copyright 2009 american foundry society, … · increase the nodule count in ductile iron. m....

53International Journal of Metalcasting/Winter 09

AFS Sponsored ResearchSiC–THE MOST EFFICIENT ADDITION TO

INCREASE THE NODULE COUNT IN DUCTILE IRON

M. Popescu, R. Zavadil, and M. SahooCANMET Materials Technology Laboratory, Ottawa, Ontario, Canada

Copyright 2008 American Foundry Society

Abstract

Introduction

Grey and ductile iron foundries are familiar with a condition where iron held for long periods in the furnace, after normal processing, presents undercooled microstructures leading to abnormal values for properties such as hardness and tensile strength. Given that this situation is encountered where the iron is held over the weekend, the iron presenting this condition is called “Monday Morning Iron”. Since the iron is not responsive to inoculation, another frequent name is “Dead Iron”. The aim of this research was to find a solution to restore the nucleation potential of an iron with this condition. The research was initiated within the AFS Cast Iron Division and has been carried out in two phases. The objectives of the first phase was to investigate the effect of holding temperature and time on the nucleation potential of the iron and to better understand the conditions promoting “Monday Morning Iron”. The investigation was done on both grey and ductile irons. The results of this investigation reinforced the importance of holding temperature and time in the furnace.1,2 The negative effect of holding on the nucleation potential of the iron depends on the initial state of iron and is more powerful at higher temperatures where the effect is more obvious and takes place more rapidly if the iron initial nucleation capability is poor and the heat is overheated. The result is the formation of undercooled graphite and ferrite in grey iron and lower nodule count and heterogeneous distribution in ductile iron. The investigations in the first phase indicated that changes in the microstructure and chilling tendency of the base iron are reflected in the nodule count and distribution and chilling tendency of the ductile iron. However, more data is necessary to provide information of practical interest for a systematic and thoroughly controlled foundry testing in ductile iron production. This would constitute an efficient way to identify

“Monday Morning” syndrome and to take action to restore the iron nucleation potential before the Mg-treatment and pouring the pieces.

The objective in Phase II was to identify in laboratory conditions the most efficient way to increase the nucleation potential of iron presenting increased undercooling tendency. Two different methods were explored for this purpose. One method was to treat the base iron with selected metallurgical additives. Based on the data published in the literature the following agents were selected for this study: SiC, a mixture of SiC with FeSi5%Mg (Mixture X), a mixture of SiC with foundry grade FeSi75% (Mixture Y), SiCa, crystalline graphite and FeS. These additives were considered to provide the liquid iron with active elements to form silicate sulphides that are known to constitute suitable sites for graphite nodule nucleation. Their effect on the iron state was monitored and compared with the iron initial state in both base and ductile irons. Changes in the base iron microstructure, chilling tendency and hardness before and after addition were investigated and the data evaluated against the microstructure, chilling tendency, shrinkage behaviour and mechanical properties of ductile iron produced before and after the addition. The second group of experiments run in the laboratory conditions consisted of using different melting techniques such as ductile iron additions to the base iron at the end of melting process or/and holding the iron with undercooled tendency at a higher temperature than normal (1565ºC or 2850 ºF). Addition of ductile iron returns to already molten iron was performed with the aim to improve the base iron nucleation potential through dilution. The second technique used was intended to dissipate the nuclei promoting undercooled microstructure.

In the first phase of the AFS-sponsored research project on “Restoring Techniques for “Monday Morning Iron”, the investigation focused on the extent to which prolonged holding of the base iron in an induction furnace affected the quality of ductile iron. The objective in Phase II was to find a way to increase the nucleation potential of iron having a high chilling tendency. The technique consisted of adding SiC, SiCa, crystalline graphite, etc., to a base iron in the furnace before the Mg-treatment with the aim to supply the iron with active elements considered suitable sites for graphite nodule

nucleation. The results of this investigation demonstrated that SiC is the most efficient addition to increase the iron nucleation potential. Based on these results, tests were run under commercial foundry operating conditions. This report presents and discusses the results of investigation performed in the MTL laboratory with detailed analysis of chilling tendency, microstructure and shrinkage propensity on test samples from both base and final ductile irons.Keywords: ductile iron, inoculation, Monday Morning Iron, nodule count, chill, nucleation, undercooling.

Copyright 2009 American Foundry Society, www.afsinc.org. Reprinted with permission.

54 International Journal of Metalcasting/Winter 09

It was initiated based on the positive results obtained on the grey iron in the first phase of the project. Details of the two groups of experiments can be found in a CANMET report.3

This paper focuses on the most efficient additive(s) to increase the nodule count in both base and ductile irons in laboratory conditions. By increasing the number of graphite nodules during solidification, the rate of release of latent heat due to the graphite crystallization increases, and the end of freezing temperature is raised above the cementite liquidus line preventing carbide formation. Following the laboratory experiments, tests were conducted under commercial foundry operating conditions to verify the data obtained in laboratory.

Experimental Procedure

The laboratory experiments were performed in coreless induction furnaces lined with alumina on a typical base iron composition: 3.85% C; 1.85% Si; 0.3% Mn; max 0.03% P and 0.01% S and following a common melting practice. Based on preliminary investigations that had shown that if the graphitization ability of the charge materials is poor, the nucleation potential of the iron obtained after remelting will be certainly poor, all the heats were produced with 50% highly carbidic ductile iron returns in the charge. The nodularization treatment was completed in a tundish ladle with 1.3% FeSi-6% Mg- 1%Re along with 0.3% foundry grade ferrosilicon as inoculant.

The metallurgical additives selected to be investigated as potential sources of active elements providing suitable sites for graphite nodule nucleation are the following: SiC, a mixture of 50% SiC with 50% FeSi5%Mg (Mixture X), a mixture of 50% SiC and 50% foundry grade FeSi75% (Mixture Y), SiCa,

FeS and crystalline graphite. The metallurgical additives were introduced into the molten iron in the furnace, wrapped in a mild steel foil to facilitate sinking and to avoid oxidation or sticking to the crucible walls. Their effect was monitored by comparison with the ductile iron initial state on test samples taken before and after addition for each experiment. The following test samples were poured: chill wedges to determine chilling tendency, 25 mm diameter rods for microstructural examination and tensile test bars. Pin samples were taken from base iron before and after addition to determine variations in total oxygen and nitrogen contents. Thermal analysis data was recorded to determine undercooling and recalescence. Changes in the microstructure, chilling tendency and mechanical properties of ductile iron were investigated and the data 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.

Results and Discussion

Microstructure

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: the nodule number meeting the image analysis criteria increased by 64%. After the addition of SiC diluted with FeSi75% (0.2%, Mixture Y) the nodule count increased by 15%. 0.05% SiCa and/or 0.1% crystalline graphite produced an increase in nodule count by only 3.5% and 2.5% respectively. Mixture X and/or FeS had a negative effect on the nodule count: the nodule count decreased 19%. Variation of total graphite separations and the density of the nodule meeting the criteria established for image analysis are shown in Fig. 1.

Figure 1. Variation of total graphite content and nodule number.

Den

sity

, Nod

ule

no./m

m2



Figure 2. Nodule size distribution in ductile iron produced before and after the additions. (Figure cont’d on next page.)

Before After

0.3% SiC

0.2% Mixture X

0.2% Mixture Y



Before After

0.05%SiCa

0.01%FeS

0.1% crystalline graphite

Figure 2. Nodule size distribution in ductile iron produced before and after the additions. (Cont’d from previous page.)



Before After

0.3% SiC

0.2% Mixture X

0.2% Mixture YFigure 3. Effect of various additions on ductile iron chilling tendency. (figure continued on next page.)



0.1% crystalline graphite

Before After

0.05%SiCa

0.01%FeS

Figure 3. Effect of various additions on ductile iron chilling tendency. (Continued from previous page.)



SiC had a positive effect, as well, on the nodule characteristics: 0.3%SiC addition produced an increase of the Nodularity count by 21%, Sphericity by 2.4% and Roundness by 2.4%. The addition of 0.1% crystalline graphite had an insignificant effect on nodularity and roundness, while Mixture X and FeS additions affected adversely the nodule characteristics.

The frequencies of nodule size variation in each test sample, poured before and after addition, as shown in Fig. 2. SiC alone or mixed with FeSi75% produced also an improvement of the nodule size distribution: the small (4-16 μm) nodule number increased and the large (16-64 μm) nodule number decreased (Fig. 2). Previous published research has shown that the skewed distribution of the nodule size and the increase of the number of the small and late nucleated nodules affect positively the ductile iron properties and that the large nodule numbers are detrimental to ductile iron properties including shrinkage tendency.1,2

Examination of the etched microstructures indicates that the average amount of ferrite and pearlite in the reference ductile iron samples were about 32 % and 58 % respectively. Addition of SiC mixed with Fesi75% had the most beneficial effect on ferrite formation followed by the crystalline graphite, which increased by 20% and 14% respectively. SiC produced only 3% increase in ferrite content.

Analysis of the chill wedges taken from ductile iron (Fig. 3) indicates less carbides in the samples after addition of SiC, Mixture Y and crystalline graphite respectively. The chill

wedge samples taken after SiCa, FeS and Mixture X indicate an increase in carbide content.

Thermal analysis data indicate that the initial state of iron varied from heat to heat presenting different degrees of undercooling. Undercooling of the base iron(s) varied from 21.84°C (71.31°F) to 9.11°C (48.4°F) and ductile iron between 27.47°C (81.45°F) and 18.66°C (65.59°F) . Thus, pouring reference samples for each heat was mandatory for making a correct assessment on the effect of investigated additions on the nucleation potential of the iron.

Variations of undercooling and recalesence of the base and ductile irons determined on the samples taken before and after each addition are represented graphically in Figs. 4 and 5. Undercooling is defined as the difference between the gray iron eutectic temperature (TE

gray =

1153 +6.7*Si) and the actual low eutectic temperature (TE

low). It characterizes the iron tendency for undercooled

microstructure. A high undercooling also means a longer time before freezing starts and in consequence an increased risk for macro shrinkage and outer sunk. Data show that SiC alone or mixed with FeSi75% has a beneficial effect on undercooling: the decrease in the base iron was about 22%; in ductile iron the decrease was 7.4% and 0.6% respectively for the two additions. FeS produces as well a decrease of undercooling even though the nodule count decreased. All other metallurgical additives were followed by an increase of undercooling in both base and ductile irons by comparison with the initial state of iron.

Figure 4. Undercooling variation in the base and ductile irons.

Und

erco

olin

g, °

C



Figure 5. Recalescence variation in the base and ductile irons.

Data from thermal analysis proved as well the correlation between the undercooling tendency of the base iron and that of ductile iron: an increase in undercooling of the base iron produces an increase of undercooling in ductile iron and vice-versa. Data on recalescence indicate that only SiC, alone or mixed with FeSi75%, and FeS had a beneficial effect leading to a decrease in recalesence in both base and ductile irons. A high recalescence in ductile iron is often an indication of low nodule count4 but the correlation did not always work.5 When comparing the effect of metallurgical additions on recalesence versus nodule count, data show that when the recalesence decreases, the nodule count increases and vice versa. Exception is FeS addition showing that while recalesence decreased in ductile iron from 4.57°C to 1.62°C (40.23°F to 34.92°F), the nodule count decreased from 164 nodules /mm2 to 132 nodules/mm2. Lower nodule count after FeS addition could be caused by a lower Mg available for nodularization after the addition of 0.017%FeS since the quantity of FeSi Mg was deliberately the same in all other experiments.

Data analysis show a good correlation between the trend of undercooling measured by thermal analysis and the variation of the amount of carbides in the ductile iron wedges with the exception of the samples taken after the crystalline graphite. The amount of carbides in the wedges poured after the graphite addition is smaller while thermal analysis data indicate an increase in undercooling.

The gas test results show that the addition of SiC, mixture Y, FeS and crystalline graphite produced an increase of total oxygen content in the base iron between 4 and 9 ppm. Data analysis shows that there is a correlation between the increase of total oxygen content and the increase of nodule number in ductile iron, with the exception of the heat with FeS addition where the nodule number decreased. Thus, the increase in total oxygen content in the base iron is not a sufficient condition to generate an increase of nodule number in ductile iron. All the additions made produced an increase in the nitrogen content in base iron between 1 and 3 ppm, but these variations could not be related with the changes in nodule number of ductile iron.

Rec

ales

cenc

e, °

C



Mechanical Properties

Based on microstructural and thermal analysis results, the samples produced with SiC alone and mixed with FeSi75% and 0.1% cristalline graphite were selected for tensile tests. Four specimens were tested for each condition. The results indicate that the addition of SiC caused an increase of elongation from 16 to 23%, tensile strength from 511 to 522 MPa and yield strength from 333 to 346 MPa. Also, the tensile properties of the samples produced with SiC were more consistent, with a relatively lower standard deviation by comparison with the reference samples. The addition of SiC diluted with FeSi75 resulted in very little increases in elongation and tensile strength. The graphite addition resulted in increased tensile properties as well: % elongation from 19 to 21%, ultimate tensile strength from 545 to 552 MPa, and yield strength from 348 to 357 MPa.

In conclusion, the results of the investigation under laboratory conditions show 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, improving undercooling tendency and upgrading tensile properties, primarily % elongation.

Technology Demonstration

Based on the laboratory results, ductile iron tests were organized in an industry. Unfortunately for this study the foundry did not have iron with increased undercooling tendency. The experiments were run in an induction furnace on iron with the typical chemical composition: 3.67% C, 2.43% Si, 0.27% Mn, 0.026% P, 0.046% Mg. After melting the 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 addition.

The as-polished microstructures of ductile iron samples poured for reference had an average of 156 nodules /mm2, 32% ferrite and only 0.14% carbides. After 0.3%SiC addition the nodule count increased to 241 nodules /mm2. The nodule roundness improved from 0.708 to 0.717 and sphericity from 0.880 to 0.891. Typical microstructures of the samples poured before and after SiC addition are shown in Figures 6 and 7 respectively. The etched microstructures indicate a much higher increase in ferrite content than in the samples produced in laboratory: the ferrite content increased from 32% to 51%. The carbide content already very low, decreased to 0.10%.

Before: 156 nodules/mm2 Before: 37% ferrite, 53.4% pearliteFigure 6. Typical microstructures in as-polished and etched samples taken before the SiC addition.

After: 241 nodules/mm2 After: 51.1% ferrite, 39.4%pearliteFigure 7. Typical microstructures in as polished and etched samples taken after the SiC addition.



Figure 8. Nodule size distribution before and after 0.3%SiC addition.

Before After

Before

AfterBase iron Ductile iron

Base iron Ductile ironFigure 9. Chilling tendency in the wedges poured from the base and final iron, before and after 0.3% SiC addition.



SiC proved to be beneficial with respect to the frequencies of nodule size as shown in Fig. 8 the large nodule number, with the size between 32-64 μm, decreased from 19% to 11%.

Chilling tendency was quantified visually by comparative examination on both base and ductile irons 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 chilling tendency of both base and ductile irons (see Fig. 9).

Unfortunately the data for thermal analysis were collected with different equipment and the data did not correlate with the trend shown by the microstructural examination.

The tensile properties determined on test specimens machined from 30 mm diameter rods indicate an increase in % elongation from 8.7% to 10.1% and a decrease of UTS from 528 MPa to 505 MPa. This reflects on the increase of ferrite content following the SiC addition.

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

Conclusions

1. SiC is the most efficient addition to obtain an increase of the iron nucleation potential and at the same time to obtain a decrease of the undercooling tendency of the iron.

2. The beneficial effect of the SiC addition on the microstructure is reflected in tensile test results: the ultimate tensile strength increased by about 2%, the yield strength by 4% and the elongation by 44%. In addition, the standard deviation for the tensile properties diminished.

3. The effect of SiC diluted with FeSi75% (Mixture Y) was beneficial to the iron quality to a smaller extent in comparison with SiC alone.

4. The effect of crystalline graphite addition to the iron with initially increased undercooling tendency was negative.

5. The effect of SiC in combination with FeSiMg (Mixture X) was negative with respect to total acceptable nodules.

6. The effect of 0.05% SiCa addition was positive on the nodule count but negative on undercooling and recalescence.

7. The effect of FeS addition on undercooling was positive in both base and ductile irons.

8. There is a correlation between the undercooling tendency of both the base and ductile irons: an increase in undercooling in base iron led to an increase of undercooling in ductile iron.

9. Data also confirmed the correlation between recalesence and nodule count: an increase in recalescence indicates an increase in nodule count.

10. The chill wedge tests and the cell number of the base iron could not provide sufficient information to predict the undercooling tendency of the ductile iron.

11. Data on the oxygen content compared with the nodule number variation indicate that an increase in total oxygen level in the iron is not a sufficient condition to determine an increase in nodule number.

Acknowledgement

The authors would like to express their gratitude to AFS for funding this work, Brillion Foundry for supplying ductile iron with undercooled microstructure, Rio Tinto Iron and Titanium for machining the tensile bars, R. Eagleson for mechanical testing and all the members of the Experimental Casting Laboratory for melting and casting.

REFERENCES

1. Popescu, M., Thompson, J., and Zavadil, R., “Restoring Techniques for Monday Morning Iron, Phase I”, Report MTL 2001-4(TR-R).

2. Popescu, M., Zavadil, R, Thompson, J. P., Sahoo, M. and Gassere, P., “Studies to Improve Nucleation Potential of Ductile Iron When Using Carbidic Ductile Iron Returns”, AFS Transactions, vol. 115, pp 591-608 (2007).

3. Popescu, M., Zavadil, R, Thompson, J. P., and Sahoo, M, “Restoring Techniques for Monday Morning Iron, Phase II”, Report MTL 2005-14(TR-R).

4. ATAS Verifier User’s Guide-NOVACAST Foundry Technology, 1994-1998.

5. D. Sparkman, “The Thermal Analysis of Ductile Iron”, Copyright Foundry Information Systems, 1997.


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