Cast iron inoculants research, before pouring molten iron, under certain conditions (such as certain overheating temperature, chemical composition, appropriate addition method, etc.), a certain amount of substance (inoculant) is added to the molten iron to change the solidification process of the molten iron, improve the cast structure, and thus achieve the purpose of improving performance. This is called inoculation treatment.

This change in inoculation treatment is often difficult to explain by subtle changes in chemical composition. In industrial production, inoculants have been widely used, but how to use inoculants efficiently still needs to be studied. This paper summarizes and analyzes the inoculation treatment of cast iron through literature research. Below is the details of Cast iron inoculants research

I. Inoculation effect of elements in commonly used cast iron inoculants

(1) Si can promote graphitization, increase the ferrite content on the one hand, and strengthen the ferrite on the other hand. It is often used together with Ca to enhance the effect, and the content is generally selected as ωSi=2.6%~3.2%.

(2) Ca is usually used together with Si, and the applicable temperature is 1370~1430℃, but excessive addition is prone to slag eye defects.

(3) Ba can inhibit the decay of inoculant and increase the amount of ferrite. Ferrosilicon containing Ba not only has good decay resistance, but is also effective in preventing white cast iron.

(4) Al can effectively promote graphitization, but it is easy to cause subcutaneous pores. It is often used together with ferrosilicon to improve the effect.

(5) Mn has the effect of promoting pearlite. It has a good effect when it works with nitrogen. It can reduce the melting point of the inoculant (Zr series). Generally, ωMn = 0.2%~0.4% is selected.

(6) RE has a great influence on its effect. When ωS = 0.035%~0.1% in the molten iron, the effect is better. It can refine graphite and inhibit white cast iron.

(7) Sr has the effect of inhibiting white cast iron and enhancing graphitization. It has poor effect when Ca and Ce elements are present. Fe-Si-Sr series inoculant is used in gray cast iron. The effect is good when ωS＜0.05%.

(8) Zr can promote graphitization. Fe-Si-Zr inoculant is difficult to dissolve and has an effect on preventing nitrogen pores. Mn element has a certain promoting effect on its dissolution.

(9) Bi increases the number of graphite nodules, reduces the tendency of white cast iron, increases the content of ferrite in the matrix, and often works together with Fe-Si to refine the grains. In addition, Bi-Cu inoculant can promote the growth of graphite core during annealing.

2. Selection of effective inoculant elements

1. General selection principles

(1) Inoculant elements that effectively prevent white cast iron are: Ba, RE, Sr., Si.

(2) Inoculant elements that effectively promote graphitization are: Ba, Ca.

(3) Elements that can inhibit or slow down the decline of inoculant are: Ba, Ca, RE.

(4) Elements that promote the melting of inoculant into molten iron are: Mn, Mg.

2. Special selection principles

(1) To prevent cracks in large thick-walled gray castings, rare earth silicon-iron-magnesium alloy can be used. Its composition is ωRE=13.24%~13.69%, ωMg=12.76%~13.11%, ωSi=38%~44%, ωCa<4%, and the addition amount is 0.25%.

(2) To produce ferritic ductile iron castings, FeSi72 inoculant can be used. The particle size is 2~6mm and the addition amount is 1.5%. At the same time, 0.16% pure aluminum is added for composite strengthening inoculation, which has a better effect.

(3) To prevent ductile iron from inoculating and declining, FeSi75 with a particle size of 3~8mm can be added in the first inoculation, the particle size added in the second inoculation is 0.5~2mm, and the powder method is used in the third inoculation, and the addition amount is 0.3%~1%.

(4) For large-section high-carbon equivalent and high-strength cast iron, an inoculant with a titanium content of 0.08% to 0.15% can be selected; for high-carbon equivalent thin-walled gray iron castings, a silicon-barium rare earth alloy inoculant should be selected, which can reduce the tendency of white cast iron, reduce the hardness of the casting, and inhibit the decay of inoculation. The effect is good, and the addition amount is 0.3% to 1%.

(5) RE-Si inoculant has a strong ability to eliminate white cast iron, but has a greater tendency to form D and E type supercooled graphite, which makes it difficult to reduce the hardness of the casting. Si-Ba inoculant has a strong ability to eliminate D and E type supercooled graphite and promote the formation of A type graphite. Therefore, the two inoculants are used together for a good effect.

(6) The sulfur content in the molten iron has a great influence on the inoculation treatment. In molten iron with high sulfur content, graphite (such as electrode scraps) and N (such as Fe-Mn-N) inoculants are generally used for good results; in molten iron with low sulfur content, Ce, Ca, Ba, and N (such as RE-Si, Ca-Si-Ba, Fe-Mn-N, and Ca-Si-Sr) inoculants are used for good results.

3. Amount of inoculant added

(1) Ductile iron generally requires more inoculant than gray cast iron.

(2) If the inoculant is poured for a long time after inoculation, the amount added should be increased due to the decline of inoculation.

(3) Thin-walled castings are prone to white spots, so the amount of inoculant added should be increased accordingly.

(4) When there is a lot of slag in the molten iron, the inoculant will be wrapped up and difficult to melt once it comes into contact with the slag, so the amount added should be increased accordingly.

(5) Excessive addition of inoculant will cause problems such as excessive slag, lower molten iron temperature, and shrinkage cavities caused by eutectic cells. If the inoculant is too large, it will not be able to uniformly inoculate the molten iron, and the unmelted inoculant will be poured into the mold cavity; if the inoculant is too small, it will be easily oxidized to cause slag, and it is also easy to cause inoculation decay.

(6) The inoculant used in the flow inoculation method is relatively small, generally 0.2~0.8mm, and the amount added is 0.05%~0.2%; the block size of the inoculant in the mold is generally required to be ≤0.25mm, and the amount added is ≤0.1%;

The block size added by other inoculation methods is generally 1~10mm, and the amount added is 0.1%%~0.8%.

4. Temperature of inoculation treatment and its effect test

The theoretical graphite eutectic temperature is 1153℃+6.7×Si%. When the theoretical graphite eutectic temperature is greater than the actual minimum eutectic temperature of the uninoculated molten iron by 20~40℃, inoculation treatment is generally required.

So, cast iron inoculants research the main target is the effect of cast iron inoculation treatment can be compared by measuring the mechanical and physical properties of the castings. The structure of its metallographic structure should also be analyzed to determine the optimal inoculation treatment.