Research & Application of High-strength and high-toughness ductile iron Materials for Wind Power Castings

In order to obtain low-temperature ductile iron materials with higher strength and toughness, alloying elements Ni and Nb were added to the QT400-18AL material for alloying. Through the tests of the mechanical properties, metallographic structure, low-temperature impact energy and fatigue performance of the samples, it can be known that Ni and Nb alloying can significantly improve the mechanical properties of QT400-18AL material. Meanwhile, by using this alloyed QT400-18AL material, the dual goals of improving the quality of castings and reducing costs have been achieved.

Advantages of Nodular cast iron

Nodular cast iron,with wear resistance shock absorption, is not sensitive to gaps, and the characteristics of the casting performance, and lower production cost, short production cycle, it is an ideal material for wind castings. At present, the dimensions of wind power castings are increasing as wind turbine units continue to become larger and heavier. The cross-sectional thickness of some parts has exceeded 200 mm. The solidification and cooling rate of thick and large-section castings is slow, which easily leads to poor shape and a small quantity of spherical graphite and even causes problems of fragmented graphite. The matrix structure will also form severe composition segregation, resulting in the generation of a large number of carbides that are difficult to eliminate even through long-term heat treatment. Moreover, the castings have coarse grains and are more prone to defects such as black spots and shrinkage porosity.

In recent years, generally adoption the strict control of chemical composition, improving the casting process after heat treatment, especially the forced cooling measures, such as to improve the organization, has had a good effect. However, it is almost impossible to enhance the quality of wind power ductile iron parts merely by improving processes and equipment, especially without increasing costs. At present, domestic wind power uses ductile iron casting materials mainly for qt400-18 al; its mechanical performance is the tensile strength of 360 MPa, yield strength of 220 MPa or greater, or more break elongation 12.5% or higher, and 30 ℃ minimum impact absorbing energy of: 10 J (on average) and 7 J (single). The materials of high-strength ductile iron castings applied abroad are solid-solution strengthened ferrite materials EN-GJS-500-14 and EN-GJS-600-10, and their mechanical properties are much higher than those of QT400-18AL. However, this material has no requirements for low-temperature impact energy, and its application in the wind power field poses risks. Therefore, it is necessary to develop a high-strength, high-toughness and low-temperature ductile iron material for wind power castings.

The technicians try to improve the wind power by using the method of alloying with the strength and toughness of ductile iron castings to reduce the casting size and thick solution caused by the slow cooling rate of castings and the thick casting quality problem. 1, the alloying materials formula nodular cast iron is of great help to improve the mechanical properties of, however, wind power castings for low-temperature impact energy requirements. Therefore, the added alloying elements need to be comprehensively considered.

Mainly in qt400-18 al material, on the basis of adding alloy elements of Ni, Nb, and Zr three elements; they basically have the following advantages: (1) Ni can reduce nodular cast iron brittle transition temperature, adding a certain amount of can improve the low-temperature performance of nodular cast iron. (2) the Nb is widely used in metallurgical industry, to increase the Nb can improve the mechanical properties of nodular cast iron, and does not affect its performance in low temperature. (3) Increasing a small amount of Zr can have the effect of DNA, purifying and refining grain, and is advantageous to the low-temperature performance of nodular cast iron.

1.The specific formula is shown in Table 1

Table 1 Material Formula (Mass Fraction, %)

2.Material trial production

2.1 material preparation

(1) adding iron, back to the furnace charge and scrap steel furnace for iron smelting.

(2) to increase carbon of liquid iron, the carbon content of control satisfies the requirement of table 1.

(3) after carburization of the liquid iron spheroidization and inoculation and alloying processing: according to table 1 quality percentage of Nb and Ni; Zr was added through silicon-zirconium inoculants.

(4) the ball, inoculation and alloying processing of iron liquid after pouring, wind power castings with high strength and high toughness ductile iron material.

2.2 try block test block size: 300 mm * 300 mm * 300 mm, with 70 mm thick test block. The pouring and unboxing conditions are shown in Figures 1 and 2.

3. Test results

3.1 Mechanical Properties

The test was conducted in accordance with “GB/T 228-2002 Test Method for Tensile Testing of Metallic Materials at Room Temperature”, and the results are shown in Table 2.

Table 2 Mechanical Properties