Analysis on the problems of inoculation treatment of cast iron

Before pouring, a certain amount of substance (inoculant) is added to the molten iron under certain conditions (such as certain superheat temperature, chemical composition, appropriate adding method, etc.) to change the solidification process of the molten iron, improve the as cast structure, and achieve the purpose of improving the performance, which is called inoculation treatment. It is difficult to explain this change in inoculation treatment with the subtle change of chemical composition. In industrial production, inoculants have been widely used, but how to use them efficiently remains to be studied. This paper summarizes and analyzes the inoculation treatment of cast iron through literature research.

I. inoculation of elements in common iron inoculants

(1) Si can promote graphitization, on the one hand, increase the content of ferrite; on the other hand, it can strengthen ferrite by solid solution. It is often used at the same time with CA to enhance the effect. Generally, the content of ω Si = 2.6% ~ 3.2%. (2) Ca and Si are usually used at the same time, and the suitable temperature is 1370 ~ 1430 ℃, but the slag hole defects are easy to be produced by adding too much ca. (3) Ba can inhibit the incubation decline and increase the amount of ferrite. Ferrosilicon containing Ba not only has good anti-aging property, but also is effective in preventing white mouth. (4) Al can effectively promote graphitization, but it is easy to cause subcutaneous pores. It is often used with ferrosilicon to improve the effect. (5) Mn has the effect of promoting pearlite, and the effect is good when it works with nitrogen, which can make the melting point of inoculant (Zr system) drop. Generally, ω Mn = 0.2% ~ 0.4%. (6) The sulfur content in re liquid iron has a great influence on its effect. When ω s = 0.035% ~ 0.1% in re liquid iron, the effect is better. It can refine graphite and inhibit white mouth. (7) SR has the effect of inhibiting white mouth and enhancing graphitization, but the effect is poor when there are Ca and CE elements. Fe Si SR System inoculant is used in grey cast iron. When ω s < 0.05%, the effect is good. (8) Zr can promote graphitization. Fe Si Zr inoculant is difficult to melt, which can prevent nitrogen pores, and Mn element can promote its melting. (9) Bi increases the number of graphite spheres, reduces the tendency of white mouth, increases the content of ferrite in the matrix, and often acts with Fe Si to refine the grains. In addition, Bi Cu inoculation can promote the growth of graphite core during annealing.

II. Selection of effective pregnant elements

1. The general selection principle (1) the inoculant elements that can effectively prevent the occurrence of white mouth are: Ba, re, Sr., Si. (2) The effective pregnant elements for graphitization are ba and ca. (3) The elements that can inhibit or slow down the incubation decline are: Ba, CA, re. (4) The elements that promote the inoculant to melt into the molten iron are Mn and Mg. 2. Special selection principle (1) rare earth ferrosilicon magnesium alloy can be selected to prevent cracks of large thick wall ash casting. Its composition is ω re = 13.24% ~ 13.69%, ω mg = 12.76% ~ 13.11%, ω Si = 38% ~ 44%, ω CA < 4%, and the addition amount is 0.25%. (2) Fesi72 inoculant can be selected for the production of Ferritic Ductile Iron Castings, with the particle size of 2-6mm and the addition amount of 1.5%. At the same time, 0.16% pure aluminum is added for the composite strengthening inoculation, and the effect is good. (3) In order to prevent nodular cast iron from inoculation decay, fesi75 with particle size of 3-8mm can be added in the first inoculation, 0.5-2mm in the second inoculation, and 0.3% – 1% in the third inoculation. (4) The inoculant with titanium content of 0.08% ~ 0.15% can be selected for large section high carbon equivalent and high strength cast iron; the inoculant with silicon barium rare earth alloy should be selected for high carbon equivalent thin-wall gray iron castings, which can reduce the tendency of white mouth, reduce the hardness of castings, inhibit the incubation decline, etc., with good effect, and the addition amount of 0.3% ~ 1%. (5) The ability of Re Si inoculant to eliminate white hole is strong, but the tendency to form D and E-type supercooled graphite is large, so it is difficult to reduce the hardness of castings. Si Ba inoculant can eliminate D-type and E-type supercooled graphite and promote the formation of A-type graphite. (6) The content of sulfur in molten iron has a great influence on inoculation treatment. In the molten iron with high sulfur content, the inoculants of graphite system (such as electrode chips) and n system (such as Fe Mn-N) are generally effective; in the molten iron with low sulfur content, the inoculants of CE, CA, Ba, n system (such as Re Si, CA Si Ba, Fe Mn n, CA Si SR) are effective. 3. The amount of inoculant (1) generally, the amount of inoculant in nodular cast iron is more than that in grey cast iron. (2) If the pouring time of the inoculated molten iron is long, the amount of inoculated molten iron should be increased because of the inoculating decay. (3) It is easy to produce white mouth in thin-walled castings, and the amount of inoculant should be increased accordingly. (4) When there are a lot of slag in the molten iron, the inoculant will be wrapped and difficult to melt once it contacts with the slag, so the amount of added slag will increase correspondingly. (5) If the inoculant is added too much, the temperature of molten iron will be reduced and the shrinkage of cattle will be produced by eutectic group. If the inoculant block is too large, it will not be able to inoculate the molten iron evenly, and the unfused inoculant will be poured into the mold cavity; if the block is too small, it is easy to oxidize and cause slag, and also prone to incubation recession. (6) The mass of inoculant used in the flow inoculation method is small, generally 0.2 ~ 0.8mm, with the addition amount of 0.05% ~ 0.2%; the mass of in mold inoculation method is generally ≤ 0.25mm, with the addition amount ≤ 0.1%; the mass of other inoculation methods is generally 1 ~ 10 mm, with the addition amount of 0.1%% ~ 0.8%. 4. The temperature of incubation treatment and its effect test theoretically, the eutectic temperature of graphite is 1153 ℃ + 6.7 × Si%. When the eutectic temperature of theoretical graphite is 20 ~ 40 ℃ higher than the lowest eutectic temperature of actual ungenerated molten iron, inoculation treatment is generally required. The effect of inoculation treatment of cast iron can be determined by comparing the mechanical and physical properties of the casting, and analyzing the structure of its metallographic structure to determine the optimal inoculation treatment.