Gray Cast Iron

7/31/2019 Gray Cast Iron

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Gray Iron-A Unique Engineering Material

By

D. E. Krause, Executive Director - 1940-1973

(The Gray Iron Research Institute, Inc.)

The Iron Casting Research Institute

REFERENCE:Krause, D. E., "Gray Iron-A Unique Engineering Material" Gray,

Ductile, and Malleable Iron Castings-Current Capabilities, ASTM STP 455,

American Society for Testing and Materials, Philadelphia, 1969, pp. 3-28.

ABSTRACT: Gray iron is the most versatile of all foundry metals. The high

carbon content is responsible for ease of melting and casting in the

foundryand for ease of machining in subsequent manufacturing. The low

degree or absence of shrinkage and high fluidityprovide maximum

freedom of design for the engineer. By suitable adjustment in composition

and selection of casting method, tensile strength can be varied from less

than 20,000 psi to over 60,000 psi and hardness from 100 to 300 BHN in

the as-cast condition. By subsequent heat treatment, the hardness can be

increased to H Rc 60.

If the service life of a gray iron part is considered to be too short, the designof the casting should be carefully reviewed before specifying a higher

strength and hardness grade of iron. An unnecessary increase in strength

and hardness may increase the cost of the casting as well as increase the

cost of machining through lower machining rates. Although the relationship

between Brinell hardness and tensile strength for gray iron is not constant,

data are shown which will allow use of the Brinell hardness test to estimate

the minimum tensile strength of the iron in a casting.

KEY WORDS: gray iron castings, casting design, foundry methods, ductileiron castings, malleable iron castings, metals, tests, evaluation

Preface:

September 7, 1990


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To: All readers of report on Gray Iron by D.E Krause

While this brief technical paper, originally presented in 1969, is still one of the best

summaries of gray iron metallurgy and properties, we call your attention to one

item on which recent research and foundry experience has shed more light. This is

the matter of manganese and sulfur effects.

In contrast to the traditional view of these elemental effects noted herein, work in

the 1980's confirms that in many cases manganese levels beyond that amount

combined with sulfur (about 1.7 times the sulfur level) tend to reduce strength and

hardness via promotion of more ferrite. However, low levels too close to this 1.7

"balanced ratio" tend to promote high and more erratic hardness and/or carbides.

Consequently, for most applications the optimum operating level for manganese

appears to be about (1.7 x % Sulfur), + 0.3% to 0.5%. For example, for an iron with0.10% Sulfur, the optimum range for Manganese would be 0.47% to

0.67%. Running toward the low end of the range would normally maintain higher

hardness and tensile strength while running toward the high end would decrease

both. This effect is also influenced by other metallurgical conditions peculiar to each

base iron so that the optimum range needs to be determined for each particular

melting operation.

We hope this clarification will be informative and useful to both casting producers

and users.

William F. Shaw, Executive Director, Iron Casting Research Institute

Gray iron is one of the oldest cast ferrous products. In spite of competition

from newer materials and their energetic promotion, gray iron is still used

for those applications where its properties have proved it to be the most

suitable material available. Next to wrought steel, gray iron is the most

widely used metallic material for engineering purposes. For 1967,production of gray iron castings was over 14 million tons, or about two

and one-half times the volume of all other types of castings combined.

There are several reasons for its popularity and widespread use. It has a

number of desirable characteristics not possessed by any other metal and

yet is among the cheapest of ferrous materials available to the engineer.

Gray iron castings are readily available in nearly all industrial areas and


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can be produced in foundries representing comparatively modest

investments. It is the purpose of this paper to bring to your attention the

characteristics of gray iron which make the material so useful.

Gray iron is one of the most easily cast of all metals in the foundry. It has

the lowest pouring temperature of the ferrous metals, which is reflected

in its high fluidity and its ability to be cast into intricate shapes. As a result

of a peculiarity during final stages of solidification, it has very low and, in

some cases, no liquid to solid shrinkage so that sound castings are readily

obtainable. For the majority of applications, gray iron is used in its as-cast

condition, thus simplifying production. Gray iron has excellent machining

qualities producing easily disposed of chips and yielding a surface with

excellent wear characteristics. The resistance of gray iron to scoring and

galling with proper matrix and graphite structure is universally recognized.

Gray iron castings can be produced by virtually any well-known foundry

process. Surprisingly enough, in spite of gray iron being an old material

and widely used in engineering construction, the metallurgy of the

material has not been clearly understood until comparatively recent

times. The mechanical properties of gray iron are not only determined by

composition but also greatly influenced by foundry practice, particularly

cooling rate in the casting. All of the carbon in gray iron, other than that

combined with iron to form pearlite in the matrix, is present as graphite in

the form of flakes of varying size and shape. It is the presence of theseflakes formed on solidification which characterize gray iron. The presence

of these flakes also imparts most of the desirable properties to gray iron.

Metallurgy of Gray Iron

MacKenzie[1]

in his 1944 Howe Memorial Lecture referred to cast iron as

"steel plus graphite." Although this simple definition still applies, the

properties of gray iron are affected by the amount of graphite present as

well as the shape, size, and distribution of the graphite flakes. Although

the matrix resembles steel, the silicon content is generally higher than for

cast steels, and the higher silicon content together with cooling rate

influences the amount of carbon in the matrix. Gray iron belongs to a

family of high-carbon silicon alloys which include malleable and nodular

irons. With the exception of magnesium or other nodularizing elements in

nodular iron, it is possible through variations in melting and foundry


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practice to produce all three materials from the same composition. In

spite of the widespread use of gray iron, the metallurgy of it is not clearly

understood by many users and even producers of the material. One of the

first and most complete discussions of the mechanism of solidification of

cast irons was presented in 1946 by Boyles

[2]

. Detailed discussions of themetallurgy of gray iron may be found in readily available handbooks[3-7]

.

The most recent review of cast iron metallurgy and the formation of

graphite is one by Wieser et al[8]

. To avoid unnecessary duplication of

information, only the more essential features of the metallurgy of gray

iron will be discussed here.

Composition

Gray iron is commercially produced over a wide range of compositions.

Foundries meeting the same specifications may use different compositionsto take advantage of lower cost raw materials locally available and the

general nature of the type of castings produced in the foundry. For these

reasons, inclusion of chemical composition in purchase specifications for

castings should be avoided unless essential to the application. The range

of compositions which one may find in gray iron castings is as follows:

total carbon, 2.75 to 4.00 percent; silicon, 0.75 to 3.00 percent;

manganese, 0.25 to 1.50 percent; sulfur, 0.02 to 0.20 percent;

phosphorus, 0.02 to 0.75 percent. One or more of the following alloying

elements may be present in varying amounts: molybdenum, copper,nickel, vanadium, titanium, tin, antimony, and chromium. Nitrogen is

generally present in the range of 20 to 92 ppm.

The concentration of some elements may exceed the limits shown above,

but generally the ranges are less than shown.

Carbon is by far the most important element in gray iron. With the

exception of the carbon in the pearlite of the matrix, the carbon is present

as graphite. The graphite is present in flake form and as such greatly

reduces the tensile strength of the matrix. It is possible to produce all

grades of iron of ASTM Specification for Gray Iron Castings (A 48-64) by

merely adjusting the carbon and silicon content of the iron. It would be

impossible to produce gray iron without an appropriate amount of silicon

being present. The addition of silicon reduces the solubility of carbon in

iron and also decreases the carbon content of the eutectic. The eutectic of


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iron and carbon is about 4.3 percent. The addition of each 1.00 percent

silicon reduces the amount of carbon in the eutectic by 0.33 percent. Since

carbon and silicon are the two principal elements in gray iron, the

combined effect of these elements in the form of percent carbon plus 1/s

percent silicon is termed carbon equivalent (CE). Gray irons having acarbon equivalent value of less than 4.3 percent are designated

hypoeutectic irons, and those with more than 4.3 percent carbon

equivalent are called hypereutectic irons. For hypoeutectic irons in the

automotive and allied industries, each 0.10 percent increase in carbon

equivalent value decreases the tensile strength by about 2700 psi.

If the cooling or solidification rate is too great for the carbon equivalent

value selected. the iron may freeze in the iron-iron carbide metastable

system rather than the stable iron-graphite system, which results in hard

or chilled edges on castings. The carbon equivalent value may be varied bychanging either or both the carbon and silicon content. Increasing the

silicon content has a greater effect on reduction of hard edges than

increasing the carbon content to the same carbon equivalent value. Silicon

has other effects than changing the carbon content of the eutectic.

Increasing the silicon content decreases the carbon content of the pearlite

and raises the transformation temperature of ferrite plus pearlite to

austenite. This influence of silicon on the critical ranges has been

discussed by Rehder[9]

.

The most common range for manganese in gray iron is from 0.55 to 0.75

percent. Increasing the manganese content tends to promote the

formation of pearlite while cooling through the critical range. It is

necessary to recognize that only that portion of the manganese not

combined with sulfur is effective. Virtually, all of the sulfur in gray iron is

present as manganese sulfide, and the manganese necessary for this

purpose is 1.7 times the sulfur content. Manganese is often raised beyond

1.00 percent, but in some types of green sand castings pinholes may be

encountered.

Sulfur is seldom intentionally added to gray iron and usually comes from

the coke in the cupola melting process. Up to 0.15 percent, sulfur tends to

promote the formation of Type A graphite. Somewhere beyond about

0.17 percent, sulfur may lead to the formation of blowholes in green sand

castings. The majority of foundries maintain sulfur content below 0.15


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percent with 0.09 to 0.12 percent being a common range for cupola

melted irons. Collaud and Thieme[10]

report that, if the sulfur is decreased

to a very low value together with low phosphorus and silicon, tougher

irons will result and have been designated as "TG," or tough graphite

irons.

The phosphorus content of most high-production gray iron castings is less

than 0.15 percent with the current trend toward more steel in the furnace

charge; phosphorus contents below 0.10 percent are common.

Phosphorus generally occurs as an iron iron-phosphide eutectic, although

in some of the higher- carbon irons, the ternary eutectic of iron iron-

phosphide iron-carbide may form. This eutectic will be found in the

eutectic cell boundaries, and beyond 0.20 percent phosphorus a decrease

in machinability may be encountered. Phosphorus contents over 0.10

percent are undesirable in the lower-carbon equivalent irons used forengine heads and blocks and other applications requiring pressure

tightness. For increased resistance to wear, phosphorus is often increased

to 0.50 percent and above as in automotive piston rings. At this level,

phosphorus also improves the fluidity of the iron and increases the

stiffness of the final casting.

Copper and nickel behave in a similar manner in cast iron. They strengthen

the matrix and decrease the tendency to form hard edges on castings.

Since they are mild graphitizers, they are often substituted for some of thesilicon in gray iron. An austenitic gray iron may be obtained by raising the

nickel content to about 15 percent together with about 6 percent copper,

or to 20 percent without copper as shown in ASTM Specification for

Austenitic Gray Iron Castings (A 436-63).

Chromium is generally present in amounts below 0.10 percent as a

residual element carried over from the charge materials. Chromium is

often added to improve hardness and strength of gray iron, and for this

purpose the chromium level is raised to 0.20 to 0.35 percent. Beyond this

range, it is necessary to add a graphitizer to avoid the formation of

carbides and hard edges. Chromium improves the elevated temperature

properties of gray iron.

One of the most widely used alloying elements for the purpose of

increasing the strength is molybdenum. It is added in amounts of 0.20 to


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0.75 percent, although the most common range is 0.35 to 0.55 percent.

Best results are obtained when the phosphorus content is below 0.10

percent, since molybdenum forms a complex eutectic with phosphorus

and thus reduces its alloying effect. Molybdenum is widely used for

improving the elevated temperature properties of gray iron. Since themodulus of elasticity of molybdenum is quite high, molybdenum additions

to gray iron increase its modulus of elasticity.

Vanadium has an effect on gray iron similar to molybdenum, but the

concentration must be limited to less than 0.15 percent if carbides are to

be avoided. Even in such small amounts, vanadium has a beneficial effect

on the elevated temperature properties of gray iron.

The beneficial effect of relatively small additions of tin (less than 0.10

percent) on the stability of pearlite in gray iron has been reported by Daviset al

[11]. The results of extensive use of tin in automotive engines has been

reported by Tache and Cage[12]

. Its use is particularly helpful in complex

castings wherein some sections cool rather slowly through the Ar3

temperature interval. It has been found that additions of up to 0.05

percent antimony have a similar effect. In larger amounts, these elements

tend to reduce the toughness and impact strength of gray iron, and good

supervision over their use is necessary.

Although most gray irons contain some titanium and the effect of titaniumon the mechanical properties has been investigated many times, it is only

recently that Sissener and Eriksson[13]

have reported the effect of titanium

reduced from a titanium containing slag in an electric arc furnace. With

titanium contents of 0.15 to 0.20 percent, the graphite flakes tend to

occur as Type D graphite rather than predominantly Type A, which is

generally considered desirable. They found that for irons with carbon

equivalent of less than about 3.9 percent, the addition of titanium tends

to lower tensile strength. but, for the higher carbon equivalent irons,

tensile strength is improved. Increasing the titanium content of gray iron

from about 0.05 to 0.14 percent through the use of a titanium bearing pig

iron increased the strength of a hypereutectic iron in an ASTM

Specification A 48 test bar A (7/8 in. diameter) from 22,000 to 34,000 psi.

Further work is being done with titanium additions.


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Normally. nitrogen is not considered as an alloying element and generally

occurs in gray iron as a result of having been in the charge materials.

Morrogh[14]

has reported that at higher nitrogen levels the graphite flakes

become shorter and the strength of the iron is improved. Gray irons

usually contain between 20 and 92 ppm (0.002 to 0.008 percent) nitrogen.If the nitrogen approaches or exceeds 100 ppm, unsoundness may be

experienced if the titanium content is insufficient to combine with the

nitrogen.

Effect of Section Size on Structure

All cast metals are said to be section sensitive. As the section size

increases. the solidification rate decreases with an accompanying increase

in grain size and subsequent decrease in tensile strength. The effect of

freezing rate on strength and hardness is more pronounced in gray ironthan for other cast metals. This is a result of the mechanism of

solidification. For a hypoeutectic iron, the first phase to separate on

cooling is austenite in the form of dendrites at the liquidus temperature.

As cooling progresses, the austenite dendrites grow, and the remaining

liquid becomes enriched in carbon until the eutectic composition of 4.3

percent carbon equivalent is reached. This occurs at a temperature of

approximately 2092 F depending on the silicon content. At this

temperature, eutectic austenite and graphite in the form of flakes are

deposited simultaneously.

The austenite-graphite deposition occurs at a number of centers or nuclei,

and these grow in size until all of the liquid is gone creating a cell-type

structure. During this period of cell growth, the phosphorus is rejected

toward the cell boundaries and freezes as a eutectic at about 1792F. The

presence of the phosphorus in the cell boundaries makes it possible to

clearly reveal them by etching with Stead's reagent. It has been

demonstrated that the graphite flakes grow only within the boundaries of

a cell and are interconnected. The cell size is dependent on the degree of

nucleation of the iron and the freezing rate. It will vary from about 500 to

as high as 25,000 cells per square inch.

Since graphite has a much lower density than iron, the normal contraction

which will occur when the iron changes from liquid to solid is completely

compensated for by the formation of graphite. For ASTM Designation A


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48, Class 30B iron, shrinkage is virtually absent so that sound castings are

readily produced providing the mold has adequate rigidity. The graphite

structure seen in gray iron has been completely established by the time

the iron is solidified. Upon further cooling, some additional carbon is

deposited on the graphite flakes until the Ar3 temperature is reached. Asa result of the high silicon content of gray iron, the transformation of

austenite to pearlite and ferrite does not occur at a fixed temperature but

takes place over a temperature range termed the "pearlite interval" and is

explained fully by Boyles[15]. Since the presence of silicon makes iron

carbide unstable, the proportion of ferrite and pearlite in the matrix after

transformation is completed will depend on the cooling rate through this

temperature range. For heavy sections and high silicon contents, the

matrix can be completely ferritic.

The graphite flake type, form, and size can be defined by following theprocedure described in ASTM Method for Evaluating the Microstructure of

Graphite in Iron Castings (A 247-67). Since graphite is a relatively soft

material, special care needs to be exercised in the preparation of a

specimen for metallographic examination. If improperly done, the true

shape of the graphite may be obscured by distorted metal that flowed

over the graphite. It is only after several etching and polishing operations

that a true representation of the graphite will be revealed.

Casting Processes

Several molding processes are used to produce gray iron castings. Some of

these have a marked influence on the structure and properties of the

resulting casting. The selection of a particular process depends on a

number of factors, and the design of the casting has much to do with it.

The processes using sand as the mold media have a somewhat similar

effect on the rate of solidification of the casting, while the permanent

mold process has a very marked effect on structure and properties.

Green sand molding is frequently the most economical method of

producing castings. Until the introduction of high-pressure molding and

very rigid flask equipment, dimensional accuracy has not been as good as

can be obtained from shell molding. If green sand molds are not

sufficiently hard or strong, some mold wall movement may take place

during solidification, and shrinkage defects develop. Although castings up


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to 1000 lb or more can be made in green sand, it generally is used for

medium to small size castings. For the larger castings, the mold surfaces

are sometimes sprayed with a blacking mix and skin dried to produce a

cleaner surface on the casting. This procedure is often used on engine

blocks.

To withstand the higher ferrostatic pressures developed in pouring larger

castings; dry sand molds are often used. In some cases, the same sand as

used for green sand molding is employed, although it is common practice

to add another binder to increase the dry strength.

The shell molding process is also used for making cores which are used in

other types of molds besides shell molds. Its principal advantage is derived

from the ability to harden the mold or core in contact with a heated metal

pattern, thus improving the accuracy with which a core or mold can bemade. In addition to the improved accuracy, a much cleaner casting is

produced than by any other high-production process. Although the

techniques and binders for hot box and the newest cold box processes

differ from those used for the shell molding process, the principle is

similar in that the core is hardened while in contact with the pattern.

Centrifugal casting of iron in water-cooled metal molds is widely used by

the cast iron pipe industry as well as for some other applications. With

sand or other refractory lining of the metal molds, the process is used formaking large cylinder liners.

For some types of castings, the permanent mold process is a very

satisfactory one, and its capabilities have been described by Frye[16].

Since the cooling or freezing rate of iron cast into permanent molds is

quite high, the thinner sections of the casting will have cementite. To

remove the cementite the castings must be annealed, and it is universal

practice to anneal all castings. The most economical composition of the

iron for permanent mold castings is hypereutectic. This type of iron

expands on solidification, and, because the molds are very rigid, the

pressure developed by separation of the graphite during freezing of the

eutectic ensures a pressure tight casting. Since the graphite occurs

predominantly as Type D with very small flakes, permanent mold castings

are capable of taking a very fine finish. For this reason, it finds extensive

use in making valve plates for refrigeration compressors. The process is


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also ideally suited for such components as automotive brake cylinders and

hydraulic valve bodies. Although the predominantly Type D graphite

structure in permanent mold castings with a matrix of ferrite have much

higher strength than sand castings of comparable graphite content, the

structure is not considered ideal for applications with borderlinelubrication. The castings perform very well, however, when operating in

an oil bath.

Unless some special properties are desired and are obtained only with a

particular casting process, the one generally selected yields castings at the

lowest cost for the finished part.

Casting Design

There are a number of requirements which must be met before the designof a casting can be considered completely satisfactory. In some respects,

the design of a casting for gray iron is somewhat simpler than for any

other foundry metal in that solidification shrinkage is at a minimum and

for the softer grades is absent altogether. With few exceptions, little

concern needs to be given to the problem of feeding metal to heavier

sections. Patternmakers shrinkage is also low. The low shrinkage

characteristics contribute to freedom from hot tears encountered with

some of the other foundry metals. These factors afford the engineer

greater freedom of design.

Although a casting must be designed to withstand the loads imposed on it,

there are many instances where deflection under load is of primary

consideration to ensure proper alignment of components under load.

There are a number of handbooks which contain information helpful to

the design engineer[17-19]. The appearance of many castings suggests,

however, that the designer has been unduly influenced by the

characteristics of flat plates and other wrought shapes. It appears he is

unable or incapable of utilizing tapered sections, long radius fillets, and

variable thickness sections which are easily obtained in a casting. Instead

of a clean design, the casting is a conglomeration of plates, ribs, bosses,

and small radii. Because of the low level of elongation values for gray iron,

the only satisfactory method of determining stress levels in a casting

under load is through the use of SR-4 strain gages. Without proper stress

analysis, the first tendency is to "beef" up the section in which failure has


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occurred. Grotto[20] has shown that such an approach does not result in

the best design and often makes the condition worse.

The molding method must be decided upon before a final casting design

can be achieved. If the casting has internal cores, they must have some

means of support and these must be provided for in the design. In using

molding methods capable of greater control over dimensional accuracy, it

is often possible to reduce section thickness. As the section thickness is

decreased and the cooling rate accordingly accelerated, the strength per

unit of cross-sectional area increases. In general, a 50 percent reduction in

casting section results in somewhat less than a 40 percent reduction in

section strength. If the castings have complex core assemblies, such as are

found in diesel engine cylinder heads, provision must be made to get the

sand out of the cored passages and to allow inspection.

With an increasing trend toward higher machining speeds and metal

removal rates, thought must be given to the manner in which the casting

is held during the machining operation so that high chucking pressures do

not distort the part. Furthermore, the design should include readily

maintained locating points. An ingate should not be placed at a locating

point because, in grinding the connection in the finishing operation, some

variation in the amount of metal removed can be expected.

The mechanical properties of gray iron are dependent on cooling rate.Some care needs to be exercised in avoiding extreme ranges in section

thickness, or hard edges will be found at the extremities of the thin

sections and too low hardness in the heavy sections. It may be desirable to

increase the thickness of the lightest sections to avoid this condition.

Sometimes a bead along the outer edges of a flange may be helpful. lf the

casting is to be used in an application where vibration is a problem,

consideration needs to be given to the damping capacity of the casting.

Although gray iron has quite a high damping capacity, casting design to

avoid resonance should also be considered. Small appendages on castings

should be avoided or strengthened to avoid undue breakage in the

handling, finishing, and shot blasting operations. Although the subject of

casting design has received much attention during the past 10 years, a

great deal more needs to be done in the field of gray iron casting design.

Mechanical Properties of Gray Iron


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Properties of principal interest to the designer and user of castings are:

resistance to wear; hardness; strength; and, in many cases, modulus of

elasticity. Some of the relationships between these properties are quite

different for gray iron as compared with steel. The variable relation

between hardness and tensile strength in gray iron appears to confusethe engineer when most of his experience may have been with other

metals.

The excellent performance of gray iron in applications involving sliding

surfaces, such as machine tool ways, cylinder bores, and piston rings, is

well known. The performance in internal combustion engines and machine

tools is remarkable when one considers the ease of machining gray iron.

Gray iron is also known for its resistance to galling and seizing. Many

explanations have been given for this behavior, such as the lubricating

effect of the graphite flakes and retention of oil in the graphite areas. Thisis very likely true, but it is also possible that the graphite flakes allow some

minor accommodation of the pearlite matrix at areas of contact between

mating surfaces. It is seldom possible to obtain perfect fits, and, ordinarily,

high spots in mating metal surfaces may result in high unit pressures

causing seizing.

The Brinell hardness test is the one most frequently used for gray iron,

and, whenever possible, the 10-mm ball and 3000-kg load is preferred. If

the section thickness or area to be tested will not withstand the 3000-kgload, a 1500-kg load is frequently used. The hardness values obtained with

the lower load may differ appreciably from those obtained with the higher

load, and this possibility is pointed out in ASTM Test for Brinell Hardness

of Metallic Materials (E 10-66). For gray iron, the difference in hardness

values may be as great as 35 BHN, and, if a difference exists, it is always

lower for the lower load. Since in most cases the Brinell hardness test can

be considered a nondestructive test, Brinell hardness is used as an

indication of machinability, resistance to wear, and tensile strength. For

light sections, such as piston rings and other light castings having a smallgraphite size, the Rockwell hardness test is often satisfactory.

The Brinell hardness test is actually a specialized compression test and

measures the combined effect of matrix hardness, graphite configuration,

and volume of graphite. The Brinell hardness of gray iron with an entirely

pearlitic matrix may vary from as low as 148 to over 277 depending on the


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fineness of the pearlite and to a greater extent on the volume of graphite

present. Over this range of hardness, the actual hardness of the pearlite

may vary from about 241 to over 400 Knoop hardness as determined by

microhardness measurements.

Virtually, all specifications and standards for gray iron classify it by tensile

strength. The tensile strength of gray iron for a given cooling rate or

section size is very much dependent on the amount of graphite in the iron.

The carbon equivalent value for the iron will give a close approximation to

the amount of graphite present. The tensile strength is also very much

influenced by cooling rate, particularly through the eutectic solidification

interval, and is generally related to section size. In recognition of the

effect of section size on strength, ASTM Specification A 48 not only

classifies the iron by strength but also requires selection of the size of the

test bar in which the strength is to be obtained.

The majority of purchasers of gray iron castings rely on the Brinell

hardness test to determine if the casting meets specifications. The

variable relation between Brinell hardness and tensile strength for gray

iron is confusing to materials engineers, who are accustomed to the fixed

relation of Brinell hardness to tensile strength for wrought steel of about

492. For gray iron, the ratio will vary from as low as 140 for low-strength

irons to over 250 for gray irons having a tensile strength of over 60,000

psi. In recognition of the wide use of the Brinell hardness test forestimating the strength of the iron in the casting, Division 9 of the Iron and

Steel Technical Committee (ISTC) of the Society of Automotive Engineers is

in the process of revising SAE J431 a. Gray iron for automotive castings, in

which gray iron castings meet various strength levels, will be specified by a

minimum Brinell hardness.

There have been numerous papers dealing with the subject of the

correlation of Brinell hardness with tensile strength. Probably the most

extensive report was that prepared by MacKenzie[21] from data obtained

for the "Impact Report" for ASTM Committee A-3 (now A-4). The report by

him was widely published and showed considerable scatter. He felt that

some of the scatter could have been a result of the manner in which the

Brinell hardness measurements were made. When he selected data taken

from the shoulders of the tension test specimens, the correlation was very

much better.


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Many users and particularly engineers are critical of cast metal properties

obtained from test bars. The situation for gray iron is very much different

from that of the other cast metals. Whereas the other ferrous metals,

particularly steel and nodular iron, use test bars with an unusually high

ratio of riser to test bar quite unlike the relation used for a commercialcasting, test bars for gray iron are quite simple castings and gated very

much like commercial castings. This can be done since there is either very

little shrinkage or none in gray iron. Careful investigations have shown

that if the test bar has the same thermal history as the section in the

casting under consideration, hardness and tensile strength will be similar.

In a casting with varying section sizes, the properties in the casting will

only be the same where solidification and cooling rates are the same. It is

possible to predict the tensile strength in other parts of the casting if the

Brinell hardness is determined.

Since it was easier to evaluate the effect of section size and composition

of gray iron in cylindrical castings, this was done in three foundries from

150 ladles of regular production iron cast into bars from s/s to 6 in.

diameter. The molds were similar to those used for commercial castings.

The sizes of the bars were as follows:

Although the castings were made under normal production conditions, all

phases of the operations were observed more thoroughly than usual. All

testing was done in a research laboratory with properly calibratedequipment and with qualified operators. Dimensions of tension test

specimens conformed to ASTM Specification A 48. The tension test

specimens were machined from the center of the casting for all sizes and,

in addition, were machined from a position about 3/4 in. from the outside

for the 4 and 6-in castings.

Diameter, in. Length, in.5/8 8

7/8 15

1.2 21

2 & 3 10

4 6


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6 18

The tension test specimens had a reduced section diameter of 0.75 in.,

with the exception of the test specimen from the 7/8-in. casting which had

a diameter of 0.5 in. in the reduced section. Brinell hardness tests were

made with a 3000-kg load and 10-mm ball. The hardness measurements

were taken on a cross section of the casting corresponding with the

position from which the tension test specimen was taken.

Foundry F normally produces light to medium weight castings, such as

small compressor heads and bodies, air conditioning component castings,

valve and pressure regular bodies, manifolds, and other types of auto-

motive castings. Since section sizes seldom exceed 1 in., the testing is

confined to 7/8 and 1.2-in. bars. Some of the irons are alloyed with one or

more of the elements (copper, chromium, and molybdenum) in small to

moderate amounts. The data are shown in Fig. 1. The proposed minimum


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Brinell hardness being considered by the SAE Division 9 ISTC Committee is

also shown. With only two exceptions, the values are all above the line.

Foundry S is a jobbing foundry specializing in truck and marine diesel and

gasoline engine blocks and heads together with related items, such as

flywheels, manifolds, transmission cases, and clutch housings. Since

heavier sections than in Foundry F are being made, test bar castings up to

4 in. diameter are cast. The data obtained are shown in Fig. 2. The alloyed

irons are at a higher strength level. Note that test specimens cut from

near the outer surface of the 4-in.-diameter bars show a higher strength

for a given hardness than test specimens machined from the center of the

4-in. bars. This is generally a result of a larger graphite flake size in the

center of the bar. Also, note that all of the values are above the SAE line.

Foundry W produces medium to heavy castings for large gas line

compressors, engines, pumps, flywheels, and related items with sections

up to 4 in. The complete range of test bar sizes are cast at this foundry.

The data obtained are shown in Fig. 3. The scatter in values becomes


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somewhat larger at the higher strength levels. Note that the inoculated

irons are higher in strength than the base iron bars, which accounts for

the increase in range of tensile strengths for a given hardness. Some of the

tensile strength values falling below the SAE line are from the center of

the 6-in.-diameter sections and have a rather large graphite flake size.

Some casting users specify a minimum tensile strength at some

designated location in the casting. This is particularly true for such castings

as hydraulic pump bodies, high-duty diesel engine cylinders, pistons and

heads, and other highly stressed castings. Data obtained from production

castings are shown in Fig. 4. Also shown on this figure for comparison are

data obtained from tension test specimens cut from annealed permanent

mold castings. These castings will be hypereutectic in composition withType D graphite and a ferritic matrix. These irons have a higher strength

for a given hardness than irons cast in sand.


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Curves showing the minimum Brinell hardness for a given tensile strength

for the irons reported, together with MacKenzie's and Caine's data, are

shown in Fig. 5. The curves are in fair agreement except for the minimum

values reported by MacKenzie. It is possible that the irons for Foundries F,

S, and W had a higher concentration of residual alloying elements, whichwould tend to keep the matrix pearlitic with accordingly higher strength.


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It is sometimes necessary to machine tension test specimens with a

reduced section of 0.357 in. diameter from a casting, since the casting

shape does not allow making a larger size specimen. Some casting usershave raised the question of the reliability of the smaller specimens. Over a

period of years, several size specimens have been taken from the same

casting, and it has been found that, if machining is carefully done, the

results are reliable. The data in Fig. 6 are representative and have been

obtained from a small automotive clutch plate. The hardness and tensile

strength data for this casting show that a 5/8-in. plate has a similar cooling

rate to a 1.2-in. test bar.


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In trying to predict tensile strength in a casting from Brinell hardness,

there are more factors involved than mere section thickness. For sections

from which heat flow during cooling is unimpeded, a very good hardness-

tensile strength relationship can be established. For more complex

castings, such as diesel engine cylinder heads having many cored

passages, the cooling pattern may be complicated. The section within the

head may freeze fairly rapidly, but, after the eutectic temperature intervalis passed, there is a heat build up, and the section may cool more like a

simple section two to four times as thick. For such cases, a correlation

needs to be worked out for each type of casting.

Steel shows a rather minor influence of tensile strength and hardness on

the modulus of elasticity, since it is mostly in the range of 29,000,000 to

30,000,000 psi. For gray iron, the modulus of elasticity not only varies with

tensile strength but also with the stress level. As a result of these factors,

the modulus of elasticity will vary from around 12,000,000 psi for a very

soft iron to over 20,000,000 psi for a high strength iron. The stress-strain

curve for gray iron in tension is almost a curved line from the origin. This

has been reported by many investigators, and Morrogh[14], in reporting

some work by Gilbert, suggests that the curve is a result of some volume

changes in the spaces occupied by the graphite. They have also shown

that some microcracking takes place between flakes. Some investigators


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have used resonant frequency measurements and also sound velocity

measurements which are dependent on modulus of elasticity to predict

tensile strength.

In machine tool and other applications where maximum stiffness of a

structure is desired, a high modulus of elasticity is desirable. There are

other applications, notably those involving thermal fatigue for which a low

modulus of elasticity is wanted to minimize the increase in stress levels

associated with expansion resulting from temperature increases under

operating conditions. High-duty brake drums are an example of this type

of situation. It has been found that a rather high-carbon iron (3.60 to 3.92

percent) will give better service than a lower carbon iron. The higher-

carbon irons nearly always have a lower modulus of elasticity.

Unfortunately. the tensile strength tends to be low with such high-carbon

irons. and it becomes necessary to add an alloy to strengthen the matrix.

Materials engineers often look on percent elongation as obtained from

tension test specimens as a measure of the ductility of the material. With

this concept. gray iron would not be considered ductile.

Nevertheless. gray iron in the form of commercial castings will

satisfactorily withstand a considerable amount of moderate shock loading.

With careful control of melting practice and selection of raw materials,

Collaud and Thieme[10] have reported irons with 2.4 percent elongationat fracture under load and by ferritizing such an iron have obtained 5.4

percent elongation. Gray irons of the same tensile strength may show

differences of 50 percent in regard to breaking energy absorbed in shock

loading. Although gray iron is said not to be notch sensitive, this is most

likely a result of being fairly well saturated with notches in the form of

graphite flakes so that the presence of another notch does not materially

affect the behavior on impact.

Heat Treatment of Gray Iron

Although the majority of gray iron castings are used in the as-cast

condition, gray iron is heat treated for a variety of reasons, such as to

relieve res

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Gray Cast Iron