In this article, a study was conducted to verify an effective method for improving the nucleation potential of ductile iron under the trend of undercooling. An experimental study has been completed to identify the most effective additive for increasing the number of nodular graphite particles. A study was conducted to verify data under casting production conditions.

A key factor in improving the performance of ductile iron is to increase the number of ductile iron particles, especially under the trend of undercooling. By increasing the number of graphite particles during the solidification process, the rate at which latent heat is released due to graphite crystallization can be increased, and the temperature at which solidification ends above the liquidus of the cementite can be raised to prevent the formation of carbides.

We have recently confirmed six additives and conducted a comparative study on their effects in inducing the formation of nodular graphite particles. In both the laboratory and the foundry, it has been proven that one additive consistently outperforms the other five.

Laboratory testing

The experiment was carried out on a coreless induction furnace lined with aluminum oxide using ordinary melting operations. The raw material used was pig iron, which contained the following components: 3.85% C, 1.85% Si, 0.3% Mn, less than 0.03% P and less than 0.01% S. Preliminary studies have shown that the graphitization ability of the furnace charge is crucial for nucleation potential, and all heat can produce up to 50% (Fe3C) carbide ductile iron from the charge. Spheroidization treatment was completed in the cover package by adding a 1.3% mass fraction of ferrosilicon alloy, a 6% magnesium alloy containing 1% rare earth elements, and a 0.3% cast grade ferrosilicon alloy as a modifier.

Silicon carbide is used as a metallurgical additive to promote graphite nucleation

1) Silicon carbide (SiC);

2) Mixture X; C is composed of 50% SiC, 50% ferrosilicon alloy, and 5% Mg;

3) A mixture composed of mixture Y (50% SiC and 50% 75 ferrosilicon);

4) Silicon calcium SiCa;

5) Ferrous sulfide FeS and crystalline graphite.

The research results indicate that adding 0.3% SiC to the original iron is the most effective metallurgical additive for increasing nucleation potential, reducing undercooling trend, and improving tensile properties mainly based on elongation in ductile iron.

Before being added to the molten iron in the furnace, these metallurgical additives are wrapped in low-carbon steel foil to promote sinking, reduce oxidation, and adhesion on the furnace wall. Monitor the effectiveness of these additives by comparing the initial state of ductile iron samples obtained before and after addition in each experiment.

The poured samples contain chilled wedges to determine the tendency of white spots, 25mm diameter specimens for microstructure inspection, and specimens for tensile testing. Obtain sales samples from the raw iron before and after adding additives to determine the total changes in oxygen and nitrogen content. Record thermal analysis data to determine undercooling and reignition. Studied the changes in microstructure, white tendency, and mechanical properties of ductile iron, and evaluated experimental data. Tensile tests were only conducted on samples cast with ductile iron that increased the number of ductile iron particles and improved the white spot condition after adding additives.

The image analysis data of the polished microstructure showed that the highest amount of graphite separation and the most suitable nucleation morphology were obtained from the ductile iron with 0.3% SiC added. This additive met the graphic analysis conditions and increased the number of nucleation by 64% (as shown in Figure 1). After adding mixture Y, the number of nodular graphite particles increased by 15%. The addition of 0.05% SiCa or 0.1% crystalline graphite increased the number of nodular graphite particles by 3.5% and 2.5%, respectively. The mixture X and FeS have a negative effect on the number of nodular graphite particles, reducing it by 19%.

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In addition, SiC has a positive impact on nucleation characteristics; The addition of 0.3% SiC additive increased the spheroidization rate by 21%, the sphericity by 2.4%, and the roundness by 2.4%. 0.1% crystalline graphite is not important for spheroidization rate and roundness, but mixtures of X and FeS have adverse effects on nucleation characteristics.

SiC alone or mixed with 75% ferrosilicon can improve the size distribution of graphite spheres, increase the number of small graphite spheres (4-16 microns), and reduce the number of large graphite spheres (16-64 microns). The previously published research results indicate that the skewed distribution of graphite ball size and the reduction in the number of small and late nucleated graphite balls have a positive impact on the performance of ductile iron; A large number of ductile iron particles is harmful to the performance of ductile iron and can increase the trend of shrinkage and porosity.

The inspection results of the microstructure after etching show that the average content of ferrite and pearlite in the reference ductile iron sample is 32% and 58%, respectively. The conclusion is:; The SiC additive mixed with 75% silicon iron is the most favorable for the formation of ferrite, followed by crystalline graphite additive. They can be increased by 20% and 14% respectively. SiC alone can add 3% ferrite to the original iron.

The analysis of the chilled wedge block (as shown in Figure 2) shows that the addition of SiC, mixture Y, and crystalline graphite reduces the amount of carbides in the sample, while the addition of SiC, FeS, and mixture X increases the amount of carbides in the sample.