1. Selection of raw materials
Due to the inherent performance characteristics of wind power ductile iron castings, the requirements for raw materials are relatively strict. Pig iron with low Mn, low P, low S and low Ti should be selected. Generally, the trace elements should not exceed their maximum limit, and the total should not exceed 0.06%, as shown in Table 1.
| Element | P | S | Mn | Ti | Cr | V | Mo | Sn | Sb | Pb | Bi | As | B | Al | Te |
| Content | <0.04 | <0.025 | <0.10 | <0.030 | <0.05 | <0.03 | <0.01 | <0.02 | <0.002 | <0.002 | <0.001 | <0.02 | <0.002 | <0.02 | <0.003 |
Scrap steel shall be high-quality carbon steel with low nitrogen content, free of oil and serious corrosion. Strictly control Cr, V, Ti, Mo, Pb, Sb, Sn, B and other trace elements to prevent excessive anti spheroidizing elements and segregation elements from being brought into the molten iron.
2. Chemical composition
(1) C and Si
Both C and Si are elements that promote graphitization. Too high carbon equivalent is easy to produce graphite floating, but too low carbon equivalent will reduce the fluidity of molten iron, and it is easy to produce shrinkage, porosity and other defects. In order to obtain better casting properties, the carbon equivalent is often selected near the eutectic point, and the general carbon content is 3.60% – 3.90%.
Si is an important element to promote graphitization and form ferrite matrix, but it is also an extremely sensitive element in thick and large section ductile iron. High silicon content is one of the main reasons for fragmented graphite. In addition, too high silicon content is easy to cause brittle transformation. The brittle transition temperature increases by 5.5 ~ 6.0 ℃ for every 1% increase in silicon content. Therefore, the silicon content cannot be too high when producing wind power castings. However, without nickel, the strength of ferritic matrix nodular cast iron is mainly obtained by the solid solution strengthening of Si. Therefore, the silicon content in wind power nodular cast iron should be limited to 1.8% – 2.2%. In production practice, the final silicon content of wind power nodular iron castings of qt400-18al is the upper limit, while the final silicon content of wind power nodular iron castings of qt350-22al is the lower limit.
Figure 1 shows the relationship between the silicon content [w (MN) 0.25% ~ 0.27%] and the tensile strength and -20 ℃ impact toughness of the 70 mm cast test block of 1.5 MW wheel hub with the material brand qt400-18al produced by a factory. It can be seen from Figure 1 that with the increase of final silicon content, the tensile strength increases slightly, while the impact toughness at -20 ℃ decreases slightly.
（2）Mn、P、S
For nodular iron castings of wind power, Mn, P and s should be strictly limited. For wind power ductile iron castings with low-temperature impact performance requirements, the low-temperature transition temperature will increase by about 10 ~ 12 ℃ for every 0.1% increase in manganese content. Therefore, it is required to strictly limit the manganese content. When there is a large amount of recycled ductile iron, considering the economic benefits, when producing castings of qt400-18al brand, the manganese content can be relaxed to about 0.25%, and the manganese content of castings of qt350-22al brand can be controlled below 0.15%.
In ductile iron, P and s are strictly restricted elements. P is easy to segregate and form phosphorus eutectic. Phosphorus eutectic is easy to be polygonal distributed at the boundary of eutectic group, which sharply deteriorates the properties of nodular cast iron. P significantly increases the brittle transition temperature. When the phosphorus content increases by 0.01%, the brittle transition temperature increases by 4.0~4.5 ℃, which seriously reduces the plasticity and impact toughness. The phosphorus content is mainly guaranteed by pig iron. The production of low-temperature wind power nodular iron castings requires that the phosphorus content be controlled below 0.04%, and the lower the better.
S is an anti graphite spheroidizing element. A considerable part of RE and Mg added to molten iron is combined with s, and the rest of RE and Mg can be spheroidized. The production practice shows that after adding nodulizing agent, only when the sulfur content in molten iron drops below 0.02%, and there is a certain amount of residual magnesium and rare earth, can good nodulization be ensured. However, the study also found that the sulfur content of molten iron should not be too low (below 0.005%), otherwise it is not conducive to graphite nucleation, lack of sulfide graphite nucleation, reduce the inoculation effect, and carbides will increase. Therefore, the sulfur content should be strictly controlled, and the sulfur content of raw molten iron should be controlled at 0.005% – 0.015%.
(3) Mg and CE
MG is the element with the strongest spheroidizing ability and the most widely used spheroidizing element, but its anti-interference element ability is poor, and it tends to form slag inclusion, shrinkage porosity, subcutaneous pores and other defects. Excessive residual magnesium will reduce elongation and impact toughness. The spheroidization of RE element is worse than that of Mg, and the roundness of graphite is not as good as that of Mg, but the deoxidation and desulfurization ability are stronger than that of Mg, and the anti-interference element ability is stronger. In the production of wind power nodular iron castings, good pig iron resources are generally selected, and the content of trace elements in its molten iron is low, especially the content of some anti spheroidizing elements is lower; In addition, if the content of rare earth is too high, not only the tendency of white mouth and shrinkage of molten iron is large, but also it is easy to make the graphite ball out of roundness and affect the spheroidization rate of castings. Rare earth can promote the formation of fragmented graphite. Therefore, under the condition of ensuring the spheroidization quality in production, the lower the amount of residual magnesium and residual rare earth, the better. Generally, the amount of residual magnesium is controlled at 0.030% – 0.050%, and the amount of residual rare earth is controlled at 0.01% – 0.015%.
(4) Sb and Bi
Sb can adsorb on the graphite / molten iron interface and reduce its interface energy, so as to improve the nucleation rate and growth rate of nodular graphite core; At the same time, it can also form stable complex compounds with elements such as re in molten iron and become heterogeneous cores; Adding a small amount of sb can eliminate the deterioration of graphite morphology caused by excessive CE. On the contrary, the deterioration of graphite caused by excessive sb can be eliminated by a certain amount of CE. The proportion of sb/ce is generally controlled at (1.2 ~ 1.5): 1. Table 2 shows the chemical composition and metallographic structure comparison of a 2 MW hub main shaft flange casting body, and Figure 2 shows the comparison of the effect of sb on the graphite morphology of the casting body.
| State | Si | Mn | Spheroidization rate /% | Number of graphite balls / piece · mm-2 | Ferrite volume fraction /% |
| Without Sb | 2.02 | 0.18 | With fragmented graphite | With fragmented graphite | ＞95 |
| Add 0.003% Sb | 1.99 | 0.20 | 93.41 | 98 | ＞95 |
Bi can reduce the interface energy and form a stable and long-term heterogeneous core similar to sb, which can be used as an inoculation element. BI was often used as an interfering element in the past. However, the addition of CE not only did not interfere with spheroidization, but also played a beneficial role in nodular cast iron. For example, adding a small amount of Bi to thick large section ductile iron can prevent and reduce the formation of abnormal graphite, prevent graphite distortion, increase the number of graphite balls, and improve the volume fraction of ferrite.