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Increasing nodule count through additives.

Increasing nodule count is critical to improving the performance of ductile iron, particularly when it is prone to under-cooling. By increasing the number of graphite nodules during solidification, the rate of release of latent heat due to graphite crystallization increases, and the end of freezing temperature is raised above the cementite liquidus line, preventing carbide formation.

A study recently was conducted to determine the effectiveness of six different additives in inducing nodule formation. In both the laboratory and the metalcasting facility, one of the additives consistently proved to be superior to the others.

A Sterile Environment

The laboratory experiments were performed using a common melting practice in coreless induction furnaces lined with alumina on a typical base iron with the following composition: 3.85% carbon, 1.85% silicon, 0.3% manganese, 0.03% phosphorus (maximum) and 0.01% sulfur. Because preliminary investigations had shown that the graphitization ability of the charge materials is essential to nucleation potential, all the heats were produced with 50% highly carbidic ductile iron returns in the charge. The nodularization treatment was completed in a tundish ladle by addition of 1.3% by weight of ferrosilicon 6% magnesium alloy with 1% rare earth elements, along with 0.3% metalcasting-grade ferrosilicon as an inoculant.

[FIGURE 1 OMITTED]

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The metallurgical additives investigated as potential sources of active elements for graphite nodule nucleation were silicon carbide (SIC), a mixture of 50% SiC and 50% ferrosilicon with 5% magnesium (Mixture X), a mixture of 50% SiC and 50% metalcasting-grade 75% ferrosilicon (Mixture Y), silicon calcium (SiCa), iron sulfide (FeS), and crystalline graphite. The results of the investigation showed that the addition of 0.3% SiC to the base iron was the most efficient metallurgical additive for increasing the nucleation potential (nodule count) in ductile iron, reducing under-cooling tendency and upgrading tensile properties, primarily percent elongation.

Before being introduced into the molten iron in the furnace, the metallurgical additives were wrapped in a mild steel foil to facilitate sinking and minimize oxidation or sticking to the crucible walls. The effects of the additives were monitored by comparison with the initial state of the ductile iron test samples taken before and after addition for each experiment.

The test samples poured included chill wedges to determine chilling tendency, 1-in. (25-mm) diameter rods for microstructural examination and tensile test bars. Pin samples were taken from the base iron before and after addition to determine variations in total oxygen and nitrogen content. Thermal analysis data were recorded to determine under-cooling and recalescence. Changes in the microstructure, chilling tendency and mechanical properties of the ductile iron were investigated, and the data were evaluated. Tensile testing was performed only on samples poured from ductile iron with an increased nodule number and improved chilling tendency after the addition was made.

[FIGURE 3 OMITTED]

Data from image analysis of the as-polished microstructures indicated that the best effect on the number of graphite separations and acceptable nodule morphology was obtained in ductile iron after the addition of 0.3% SiC, which increased the nodule number meeting the image analysis criteria by 64% (Fig. 1). After the addition of Mixture Y, the nodule count increased by 15%. Addition of 0.05% SiCa and/or 0.1% crystalline graphite produced increases in nodule count by only 3.5% and 2.5%. Mixture X and/or FeS had a negative effect on the nodule count, which decreased 19%.

In addition, SiC had a positive effect on the nodule characteristics; 0.3% SiC addition produced increases of 21% in nodularity, 2.4% in sphericity, and 2.4% in roundness. The addition of 0.1% crystalline graphite had an insignificant effect on nodularity and roundness, while Mixture X and FeS additions had an adverse effect on the nodule characteristics.

SiC, alone or mixed with 75% ferrosilicon, also caused an improvement in the nodule size distribution. The number of small nodules (4-16[micro]m) increased, and the number of large nodules (16-64 [micro]m) decreased. Previously published research has shown that a skewed distribution of nodule sizes and an increase in the number of small and late-nucleated nodules positively affects ductile iron properties and that large nodule numbers are detrimental to ductile iron properties, including shrinkage tendency.

The examination of the etched microstructures indicated the average amount of ferrite and pearlite in the reference ductile iron samples was 32% and 58%. The addition of SiC mixed with 75% ferrosilicon, followed by the crystalline graphite, had the most beneficial effect on ferrite formation, which increased by 20% and 14%. SiC alone produced a 3% increase in ferrite ratio in the matrix.

The analysis of the chill wedges taken (Fig. 2) indicated fewer carbides in the samples after additions of SiC, Mixture Y and crystalline graphite. The chill wedge samples taken after SiC, FeS and Mixture X additions indicated an increase in carbide ratio.

Avoiding Undercooling and Recalescence

The thermal analysis data indicated that the initial state of iron varied from heat to heat, presenting different degrees of under-cooling. Under-cooling of the base iron varied from 71.3 to 48.4F (21.84 to 9.11C), and ductile iron varied between 81.45 and 65.6F (27.47 and 18.66C) (Fig. 3).

Under-cooling is defined as the difference between the gray iron eutectic temperature (TEgray = 1153 + 6.7*Si) and the actual low eutectic temperature (TElow), and it characterizes the iron tendency to an undercooled microstructure. A high undercooling means a longer time before freezing starts and an increased risk for macroshrinkage and outer sunk. Data showed that SiC alone or mixed with 75% ferrosilicon had a beneficial effect on under-cooling; the decrease in the base iron was about 22%, and in ductile iron the decrease was 7.4% for the SiC addition and 0.6% for the ferrosilicon mixture. FeS also produced a decrease in under-cooling even though the nodule count decreased. All other metallurgical additives initiated an increase in under-cooling in both base and ductile irons compared with the initial state of iron.

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Data from thermal analysis also proved the correlation between the under-cooling tendency of the base iron and that of ductile iron. An increase in the under-cooling of the base iron produced an increase in under-cooling in ductile iron and vice-versa.

Data on recalescence indicated that only SiC, alone or mixed with 75% ferrosilicon, and FeS had a beneficial effect leading to a decrease in recalescence in both base and ductile irons (Fig. 4).

When comparing the effects of metallurgical additions on recalescence versus nodule count, data showed that the nodule count generally increased as the recalescence decreased and vice-versa.

Data analysis showed a good correlation between the trend of undercooling measured by thermal analysis and the variation in the amount of carbides in the ductile iron wedges, with the exception of the samples taken after the crystalline graphite addition. The amount of carbides in the wedges poured after the graphite addition was smaller, while thermal analysis data indicated an increase in under-cooling.

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The gas test results showed that the addition of SiC, Mixture Y, FeS and crystalline graphite produced an increase in total oxygen content in the base iron between 4 and 9 ppm. Data analysis showed that there was a correlation between the increase in total oxygen content and the increased number of nodules in ductile iron, with the exception of the heat with FeS addition in which the nodule number decreased. Thus, the increase in total oxygen content in the base iron was not sufficient to cause an increased nodule number in ductile iron. All the additions made produced an insignificant increase in the nitrogen content in the base iron between 1 and 3 ppm; these variations could not be related to the changes in the nodule number of the ductile iron.

[FIGURE 7 OMITTED]

Based on microstructural and thermal analysis results, the samples produced with SiC alone and those produced with SiC mixed with 75% ferrosilicon and 0.1% crystalline graphite were selected for tensile tests. The results indicated that the addition of SiC caused increases in elongation from 16 to 23%, tensile strength front 511 to 522 MPa and yield strength front 333 to 346 MPa. Also, the tensile properties of the samples produced with SiC were more consistent, with a relatively lower standard deviation than the reference samples. The addition of SiC diluted with 75% ferrosilicon resulted in very little increase in elongation and tensile strength. The graphite addition increased percent elongation from 19 to 21%, ultimate tensile strength from 545 to 552 MPa, and yield strength from 348 to 357 MPa.

[FIGURE 8 OMITTED]

In the Real World

Based on the laboratory results, ductile iron tests were organized in an industrial metalcasting facility. The experiments were run in an induction furnace on a regular iron with the typical chemical composition of 3.67% carbon, 2.43% silicon, 0.27% manganese, 0.26% phosphorus and 0.046% magnesium. After melting the regular charge materials, the reference samples were poured. Then 0.3% SiC was added to the furnace, and the second set of test samples was taken. Test samples were poured from both base and final irons before and after the addition.

The results obtained under industrial conditions confirmed the beneficial effect of 0.3% SiC addition on the nucleation potential of ductile iron producing an increase in nodule count and a decrease in chilling tendency of ductile iron.

The as-polished microstructures of ductile iron samples poured for reference had an average of 156 nodules /sq. mm, 32% ferrite and only 0.14% carbides. After the 0.3% SiC addition, the nodule count increased to 241 nodules/sq, ram. The nodule roundness improved from 0.708 to 0.717, and sphericity increased front 0.880 to 0.891 (Figs. 5 and 6). The etched microstructures indicated a much higher increase in ferrite content than in the samples produced in laboratory; the ferrite content increased from 32% to 51%. The carbide ratio, already very low, decreased to 0.1%.

SiC proved to be beneficial to the frequencies of nodule size. The number of nodules between 32 and 64 [micro]m decreased from 19 to 11% (Fig. 7).

Chilling tendency was quantified visually by comparative examination for both base and ductile iron on the chill wedges poured before and after the SiC additions. The ductile iron samples were polished and etched to reveal the white zone formed at the tip of the chill test samples. The chill wedges indicate that 0.3% SiC addition improved the chilling tendency of both base and ductile irons (Fig. 8).

The tensile properties determined on test specimens machined from 30 mm diameter rods indicate an increase in percent elongation from 8,7 to 10.1% and a decrease in ultimate tensile strength from 528 to 505 MPa, The yield strength was 329 MPa and did not change after SiC addition. The changes in tensile properties reflect the increase in ferrite ratio following the SiC addition.

For More Information

"Testing Ductile Iron Nodularity," MODERN CASTING, August 2007, p. 33-34.

R. Zavadil is a technologist at the Materials Technology Laboratory-CANMET, Canada. M. Popescu is a physical scientist for the lab, and M. Sahoo is manager of casting technology.
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Author:Zavadil, R.; Popescu, M.; Sahoo, M.
Publication:Modern Casting
Date:Aug 1, 2008
Words:1875
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