Selection of raw materials in the production of wind power ductile iron castings

With the rapid development of wind power industry in the world, the production of wind power castings has shown a good momentum in China's foundry industry. At the same time, the strict quality requirements of wind power castings pose a serious challenge to the technical level and management level of our foundry workers.

The working environment of wind power castings is in the harsh field. After long-term exposure to wind, sun and rain, after entering service, no quality accidents are allowed within 20 years. Therefore, the quality requirements of wind power castings should be compared with the quality requirements of nuclear power and aerospace castings. Wind power castings mainly include the hub of the blade, the gear box, the mechanical gantry and the base member. Each 1~2 MW unit requires about 15 tons of castings, and each 3~5 MW unit requires 35 to 50 tons of castings.

The material requirements for wind power castings are: QT400-18AL, QT700-2, QT350-22AL material grades. The materials of various wind power castings must meet the tensile strength, yield strength, elongation, impact toughness and hardness of the standard or user requirements, and have high internal and external integrity and density. All castings are not allowed to be repaired by welding, and they must undergo strict ultrasonic testing and magnetic particle testing. The QT400-18AL material is required to meet the low temperature impact performance of minus 20 ° C. The QT350-22AL material is required to meet the low temperature impact performance of minus 40 ° C.

In the production process of wind power ductile iron castings, the smelting and processing of molten iron is a very important and technically demanding process, which has a significant impact on the material quality of the castings and the quality inside and outside. This paper mainly describes the control of smelting technology in the production of QT400-18AL and QT350-22AL large-section wind power ductile iron castings.

1 Raw material selection

1.1 pig iron

In order to ensure that the mechanical properties of wind power casting materials meet the standard requirements and facilitate production control, the Q4-Q10 grade pig iron is generally selected, and the chemical composition content of the alloy must be strictly controlled. The composition requirements are shown in Table 1.

2 Target molten iron composition selection

The main chemical components of wind power ductile iron castings are: C, Si, Mn, P, S, Mg, RE. The scope of its selection is described below. 2.1 C

According to the characteristics of the large size of wind power ductile iron castings and the low temperature performance requirements of materials, the composition control range of C must ensure the requirements of full graphitization, and prevent the generation of graphite floating. The selection range of raw materials C is 3.5-3.8. %, the lower limit of the thick section of the casting is taken, and the upper limit of the section is small.

2.2 Si

Although Si promotes the element of graphitization, the content should be properly controlled. As the Si content increases, the brittle transition temperature increases. Therefore, the wind-powered ductile iron castings requiring low-temperature impact properties should be strictly controlled in the range of the amount of addition. Iron castings have different grades of ductile iron. The range of Si content is: a.QT400-18AL, Si 1.8~2.1%; b.QT700-2, Si 2.0~2.3%; c.QT350-22AL, Si 1.7~2.0 %.

2.3 Mn

High Mn content will promote the pearlite content, segregation will occur, and the ductility and elongation of ductile iron will decrease. Therefore, the Mn requirement is as low as possible. Considering the specific conditions of pig iron, Mn<0.3% is generally required. Production of QT350-22AL requires Mn <0.2%.

2.4 P

P is a very harmful element in cast iron. It forms a binary or ternary phosphorus eutectic and is distributed on the grain boundaries, which significantly reduces the low temperature impact properties and elongation of wind power castings, requiring control below 0.03%.

2.5 S

S is a harmful element of anti-spheroidization. If the content is high, the spheroidization rate will decrease, resulting in a decrease in strength, elongation, and impact toughness. Generally, S<0.02% is required; however, from the concept of inoculation, the S content should not be too low. Generally not less than 0.006%.

2.6 Mg

Mg is the main spheroidizing element. In order to reduce the influence of Mg on the toughness, inclusion and shrinkage tendency of ductile iron, the Mg content should not be too high, generally 0.040 to 0.060%.

2.7 RE

RE is an important element in ductile iron. RE can remove S, remove O and purify molten iron, and can also play a role in assisting spheroidization. However, if a spheroidizing agent of light rare earth is used, it should be noted that when the S content is low, the control of the rare earth content should not be too high, otherwise inclusions and graphite distortion are likely to occur; when heavy rare earth is selected, this phenomenon is greatly alleviated. It is recommended that the control range of RE be <0.025%.

3 smelting technology control

The quality of the original molten iron smelting has an important influence on the performance of ductile iron and the casting casting defects. The chemical composition control, the selection of the melting equipment and the melting process are important links. 3.1 Chemical composition of the original molten iron

According to the chemical composition required for the casting selected above, the chemical composition range of the original molten iron is shown in Table 4.

S is a harmful element of anti-spheroidization. If the content is high, the spheroidization rate will decrease, resulting in a decrease in strength, elongation, and impact toughness. Generally, S<0.02% is required; however, from the concept of inoculation, the S content should not be too low. Generally not less than 0.006%. 2.6 Mg

Mg is the main spheroidizing element. In order to reduce the influence of Mg on the toughness, inclusion and shrinkage tendency of ductile iron, the Mg content should not be too high, generally 0.040 to 0.060%. 2.7 RE

RE is an important element in ductile iron. RE can remove S, remove O and purify molten iron, and can also play a role in assisting spheroidization. However, if a spheroidizing agent of light rare earth is used, it should be noted that when the S content is low, the control of the rare earth content should not be too high, otherwise inclusions and graphite distortion are likely to occur; when heavy rare earth is selected, this phenomenon is greatly alleviated. It is recommended that the control range of RE be <0.025%.

3 smelting technology control

The quality of the original molten iron smelting has an important influence on the performance of ductile iron and the casting casting defects. The chemical composition control, the selection of the melting equipment and the melting process are important links. 3.1 Chemical composition of the original molten iron

According to the chemical composition required for the casting selected above, the chemical composition range of the original molten iron is shown in Table 4.

Table 4 chemical composition of the original molten iron

3.2 Selection of smelting equipment

There are generally two options for smelting equipment. One is the double smelting using cupola and induction furnace; the other is direct smelting by induction furnace. Each has its own characteristics, as long as the smelting process is reasonable, it can produce qualified wind power ductile iron casting products.

The double-linking can utilize the high-efficiency melting efficiency of the cupola to greatly improve the production efficiency. At the same time, the high-temperature superheating temperature in the cupola is used to refine the graphite of the original iron to eliminate the coarse genetic property of the graphite and solidify the ductile iron. Providing more cores lays a good foundation; therefore, duplexing is more suitable for production with pig iron as the main material.

Direct smelting with induction furnace, the production process is relatively simple and stable, coupled with the use of furnace carbonization process, therefore, it is more suitable for the production of a large number of scrap steel as the main material. Since a large amount of [C] microcrystalline crystal embryos are generated in the molten iron by carbon addition, it is a direct core of graphite deposition in the future, so that the ductile iron structure can be refined.

3.3 Covering the ball, gestation and treatment

3.3.1 Release temperature

The temperature of the tapping furnace is controlled to be 1520～1540°C. When the tapping temperature is reached, the spheroidizing treatment should be carried out as soon as possible to avoid long-term stay in the furnace and prevent the molten iron from losing its activity.

3.3.2 Spheroidizing package

Spheroidizing treatment package requires H (inside): φ (inside) ≥ 1.5; the treatment package is repaired or cleaned before use, the first package is baked to 600 ° C or more; the spheroidized package requires cover spheroidization treatment, It can increase the absorption rate of spheroidized elements; secondly, it can prevent spheroidized molten iron from directly impacting the spheroidizing agent; and third, it can reduce thermal energy loss and molten iron impurities. 3.3.3 Spheroidization and birth treatment

Since wind power castings are ferrite ductile iron with as-cast large section resistance to low temperature impact toughness, the general principle of inoculation is that the inoculant is added in large quantities and is used for multiple births.

The amount of spheroidizing agent added: the amount of spheroidizing agent added (depending on the sulfur content of the original molten iron) is generally 1.2 to 1.8%, and the particle size is 10 to 25 mm. The order of addition of the spheroidizing treatment: firstly adding the spheroidizing agent to the spheroidizing reaction pit to flatten and compact; adding 0.3~0.4% of the inoculant to the spheroidizing agent to level and firm; then sprinkling 0.2 on the inoculant alloy. % of the covering agent, and finally press the clean homogenous iron or steel plate and compact it.

Spheroidization and inoculation treatment: The temperature of spheroidized molten iron is generally controlled at 1420~1460 °C. The molten raw iron water is put into the pre-prepared transfer bag (the advantage of using the transfer bag is that the molten iron in the furnace can be quickly reduced to a suitable processing temperature, and the second is to quantitatively spheroidize the molten iron) 3/ 4. The molten iron is quickly added to the spheroidized bag from the spheroidal bag to be spheroidized. The spheroidization reaction time is controlled in the range of 90 to 150 seconds, and the spheroidization process is carried out in the late stage, and the package is used. Add the remaining 1/4 of the molten iron and simultaneously inject 0.4% of the inoculant into an inoculant, quickly sprinkle the slag into the clean slag, and then sprinkle 0.2-0.3% of the inoculant on the surface of the spheroidized molten iron. The second inoculant is quickly covered with a covering agent without stirring, and the temperature is prepared for pouring.

Three times of inoculation can be added 0.1~0.2% of the instant inoculant during the pouring of the pouring cup. The granular inoculant with a particle size of 0.3~0.8mm can be used or the inoculation block can also be added to the pouring cup (the block size is based on the casting weight) Ding).

4 Conclusion

With the rapid development of wind power generation, the production of wind power castings has entered a stage of large-scale production, and the market competition will become increasingly fierce. Therefore, improving the level of foundry production technology and reducing production consumption and pollution are the only roads for enterprise development. This article describes the control of smelting technology in wind power ductile iron castings, hoping to help the foundry colleagues.