Knowledge

In recent years, with the continuous development and utilization of new energy, China has vigorously developed the wind power generation industry, and the demand for wind power castings has also been increasing year by year. The working conditions of large-scale wind power castings are often harsh. Compared with conventional castings, the performance requirements for materials are higher, especially the requirements for low-temperature impact toughness. When producing ductile iron EN-GJS-400-18U-LT (equivalent to the national standard QT400-18AL), the impact toughness of the material at -20℃ is required to be greater than 12J/cm2. To ensure the product quality of wind power castings and meet the low-temperature impact performance in the as-cast state, it is necessary to make the ferrite matrix and spheroidization effect of the castings meet the standard requirements. This paper takes the medium-frequency induction furnace melting as the premise, combines the self-hardening furan resin sand molding process, and produces wind energy castings with a mass of 7500kg and a maximum wall thickness of 150mm. It analyzes the organizational control measures in the production process of large-section castings and discusses how to achieve the batch production of ferrite matrix ductile iron through the reasonable selection of chemical composition and effective control of the spheroidization and inoculation process.

 

I. Performance Requirements

 

European standard EN-GJS-400-18U-LT. ​​​​​​​

 

Metallographic structure requirements

Graphite morphology	Graphite size	Ferrite content	Cementite content	Spheroidization rate
1 or 2 ≥ 90	4-8 grade	≥290%	≤1%	≥ 90%

 

Mechanical property requirements

Tensile strength MPa	Yield strength MPa	Elongation %	Hardness HB	Impact toughness J/cm² (-20℃)
≥ 370	≥ 220	≥ 12	130 - 180	≥ 12

 

II. Test Conditions

 

(1) Main equipment: 10t medium-frequency induction furnace, thermal analyzer, spectrometer, 4XB type metallographic microscope, WE.300/600 tensile testing machine, JBDS-300Y type impact testing machine, HB-3000B hardness tester.

 

(2) Selection of raw materials: The choice of raw materials directly affects the control of composition and thus plays a crucial role in the performance of the castings. Low-sulfur and low-phosphorus Q10 pig iron is selected for the pig iron, high-quality carbon steel with low carbon content is chosen for the scrap steel, and graphite electrodes are used as the carburizer. In this way, the sulfur and phosphorus contents in the molten iron can be well controlled, ensuring the graphitization process and mechanical properties.

 

(3) Selection of spheroidizing agent and inoculant: In view of the phenomena such as spheroidization regression, graphite floatation and variation that are prone to occur in large castings, a yttrium-based heavy rare earth spheroidizing agent with anti-regression property is selected, with a particle size of 5 to 30 mm. The inoculant adopts a low-boron ferrosilicon inoculant with strong anti-regression ability and high efficiency.

 

III. Melting and Pouring Process

 

3.1 Melting

 

(1) Preparations before smelting

The ladles and spheroidizing ladles should be dried, and the smelting tools and testing instruments should be complete and in good condition.

 

(2) Melting Process

Weigh and add materials strictly according to the batching sheet. The sequence of adding materials is: carbon additive, pig iron, return iron, and scrap steel.

 

The ratio of pig iron, scrap steel and return material is 5:3:2. The melting temperature should be above 1500℃ but not exceed 1520℃. A temperature that is too high is not conducive to nucleation and will result in coarse grains. After skimming off the slag, measure the temperature and then use a thermal analyzer to detect carbon and silicon. The carbon equivalent should follow the principle of high carbon and low silicon. To prevent graphite blooming, the carbon equivalent should not be too high. Carbon should be controlled at 3.6% - 3.8% and silicon at 2.0% - 2.2%. Use a spectrometer to detect other element compositions. Manganese should be controlled at 0.18% - 0.20%. Sulfur is a serious interference element for spheroidization and can easily form inclusions when forming sulfides, so sulfur should be controlled below 0.022%. Excessive phosphorus will reduce toughness, so it should be controlled below 0.03%. Adjust the composition based on the test results. Once it meets the requirements, skim off the slag and pour the molten metal.

 

(3) Spheroidization Process

Excessive residual magnesium can easily lead to white cast iron, so the amount of magnesium should only be sufficient to ensure the best spheroidization effect. The optimal residual magnesium content was determined through experiments, and the addition amount of the spheroidizing agent should be controlled at 1.4% - 1.6% of the molten iron mass. The spheroidizing ladle should be preheated to 500 - 600°C in advance. Then, the spheroidizing agent is buried on one side of the dam, and covered with 0.2% inoculant. To prevent premature spheroidization reaction, iron filings and silicon steel sheets are placed on top of the inoculant, and finally, the ductile iron cover plate is placed and compacted. If the molten iron temperature is too high, it will increase the loss and evaporation of magnesium during melting. If the molten iron temperature is too low, the spheroidizing agent cannot melt and be absorbed by the molten iron. The molten iron should be controlled at 1450 - 1460°C when pouring out, and 0.5% - 0.6% inoculant should be added for inoculation. To prevent premature spheroidization reaction and insufficient spheroidization, the molten iron should be quickly poured into the side of the spheroidizing ladle without the spheroidizing agent when pouring out. To prevent excessive magnesium reduction due to prolonged stay time, the casting should be carried out immediately after the spheroidization reaction is completed.

 

3.2 Pouring Molten Iron

After the spheroidizing reaction is completed, the slag is removed and a covering agent is spread to maintain the temperature. Then, the pouring process begins. To prevent the loss of inoculation effect, inoculant is added during the pouring process for in-stream inoculation, with an addition amount of 0.2% of the molten iron volume. A higher pouring temperature ensures good fluidity of the molten iron, which is beneficial for feeding and reduces slag inclusion, resulting in better appearance quality of the castings. However, if the temperature is too high, the liquid shrinkage increases, making it prone to shrinkage porosity and porosity. Therefore, the pouring temperature is controlled at 1350 to 1370°C.

 

IV. Modeling Techniques

 

The self-hardening furan resin sand molding process is adopted. The characteristics of furan resin sand are large instantaneous gas evolution, good high-temperature collapsibility, and it is prone to porosity, slag inclusion and sand washing defects. The cross-sectional area of the gating system should be slightly larger than that of the clay sand process, and the inner gates should be distributed. The casting adopts the bottom pouring system. To improve the slag-blocking capacity, a filter screen is placed in the gating system. To avoid sand washing, a ceramic tube is used as the sprue, and a ceramic filter plate is placed under the sprue. To prevent shrinkage cavities and porosity, chill irons are placed at the thick sections (bottom surface). The resin sand itself has high strength and good rigidity, and the casting is less likely to have shrinkage cavity defects. Therefore, a higher pouring temperature is selected. The pouring temperature for large and thick cast iron parts should not be lower than 1320℃.

 

V. Experimental Content and Result Analysis

 

Three reference test blocks were evenly set at the parting surface. After casting, they were cut off. The impact test was conducted in accordance with GB/T229-2007 Charpy V-notch impact test method, and the V-notch test samples were cut from the center of the reference test blocks on the JBDS-300Y impact testing machine. The tensile test was carried out in accordance with GB/T228.1-2010 Metal materials - Tensile testing - Part 1: Method of test at room temperature on the WE.300/600 tensile testing machine. The hardness test was measured by the HB-3000B Brinell hardness tester. Samples were cut from different test blocks for microstructure and composition analysis. The microstructure was observed by the 4XB metallographic microscope, and the chemical composition was determined in accordance with the X series standards of GB/T229 Steel and alloys - Chemical analysis methods. According to ASTM247 standard, the spheroidization rate was greater than 90%, the spheroidization grade was grade I, the graphite size was about grade 6, the ferrite content was greater than 90%, and the cementite content was less than 1%.

 

From the test results, it can be seen that by controlling the raw materials, reasonably selecting the addition amounts of spheroidizing agent and inoculant, and adopting multiple inoculations, both the ferrite content and the graphite spheres fully meet the standards.

 

The mechanical properties of the specimens are shown in the following table.

Sample number	Tensile strength MPa	Yield strength MPa	Elongation%	Hardness HB	Impact toughness J/cm ² (-20°C)
Standard	≥370	≥220	≥12	130-180	≥12
1	385	250	24.5	138	14.67
2	380	245	25.5	136	14.33
3	380	255	23.5	139	13.88

 

The key to ductile iron having high low-temperature toughness lies in the ductile-brittle transition temperature, and the matrix structure of ductile iron has a significant impact on the ductile-brittle transition temperature. Ductile iron with a 100% ferrite matrix has the lowest ductile-brittle transition temperature and the best upper limit impact energy. As the proportion of pearlite increases, both of these properties will deteriorate. Besides the matrix structure, the spheroidization rate and the number of graphite spheres in ductile iron also have a significant effect on the impact performance. An increase in the spheroidization rate and the number of graphite spheres in ductile iron can significantly improve the impact performance. Therefore, when the ferrite content and the spheroidization effect of the castings meet the standards during the production process, the low-temperature impact performance of the castings in the as-cast state can meet the requirements.

 

VI. Conclusion

 

When using medium-frequency induction furnaces to produce EN-GJS-400-18U-LT ductile iron, there are advantages such as low alloy element burn-off, few impurities, low oxygen content, uniform and accurate chemical composition, high molten iron temperature and easy temperature control. On this basis, as long as the following key technical elements are well controlled, batch production can be carried out smoothly.

 

Chemical composition: C 3.6-3.8%, Si 2.0-2.2%, Mn 0.18-0.2%, S ≤ 0.02%, P ≤ 0.03%, residual rare earth 0.02-0.03%, residual magnesium 0.04-0.06%. Strict control of the charge is maintained to keep the purity of the molten iron.

Spheroidization treatment is carried out by the conventional dam-injection method, and the spheroidization treatment temperature is well controlled.

Double inoculation is adopted to prevent inoculation decline. The first inoculation is carried out during iron tapping and in the ladle, and the second inoculation is done in the flow.

 

Vigor team has more than 20 years experience in stainless steel casting production and the robust supply chain of different treatment. If anything we can help or any parts you want to develop, please contact us at info@castings-forging.om