1. Carbon, silicon and carbon equivalent
When the carbon equivalent is at the eutectic point, the casting process performance is the best, the shrinkage tendency is the smallest, the concentrated shrinkage tendency is the largest, and it is the easiest to obtain dense castings by feeding during solidification. This view has been accepted by most foundry workers. However, how to choose the carbon equivalent of eutectic point of ductile iron is controversial.
2. Other chemical elements
The content of residual magnesium is high, and the tendency of shrinkage is large; When the content of sulfur and oxygen is not high, the residual magnesium content should be controlled at 0.03% ~ 0.05%. A small amount of rare earth can reduce the tendency of shrinkage cavity and porosity; The residual rare earth content can be controlled in the range of 0.01% ~ 0.02%; The residual rare earth content is generally less than 0.01% when the low sulfur molten iron is spheroidized by feeding wire.
A small amount of Bi can reduce the tendency of shrinkage cavity and porosity of large section ductile iron; 0.002% ~ 0.005% Bi is beneficial to refine graphite, increase spheroidization rate and graphite ball number.
Phosphorus has a tendency to increase shrinkage porosity, the less the better.
Both too high and too low sulfur affect the precipitation time and uniformity of graphite. Too low sulfur has insufficient nucleation ability, too high sulfur precipitates too early, and the shrinkage tendency of graphite increases, which can be controlled within 0.008% ~ 0.015%.
Manganese, chromium, vanadium, titanium and other carbide forming elements have the tendency to increase the formation of shrinkage cavity and porosity. Ti ≤ 0.03% is better; Mn is selected according to different material requirements. For example, pearlite ductile iron has higher Mn content, while ferrite ductile iron has lower Mn content; For general wind power ductile iron parts, Mn < 0.2%, Cr < 0.02%, V < 0.02%; Cr ＜ 0.1%, V ＜ 0.1% for austempered ductile iron parts.
Other element content requirements: Pb ≤ 0.002%, as ≤ 0.002%, te ≤ 0.001%, sb ≤ 0.001%, B ≤ 0.001%, O ≤ 0.005%, n ≤ 0.007%.
3. Spheroidization, inoculation and pretreatment
On the premise of ensuring the spheroidization rate, the minimum amount of residual mg and re is beneficial to reduce the shrinkage tendency of ductile iron.
Good inoculation (post inoculation and multiple inoculation) can reduce the shrinkage tendency of ductile iron.
Lanthanide anti shrinkage nodularizing agent is beneficial to reduce the shrinkage tendency of ductile iron.
The anti shrinkage inoculants containing Ce, Bi, s and o are beneficial to reduce the shrinkage tendency of ductile iron.
It is beneficial to reduce the shrinkage tendency of nodular cast iron after nodularization and inoculation.
Reasonable pretreatment of molten iron is beneficial to increase the crystalline core of molten iron, reduce the undercooling and the shrinkage tendency of ductile iron.
4. Melting temperature and pouring temperature
The necessary melting temperature and high temperature standing are beneficial to ensure the metallurgical quality of molten iron and reduce the shrinkage tendency of ductile iron; The melting temperature of ductile iron can be controlled at 1 500 ~ 1 550 ℃ according to the product structure.
The heat preservation time should be appropriate. If the holding time of molten iron is too long, especially when the temperature is too high, the metallurgical quality of molten iron will deteriorate and the shrinkage tendency of ductile iron will increase.
The pouring temperature should be selected according to the casting size, wall thickness, molding material and method, and the design of pouring riser. Fig. 1 is the recommended pouring temperature for ductile iron castings with different wall thickness. High pouring temperature is beneficial to feeding, but too high pouring temperature will increase liquid shrinkage, which is unfavorable to eliminating shrinkage porosity. If the pouring temperature is too low, defects such as insufficient pouring, cold shut, pinhole and inclusion will easily appear, and shrinkage defects will also increase, especially for small and medium-sized parts. When the temperature of molten iron is lower than 1 260 ℃, graphite begins to precipitate. If the mold is not full at this time, graphite expansion can not counteract shrinkage, and more shrinkage porosity may appear in the casting.
5. Raw materials and their purity
Pure raw materials (such as high purity pig iron or ultra-high purity pig iron, recycled material and scrap with stable composition after purification) are beneficial to ensure the metallurgical properties of molten iron.
Lower gas content (such as O, N, h, etc.) is beneficial to reduce the shrinkage tendency of ductile iron.
Reducing the oxidation tendency of molten iron is helpful to reduce the shrinkage porosity of ductile iron. The content of carbide forming elements and spheroidizing trace interfering elements is not conducive to reducing the shrinkage tendency of ductile iron.
Timely and thorough slag stripping is helpful to reduce the shrinkage tendency of ductile iron.