UNEDITED COPY

Materials Technology Laboratory

Effect of Chemistry and Processing Variables on the

Mechanical Properties of Thin-Wall Ductile Iron Castings

MTL 99 - 23 (OP-J)

A. Javaid, K.G. Davis and M. Sahoo

Materials Technology Laboratory (CANMET)

Ottawa, Ontario, Canada

October, 1999

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Effect of Chemistry and Processing Variables on theMechanical Properties of Thin-Wall Ductile Iron Castings

A. Javaid, K.G. Davis and M. SahooMaterials Technology Laboratory (CANMET)

Ottawa, Ontario, Canada

ABSTRACT

The effect of melt chemistry and molten metal processing variables (i.e., pre-conditioning

of the base iron, inoculation type and practice, and pouring temperature, etc.) on the tensile and

impact properties of thin-wall ductile iron castings has been investigated. Comparison of 3 and

12 mm sections within the same casting showed that section size was the main factor influencing

tensile properties of ductile irons. While many samples from 3 mm sections showed low

elongation values, likely caused by a high pearlite content or presence of carbides, many others

showed higher elongations and superior strengths well above those required in ASTM A536

grades. At moderate to high elongations, the thin-wall samples were significantly stronger than

samples from identical irons of 12 mm section.

A direct comparison between impact values could not be made due to different test

specimen sizes, but it is clear that toughness in the two section sizes was roughly equivalent

when account was made for the total cross sectional area. The main difference between the

Impact properties in the two section sizes lay in the relative insensitivity of the thin-section

specimens to either melt chemistry or molten metal processing variables.

Of the elements contained in the iron, silicon had the greatest effect on the tensile

properties of the thin-wall sections. The same increase in silicon content of the thin-wall sections

had little effect on impact toughness. As expected, any processing variable that led to an increase

in nodule count (with a corresponding increase in ferrite content) led to greater ductility, lower

strength, and improved toughness. Of the variables studied the greatest effect was found to be

from late inoculation, base iron pre-conditioning, and the use of an inoculant containing bismuth

and rare earths.

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INTRODUCTION

Applications of ductile cast iron have increased steadily in recent years due to its

relatively low-cost production and the capability of producing a range of microstructures, each

with different mechanical properties. There has been an increasing demand for strong thin-wall

ductile iron castings to provide components with high strength to weight ratios. Reducing the

weight of ductile iron castings by producing thin-wall parts is an important method for saving

energy in vehicles. The lower weight of iron used also reduces the energy needed for melting.

Both of these aspects lead to reduced carbon dioxide emissions into the atmosphere.

The high cooling rate in thin-section ductile iron results in increased amounts of carbides

with a corresponding loss in mechanical properties, specifically ductility and toughness. There is

only limited data for "quality" thin-wall ductile irons that meet or exceed the ASTM

specifications.

The current authors have published results from a study of factors affecting the carbide

formation in thin-wall ductile iron castings1. The mechanical properties in 3 and 12 mm sections

taken from the castings made in this earlier study have now been determined and are the subject

of the current report.

The aim of the project is to evaluate and identify the most important variables affecting

the mechanical properties and carbide formation in thin-wall ductile iron castings, as well as to

suggest the most economical ways of reducing or eliminating the formation of these carbides.

LITERATURE REVIEW

A detailed literature review summarizing developments in thin-wall iron casting

technology with respect to production processes, casting parameters, mechanical properties,

dimensional tolerances and influences of metallurgical processing variables was carried out by the

Thin Wall Iron Group (TWIG)2. There was found to be only limited data available regarding the

mechanical properties of thin-wall ductile irons. Van Eldijk et al3 describe in detail the design

criteria, life endurance properties and production procedures for producing 3 mm thin-walled

ductile iron castings free of primary carbides for automotive applications. Successful results

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indicate that considerable reduction in weight and costs are realized when compared to

conventional castings. The elastic deformation and buckling force have been measured in bend

specimens from a 5 mm thick gray iron plate4. It was concluded that thin-wall gray iron castings

cannot be characterized by their tensile strength. The application of nondestructive testing

methods (ultrasonic, magnetic, or electrical) for determining mechanical properties in thin-section

cast irons has also been described 5.

EXPERIMENTAL PROCEDURE

Laboratory heats of 100 Kg (220 lb.) were produced in an alumina-lined 100 KW medium

frequency induction furnace. Details can be found in Ref 1. Variables studied included

composition, inoculation type and practice, base iron pre-conditioning, pouring temperature, and

mold materials, etc.

Shapes and dimensions of the step block castings are shown in Fig. 1. The samples for

mechanical property evaluation were obtained from the 3 and 12 mm sections of the step block

casting. Details of the tensile and impact specimens are shown in Fig. 2a and 2b.

RESULTS AND DISCUSSION

Tensile Properties

Overall Effect of Section Size on Tensile Properties

Loper and Kotschi6 have proposed the use of a “quality index” based upon the minimum

values of tensile strength and elongation, or yield strength and elongation, set forth in ASTM

A536 specifications. Quality baseline diagrams are devised by plotting the minimum values of

yield strength vs. elongation, and/or the minimum values of tensile strength vs. elongation, from

the five grades of ASTM A536 specifications, as shown in Fig. 3a and 3b, respectively. Irons

with mechanical properties that plot on or above the baseline are acceptable. The higher the

mechanical properties plot above the baseline, the higher the quality. A higher quality index is

obtained when the graphite is fully spheroidal, and when the matrix structure becomes more

uniform in composition (higher nodule count, reduced intercellular structure, etc.)7,8

. Ductile irons

with properties that plot below the quality baseline are considered unacceptable (lower

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nodularity, lower nodule count, increased intercellular structure, presence of carbides, etc.).

Except for irons containing carbides (where those carbides can be removed by suitable heat

treatment), heat treatment will not result in changes to the properties normal to the baseline, but

will result in changes in properties essentially parallel to the baseline as the pearlite content is

increased or reduced.

Evaluation of acceptability of the 3 and 12 mm section ductile irons using the yield

strength-elongation quality baseline is shown in Fig. 3a and the corresponding tensile strength-

elongation baseline is shown in Fig. 3b. It is apparent that 3 mm sections show a significant

increase in strength compared with 12 mm sections, even though a substantial difference in

elongation is realized due to the section size effect (i.e., variations in nodularity, nodule count and

matrix structure). Some of the property values from 12 mm sections falls below the quality

baseline as a result of low Mg residual (low nodularity). It was hard to maintain the spheroidal

graphite in thicker sections (>6 mm) at approximately 0.030% Mg residual.

Considering the quality index plots (Fig. 3a and 3b), it is clear that while many 3 mm

samples showed low elongation values, likely caused by a high pearlite content or the presence of

carbides, many others showed strengths well above those required in ASTM A536 grades.

Indeed, at low elongations some of the thin-wall samples showed superior strengths that can

normally be obtained in ductile iron only through heat treatment. At moderate to high

elongations, the thin-wall samples were significantly stronger than samples from identical irons of

12 mm section. Fig. 4a shows carbide free microstructure from the fractured end of 3 and 12 mm

tensile test samples within the same casting (late-inoculation with 75%FeSi containing Bi and

RE). The tensile property values of these samples plot well above the quality baseline, showing

high elongations. The property value from another 3 mm tensile test sample from the same iron

(late-inoculation with foundry grade 75%FeSi) are well below the quality baseline, showing low

elongation. The presence of the carbides, even in small amounts (see Fig. 4b), drastically decrease

the elongation values in this 3 mm section. In general, irons with properties at lower elongation

reflect lower Si (higher pearlite)/presence of carbides/lower nodule count or nodularity, while

irons at higher elongation represent higher Si (higher ferrite)/improved nodularity and higher

nodule count.

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Effect of Chemical Composition on Tensile Properties

The effect of carbon equivalent (CE) on the yield strength and tensile strength in 3 and 12

mm sections is given in Fig. 5a and 5b. Both the yield and tensile strength of 3 mm sections

decrease significantly with an increase in CE, as a result of increase in ferrite content. On the

other hand, 12 mm sections show a slight increase in yield strength with an increase in CE but the

tensile strength does not show any effect. Fig. 6a and 6b show changes in elongation with CE and

carbon on 3 and 12 mm sections. The increase in CE shows a small improvement in the

elongation of 12 mm sections as a result of increase in ferrite but the 3mm sections are not much

affected by changes in CE. The elongation of 3 mm sections reduces with an increase in carbon

content but the 12 mm sections do not show any effect. The increase in carbon content results in

a slight increase in the strength of 3 mm sections, whereas the strength of 12 mm sections do not

significantly vary despite changes in the carbon content as shown in Fig. 7a and 7b.

Silicon shows the most significant effect on the tensile and yield strength of 3mm

sections, as shown in Fig. 8a and 8b. This decrease in strength results from the increase in ferrite

content/nodule count with an increase in silicon content. The 12 mm sections show a moderate

increase in yield strength with an increase in silicon content due to the solid-solution

strengthening of the ferrite, whereas the tensile strength is not much affected by change in the

silicon level. A strong effect of silicon in increasing the elongation of both 3 and 12 mm sections

is shown in Fig. 9a.

An increase in Mg content is shown to decrease the elongation in both 3 and 12 mm

sections (Fig. 9b) as a result of carbide stabilizing effect of Mg in cast iron. On the other hand,

the increase in cerium content shows a small increase in the elongation of both 3 and 12 mm

sections as a result of increase in nodule count and thus ferrite content (Fig. 10a). Mg and Ce

does not show any effect on the strength of 3 and 12 mm sections. The small changes in Al, S,

and P content has no effect on the elongation and strength of 3 and 12 mm sections. However, a

small variation in Cr content shows a significant effect on the elongation but no effect on the

strength of 3 and 12 mm sections

Effect of Processing Variables on Tensile Properties

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An increase in the pouring temperature shows some improvement in the elongation of 12

mm sections (Fig. 10b) but the elongation in 3 mm sections and strength in 3 and 12 mm sections

does not show any effect.

Pre-conditioning of the base iron was done by the addition of 0.20% FeSi (75%), 0.20%

SiC, or 0.1%FeSi+0.1%SiC, immediately before tapping. Pre-conditioning increases the

nucleation potential, and so minimizes the potential for primary carbide formation in the final

iron and improves the elongation, as shown in Fig. 11a.

In-stream inoculation experiments, each melt was used to test three different kinds of

inoculation treatments with; 1.0% of 75%FeSi in the first tap, 1.0% of 75%FeSi with rare earths

(RE) in second tap and 1.0% of 75%FeSi with RE and Bi in third tap. In the late-inoculation

procedure, an additional 0.1% of inoculant was placed on a filter at the base of the pouring cup.

Fig. 11b shows the beneficial effect of late-inoculation on the elongation of 3 and 12 mm sections.

The late-inoculation was much more effective in increasing nodule count and reducing carbides.

In the late-inoculation experiments, two different methods for the addition of the foundry

grade 75%FeSi inoculant were used. In one, 1.0% of the FeSi was added to the metal stream

during transfer from the tundish to the pouring ladle. In the other, the same amount of inoculant

was added but at three different stages: 0.20% onto the metal stream during tapping from the

furnace into the tundish ladle, 0.30% as a cover for the Mg treatment alloy in the tundish ladle,

and 0.50% stirred into the tundish ladle after Mg treatment and right before pouring the castings

(no reladling was done to avoid loss of temperature) and finally the 0.1% late-inoculation in-

sprue addition. Fig. 12a shows the effect of method of addition of the inoculant on the elongation

of 3 and 12 mm sections. The addition of inoculants at different stages of molten metal

processing resulted in the highest nodule count, and therefore led to improved elongation.

In late-inoculation experiments, five types of inoculant were investigated in terms of their

effectiveness in increasing the nodule count and chill reduction. The best results were obtained

from 75%FeSi containing Bi and RE. Comparison of the effectiveness of these late inoculants in

increasing the elongation is shown in Fig. 12b. Fig. 13a and 13b show the effect of these

inoculants on the strength properties of 3 and 12 mm sections. There is a decrease in both yield

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and tensile strength as a result of the increase in nodule count and thus of ferrite content. FeSi

containing Bi and RE was most effective in terms of increasing the nodule count.

Tensile and yield strength shows a significant decrease in 3 mm sections as a result of increase

in nodule count (with a corresponding increase in ferrite content) due to pre-conditioning and

late-inoculation with 75%FeSi containing Bi and RE. Pre-conditioning and late-inoculation with

75%FeSi containing Bi and RE, slightly improves the yield strength but does not show any effect

on tensile strength in 12 mm sections.

Impact Properties

Overall Effect of Section Size on Impact Properties

The effect of section size on Charpy V-notched impact energy at room temperature on 3

mm and 12 mm sections is shown in Fig. 14a. For comparison purposes, the impact energies of 3

mm sections is shown four times higher, to account for the total cross sectional areas of the test

specimens (i.e., 25 mm2 for 2.5 mm and 100 mm

2 for 10 mm thick Charpy bars). It appears that

the smaller bars are less tough, but there may be an intrinsic size effect, making direct comparison

invalid. It may, however, be noted that for identical conditions (i.e. two sections of the same

casting), the 3 mm section will contain more pearlite than the 12 mm section, and so the 3 mm

sections are expected to be less tough. In fact, in some examples the improvement in toughness of

the thicker section is far less than expected, see Fig. 15a and 15b.

Effect of Chemical Composition on Impact Properties

The influence of the CE on the impact energies of 3 and 12 mm sections is shown in Fig.

14b. The impact energies of 12 mm sections decrease with an increase in CE, whereas the 3 mm

sections do not show much effect. Fig. 16a shows a similar effect of carbon on impact energies.

On the other hand, silicon shows a strong effect on the impact energies of 12 mm sections, while

the 3 mm sections show a much smaller effect (see Fig. 16b). Phosphorous as low as from 0.01 to

0.016% shows a significant embrittling effect in the 12 mm sections, as shown in Fig. 17a. The

impact properties in 3 mm sections also show a slight reduction with increase in phosphorous

content. An increase in the cerium content produces an improvement in the impact properties of

both the 3 and 12 mm sections (see Fig. 17b). This might be as a result of an increase in the

nodule count with the addition of cerium, which promotes ferrite. The increase in Mg content

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results in a slight decrease in the impact values of 12 mm sections but the 3 mm sections are not

much affected by changes in Mg. The small variations in Mn and S content has no effect on the

impact properties. On the other hand, the presence of certain residual elements such as Cr and

Al, even in very small ranges, show a significant effect on the impact properties of 12 mm

sections but no effect in 3 mm sections.

Effect of Processing Variables on Impact Properties

Pre-conditioning of the base iron and late-inoculation shows a slight improvement in the

impact values of 12 mm sections but does not show any effect in 3 mm sections. Inoculant type

and method of addition of inoculant has a significant effect on the impact properties of 12 mm

sections but no effect in 3 mm sections. The impact energies of both 3 and 12 mm sections are

not affected by changes in pouring temperature.

A direct comparison between impact values could not be made due to different test

specimen sizes, but it is clear that toughness in the two section sizes was roughly equivalent

when account was made for the total cross sectional area. The main difference between the

Impact properties in the two section sizes lay in the relative insensitivity of the thin-section

specimens to either melt chemistry or molten metal processing variables.

CONCLUSIONS

In view of the extremely high nodule count in thin-wall ductile iron, unusual mechanical

properties may be expected. Considering the quality index plots (Fig. 3a and 3b), it is clear that

while many samples showed low elongation values, likely caused by a high pearlite content or the

presence of carbides, many others showed strengths well above those required in ASTM A536

grades. Indeed, at low elongations some of the thin-wall samples showed superior strengths that

can normally be obtained in ductile iron only through heat treatment. At moderate to high

elongations, the thin-wall samples were significantly stronger than samples from identical irons of

12 mm section.

A direct comparison between Charpy impact values could not be made due to possible

intrinsic effects associated with specimen size, but it is clear that toughness in the two section

sizes was roughly equivalent when account was made for the total cross sectional area. The main

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difference between the Impact properties in the two section sizes lay in the relative insensitivity

of the thin-section specimens to either melt chemistry or molten metal processing variables.

Of the elements contained in the iron, silicon had the greatest effect on the tensile

properties of the thin-wall sections. An increase between approx. 2.5% and 3.4% led to large

decreases in tensile and yield strength, together with a large increase in elongation as a result of

higher ferrite content in the thin-wall sections. The same increase in silicon content of the thin-

wall sections had little effect on impact toughness. As expected, any processing variable that led

to an increase in nodule count (with a corresponding increase in ferrite content) led to greater

ductility, lower strength, and improved toughness. Of the variables studied the greatest effect

was found to be from late inoculation, base iron pre-conditioning, and the use of an inoculant

containing Bi and RE.

ACKNOWLEDGMENT

The authors wish to thank all the staff of the Experimental Casting Laboratory for melting

and casting, ETSD for machining the tensile and charpy specimens, Bob Eagleson for mechanical

testing, J. Thomson for assistance in experiments, and R. Zavadil for metallography.

REFERENCES

1. A. Javaid, J. Thomson, K.G. Davis, and M. Sahoo,”Factors Affecting the Formation of

Carbides in Thin-Wall Ductile Iron Castings,” To be Published in 1999 AFS Transactions.

2. J.F. Cluttino, J. Andrews, T. Piwonka,”Development in Thin Wall Iron Casting Technology,”

To be Published in 1999 AFS Transactions.

3. Van Eldijk, P C; Kaspers, J; van Lipzig, H; inst. Veldt, J,”Thin Ductile Iron Castings For

Automotive Applications: A Strong Potential,” 60th World Foundry Congress, Netherlands,

Sep 1993.

4. Nandori, G.P. Joans, and J. Dul,”Testing of Thin-Walled Gray Iron Castings by Means of

Bending Test Bars,” Kohasz Banyasz, Lapok (Ontode), Vol. 38, No 2, 1987.

5. Cech, J,”Measuring the Mechanical Properties of Cast Irons by NDT Methods,” NDT

International, Vol. 23 n 2, 1990, p 93-102.

6. C. R. Loper Jr., and R. M. Kotschi,"A New Quality Index for Ductile Cast Iron," AFS

Transactions, Vol. 82, 1974, p. 226-228.

7. C. R. Loper, Jr.,"Microstructure and Quality Control of Cast Iron," Proceedings of the 4th

International Symposium on the Physical Metallurgy of Cast Iron, Tokyo, Japan, Sept. 4-6,

1989, p. 281-291.

8. A. Javaid and C. R. Loper, Jr.,"Quality Control of Heavy-Section Ductile Iron," AFS

Transactions, Vol. 48, 1995, p. 119-134.

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LIST OF ILLUSTRATIONS

Fig. 1. Shape and dimension of the step block casting.

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Fig. 2a&b. Shape and dimension of the tensile (top) and impact (bottom) test specimens.

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Fig. 3a&b. Evaluation of acceptability of 3 and 12 mm ductile irons using the yield strength-

elongation (top) and tensile-strength elongation (bottom) quality baselines.

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Fig. 4a&b. Microstructure from the fractured end of 3 mm and 12 mm tensile test samples.

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Fig. 5a&b. Effect of carbon equivalent on the yield strength (top) and tensile strength

(bottom) of 3 and 12 mm sections.

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Fig. 6a&b Effect of carbon equivalent on elongation (top) and effect of carbon on elongation

(bottom) of 3 and 12 mm sections.

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Fig. 7a&b. Effect of carbon on yield strength (top) and on tensile strength (bottom) of 3 and

12 mm sections.

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Fig. 8a&b. Effect of silicon on the yield strength (top) and tensile strength (bottom) of 3 and

12 mm sections.

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Fig. 9a&b. Effect of silicon on elongation (top) and effect of magnesium on elongation

(bottom) of 3 and 12 mm sections.

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Fig. 10a&b. Effect of cerium on elongation (top) and effect of pouring temperature on

elongation (bottom) of 3 and 12 mm sections.

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Fig. 11a&b. Effect of pre-conditioning on elongation (top) and effect of method of inoculation

on elongation (bottom) of 3 and 12 mm sections.

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Fig. 12a&b. Effect of method of addition of inoculant on elongation (top) and effect of type of

inoculant on elongation (bottom) of 3 and 12 mm sections.

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Fig. 13a&b. Effect of type of inoculants on yield strength (top) and on tensile strength

(bottom) of 3 and 12 mm sections.

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Fig. 14a&b. Effect of section size on impact energy at room temperature (top) and the effect

of the carbon equivalent on the impact energies of 3 and 12 mm sections (bottom).

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Fig. 15a&b. Microstructure from the fractured end of 12 mm and 3 mm impact specimens.

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Fig. 16a&b. Effect of carbon (top) and effect of silicon (bottom) on the Charpy V-notched

impact energies of 3 and 12 mm sections.

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Fig. 17a&b. Effect of phosphorous (top) and effect of cerium (bottom) on the Charpy V-

notched impact energies of 3 and 12 mm sections.