and II and the original scheme are the change of Si in the original molten iron and the change of total inoculation amount caused by the change of charge proportion, the constant proportion of nodularizing agent, the constant composition and temperature of the molten iron;

(3) The main difference between the test scheme III and the original scheme is only the change of charge proportion, while the composition, temperature, nodularization and inoculation amount of the original molten iron and ductile iron remain unchanged;

(4) Low S and low N carburizing agent after high temperature graphitization was used to adjust the C content of the original molten iron in test scheme I, II and III;

(5) The production process, sand, sand core and production control process remain unchanged;

(6) In addition to the Si content in the production of solution strengthened ductile iron, the other elements in the original molten iron are controlled as follows: C 3.30% ~ 3.45%, Mn ≤ 0.50%, P ≤ 0.040%, s ≤ 0.020%, Ti ≤ 0.045%, Cr ≤ 0.05%, and the other elements in the final molten iron are controlled as follows: C 3.15% ~ 3.35%, Mn ≤ 0.50%, P ≤ 0.040%, s ≤ 0.015%, Mg 0.035% ~ 0.055%, re 0.01% ~ 0.02%.

(7) Spheroidizing and inoculation process: the molten iron is treated by spheroidizing with the method of punching. The amount of spheroidizing agent in the four schemes is 1.05%, the grade of spheroidizing agent is mg6re2, and the particle size is 5 ~ 25 mm; The inoculation process is three times, the amount of secondary inoculation is 0.2%, the inoculant is fesi75a11.5a, and the particle size is 2 ~ 3 mm; 0.07% ~ 0.10% of the inoculant was inoculated with fesi75a11.5a.

(8) The original molten iron temperature refers to the high temperature standing temperature rather than the tapping spheroidization temperature, and the final molten iron temperature refers to the temperature of the first pouring box. With the increase of the proportion of recycled material, the tendency of shrinkage porosity of some products is increasing. The main reason may be that the oxides and sulfides of Mg and re caused by the long-term large-scale turnover of the return material affect the purity of the molten iron and reduce the metallurgical properties of the molten iron. For ductile iron hydraulic parts, the proportion of recycled material should not exceed 60%.

The above problems may be related to the control of raw materials and molten iron composition. In fact, as long as the composition of the original liquid iron and the spheroidization and inoculation treatment are strictly controlled, the composition of the recycled material will be exactly the same as that of the nodular iron castings, which will not have a substantial impact on the quality of the nodular iron castings. The main raw materials for melting ductile iron are pig iron, scrap and recycled material. Recycled charge (riser and waste casting) is the cheapest charge with known composition. Generally, the proportion of recycled charge should be kept stable in production, otherwise it is necessary to purchase or sell recycled charge. Pig iron and scrap steel should not only keep the quality and source basically stable, but also keep the price stable and reasonable. The amount of pig iron and scrap is usually adjusted according to composition, source and cost. In foreign countries, synthetic cast iron is not specially used to produce nodular cast iron, but it often contains a certain proportion of pig iron, because a certain amount of pig iron is beneficial to graphite nucleation and obtaining more graphite balls. During the remelting process, both active magnesium and rare earth will oxidize into the slag and no longer have any effect. It should also be noted that the main problem of the recycled material is the high silicon content.

The return material mainly comes from the riser and waste castings. For small thin-walled parts, such as automobile parts, the proportion of gating and riser is large, and the process yield (the ratio of the casting weight in process design to the total weight of casting with gating and riser) is small, usually 40% ~ 60%. The production rate of thick and large parts is high, and the production rate of large parts can reach 80%, even more than 90%. For example, the process yield can be increased from 45% to 60%, and the proportion of riser can be reduced from 55% to 40%. Optimization and strict control of the whole casting process can reduce the scrap rate to improve the yield (the ratio of the total finished casting weight to the total molten iron weight), reduce the scrap rate, reduce the cost and reduce the amount of recycled materials.

The latest production experience has proved that the high quality nodular cast iron can be produced stably by using customized ultra-high purity and high purity pig iron and recycled materials and melting the original molten iron without scrap, and the scrap rate is reduced and the processing performance is improved. The main reason is that the sulfur, phosphorus and other harmful spheroidizing interference elements and elements promoting carbide formation in customized high-purity and ultra-high-purity pig iron are very low, relatively few spheroidizing agents can be added, and it is easy to obtain high spheroidizing rate, thus reducing the tendency of shrinkage porosity and porosity. In addition, due to the absence of scrap, there are no potential elements to promote the formation of carbides, and the elimination of high hardness carbides at the boundary of eutectic cluster and carbon nitrogen compounds of titanium, which not only improves the mechanical properties, but also significantly improves the machinability. All of these reduce the production cost and improve the economic efficiency. Some data show that the super large scale ductile iron nuclear waste tank is successfully produced by spheroidizing the original ductile iron melt completely melted with ultra-high purity pig iron with low magnesium and rare earth spheroidizing agent and inoculating with inoculant. Good spherical graphite is obtained, and there is no fragmented graphite when bismuth and antimony are not added specially to eliminate the fragmented graphite elements, The performance fully meets the requirements.

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