KAYABA TECHNICAL REVIEW No. 64 APR. 2022

EditorialMicrostructure and Mechanical Propertiesof Cast Iron
HIRATSUKA Sadato*

1. Introduction
Cast iron is an important structural material of castings.
Japan produced about 3 million tons of cast iron in 2020.
Of the annual production, 63.7% was used for transportation machinery (including automobiles, railroads andships), 23.7% for general or electric machinery (including
industrial machinery and equipment, construction
machinery, metal working tools & machine tools, andelectric machinery), 7.8% for cast iron pipes, and theremaining 4.7% for other applications (including couplings, kitchen appliances and craftwork) 1).
Fig. 1 shows an example of hydraulic equipment for
regular/compact excavators manufactured by Kayaba
Industry Co., Ltd. 2) The compact hydraulic excavator has
control valves whose body is made of cast iron.
Both cast iron and steel are classified into iron-carbon(Fe-C) alloys. They are differentiated from each other by
carbon content. Steel has a carbon content of not morethan approximately 2 mass% (hereinafter expressed in"%" only) while cast iron has a higher carbon content thanthe level. Practical cast iron is usually considered asFe-C-Si alloys because of its carbon content as high as 3%
to 4% and its silicon (Si) content of around 1% to 3%.
With a higher percentage of carbon than 2%, carbide
forms in iron (Fe) in the cast iron. In this sense, cast ironmay be considered as a composite material consisting ofsteel and graphite in various forms.
Boom Cylinder
Arm Cylinder
Piston Pump
Double-acting Cylinder
BucketCylinder
Variable Displacement
Travering Motor Piston Pump

Control Valve

Swing Motor

Tractional Motor (with Reducer)

Sectional Valves for
Compact Excavators

Monoblock Valves for
Excavators
Swing Motor (with Reducer)

Fig. 1

Example of hydraulic equipment for regular/
compact excavators*Professor, Research Center for Casting Technology, Iwate University

The lower carbon content of cast iron compared to steel
results in a substantially low solidifying (or melting) temperature and good fluidity. Cast iron also features good
castability because of its low solidification shrinkage orless shrinkage cavities attributable to volume expansion
taking place when the graphite is crystallized duringsolidification. In addition, the presence of graphite and thehigh silicone content give cast iron excellent industrial
properties including vibration absorption, machineability,wear resistance, thermal conductivity, corrosion resistance and oxidization resistance.
This paper describes the microstructure and mechanicalproperties of cast iron that may affect its characteristics.
2. Microstructure of Cast Iron
The microstructure of cast iron greatly varies by thechemical composition of the molten iron, the solidification condition, and heat treatment given. What microstructure the solidified cast iron will have is a critical issue
because cast iron is used in the as-cast condition in mostcases; however, this does not matter a lot for steel because
it is subjected to deformation processing, such as rolling,and heat treatment after solidification. The microstructureof cast iron can be roughly divided into graphite andmatrix microstructures. The combination of these two
microstructures greatly affects the mechanical, physicaland chemical properties of the cast iron.
2.1 Graphite Microstructure
Fig. 2 shows how graphite can appear in iron castings.
This classification was proposed by the technical committee in the World Foundry Congress held in 1962 in theU.S. 3)
Form I is flake graphite. Cast iron with graphite flakes isclassified by the Japanese Industrial Standard (JIS) G5501
into gray cast iron. Form II is spheroidal graphite withpointed ends that is likely to appear when a spheroidizing
agent such as magnesium (Mg) is added in an excessive
amount during production of nodular graphite cast iron.
Form III is quasi-flake graphite that may appear with alack of graphite spheroidizing agent. This type of graphite
forms in CV graphite cast iron. Form IV is massive graphite that may appear in malleable cast iron or graphite steel.
Form V is quasi-spheroidal graphite. Form VI is com-―3―

Microstructure and Mechanical Properties of Cast IronFig. 2Classification of graphite forms

pletely spheroidal graphite that often appears in nodulargraphite cast iron. Among these, the forms II to VI are
uniform graphite in a random distribution in most cases.
However, form I, namely, flake graphite, changes in formand distribution depending on the chemical compositionof the cast iron, its melting history and/or cooling rateduring solidification. American Society for Testing Materials (ASTM) A247 classifies this category of graphite
into Types A to E by distribution as shown in Fig. 3. TypeA shows random flake graphite in a uniform distribution,
which is the most optimally distributed flake graphite.
Type B is called rosette flake graphite. Type C shows
hypereutectic cast iron with coarse pro-eutectic graphite.
Type D is called eutectic graphite that is likely to appear
in titan (Ti)-added or undercooled molten iron. Type E iscalled interdendritic flake graphite.

having a body-centered cubic crystal structure with adensity of 7.9, a tensile strength of 200 MPa to 400 MPa,
and a Brinell hardness of 90 HB to 150 HB. Ferrite in ironcastings is called silico-ferrite because silicon (Si) incorporates into the ferrite to form a solid solution. The ferrite
matrix with more silicon offers higher tensile strength andhigher hardness.
(ii) Cementite
Cementite is a rhombic composition consisting of three
Fe atoms and a C atom with a density of 7.7 and a Brinell
hardness of 550 HB. This is the hardest and brittlest
among all the cast iron microstructures.
(iii) Pearlite
Austenite breaks down (transforms) into ferrite andcementite during metastable eutectoid transformation.
These two phases are generated in the form of a stack ofsheets alternately overlayed with each other. They can be
identified as a layered or banded structure under an optical
microscope. It is a very tough microstructure with adensity of 7.8, a tensile strength of 800 MPa to 900 MPa,
and a Brinell hardness of 200 HB to 240 HB.
Nodular graphite cast iron normally has a so-called
bull's-eye microstructure in which ferrite and pearlite
coexist as shown in Fig. 4.

FerritePearlite

Fig. 4

Fig. 3

Types of flake graphite by distribution[ASTM A247]

2.2 Matrix Microstructure
The matrix microstructure of cast iron varies by thecooling rate during solidification, the alloy element
content, and/or heat treatment given. This section introduces three typical matrices:(i) Ferrite
The ferrite matrix, which is also known as α-Fe or αsolid solution, is soft iron with a trace quantity of carbon

GraphiteBull's eye microstructure ofnodular graphite cast iron

3. Mechanical Properties of Cast Iron
Mechanical properties of material including tensile
strength, hardness, elongation and fatigue strength arecollectively called generalized strength. Any of theseproperties is a measure of resistance to deformation orfracture.
The strength of a metallic material is decided by its
microstructure. In the case of cast iron, the strength isgoverned by the graphite, matrix and other microstructures.
3.1 Strength of Flake Graphite Cast Iron
As compared to the matrix, the graphite microstructure―4―

KAYABA TECHNICAL REVIEW No. 64 APR. 2022

(a) Ductile matrix(b) Brittle matrix

Fig. 5Plastic deformation andductile cracks in matrix
connections

Brittle cracks at theends of graphite
flakesStress α, MPa

can easily fracture with a substantially lower force becauseof its tensile strength of around as low as 20 MPa. Therefore, the strength of cast iron is decided by the strength,
form, and continuity of the matrix except graphite particles.
In other words, cast iron with fine graphite (Type A orE) may offer a high strength. Those with long-type graphite (Type A, B or C) usually have a low strength becauseof their low continuity of the matrix.
Fig. 5 illustrates how two types of flake graphite castiron can fracture when they are applied with a tensile
force: one has a ductile matrix (a) and the other a brittlematrix (b). 4) The former (a) with a ductile matrix will havea number of ductile cracks and partial ruptures. These will
be connected with each other along the graphite flakes,
thereby accelerating rupture.
The latter (b) with a brittle matrix in turn will accelerate
rupture when brittle cracks occurring at the ends of thegraphite flakes propagate.

Strain ε, %Fig. 6

Stress-strain curves of flake graphite cast iron(a) Ductile matrix

Voids in the matrix due
to plastic deformation

Voids grow and merge tocause ductile fracture(b) Brittle matrix

Cleavage cracks aroundgraphite particles

Cracks propagate andmerge to cause a brittle
fracture

Accumulated ruptures inmatrix connections leadto final rupture

Cracks propagate to beconnected with eachother, resulting in a brittle
fracture

Fig. 7

How flake graphite cast iron fails withtensile fracture

Fig. 6 shows stress-strain curves of flake graphite castiron. 5) The continuously warped curves include noobvious yield behavior. Determining the 0.2% yield point(or yield strength) will not make sense to flake graphite
cast iron. Rather, it is necessary to define the yield strength
against a small distortion as low as 0.05% or even smaller.
3.2 Strength of Nodular Graphite Cast Iron
Nodular graphite cast iron has a matrix with substantially higher continuity than that of the flake graphite type.
The extent of continuity hardly varies by minor changes
in form (the degree of spheroidization) or particle size ofgraphite as long as the degree of spheroidization is higher
than a certain level (generally 0.7).
Fig. 7 illustrates how two types of nodular graphite castiron can fracture when they are applied with a tensile
force: one has a ductile matrix (a) and the other a brittlematrix (b). 6)

How nodular graphite cast iron fails withtensile fracture

The former (a) with a ductile matrix involves sufficient
plastic deformation. Many cracks occurring around thegraphite particles merge into voids as deformation progresses. Voids also occur in the grain boundaries andinclusions within the matrix. These voids are connectedwith each other and merge into bigger voids, resulting in arupture.
In this rupture development process, cracks tend todevelop in a zigzag manner bridging the closest graphite
particles or the weakest parts of the largest graphite particles.
For the latter (b) with a brittle matrix in turn, cleavage
cracks occurring around the graphite particles will growto cause rapid crack propagation, resulting in a rupture.
Fig. 8 shows the relationship between the tensile
strength and the carbon equivalent (CE) value of nodular,
compacted and flake graphite cast iron. 7) According to thefigure, the strength of flake graphite cast iron greatly
varies by the CE value while that of nodular graphite cast―5―Microstructure and Mechanical Properties of Cast Iron

Tensile strength

Fig. 10 R elationship among pearlite area, tensile strengthand elongation of nodular graphite cast ironCE, mass%

Effect of CE value on the tensile strengthof a variety of cast ironFig. 9 shows stress-strain curves of four types of nodular
graphite cast iron with different matrix microstructures. 8)
Specifically, these curves indicate the stress-strain relationship of austempered ductile irons (ADI) with ferrite
matrix (curve 1), with ferrite and pearlite matrix (bull'seye microstructure) (curve 2), with pearlite matrix (curve3), and with bainite matrix (curve 4). The curve 1 forStress α, MPa

3. FCD700, Pearlite
2. FCD500,Ferrite + Pearlite

1. FCD350, Ferrite

Strain ε, %Fig. 9Stress-strain curves of nodular graphite cast ironElongation

Pearlite area, %Flake graphite

4. ADI (FCAD900)

Nakamura

NakamuraCompacted graphite

Fig. 8Combined type

Elongation, %Tensile strength, MPa

Nodular graphite(FCD 500 equivalent)

ferrite matrix shows high elongation. As pearlite occupies
a larger area of the matrix in order of the curves 2, 3, thestress-strain curve tends to show higher stress.
Fig. 10 shows the correlation among pearlite area,tensile strength and elongation of nodular graphite castiron. 9) As the pearlite area increases, the tensile strength
goes up. By contrast, as the pearlite area slightly increases,
elongation rapidly decreases and then tends to decrease
gradually.

Tensile strength, MPa

iron seldom depends on the CE value. The figure also
implies that compacted graphite cast iron is almost
midway between the two. This is because the strength offlake graphite cast iron heavily depends on the matrix
continuity (i.e., to what extent the matrix among graphite
flakes is sound with no break), in other words, the formand distribution of graphite flakes. For nodular graphite
cast iron consisting of distributed independent spheroidal
graphite particles (approximately 10 vol%) and a matrix(approximately 90 vol%) in turn, the matrix shows considerably higher continuity than for flake graphite castiron. Naturally, the strength of nodular graphite cast ironheavily depends on the matrix microstructure and is
seldom affected by the CE value.

4. In Closing
Cast iron can be considered as a composite material
consisting of graphite and steel. Its strength is governedby the graphite, matrix and other microstructures.
The strength of flake graphite cast iron is primarily
affected by the form of graphite flakes, namely, the matrix
continuity between graphite flakes, and secondarily by thematrix microstructure.
In nodular graphite cast iron in turn, the matrix continuity is substantially higher than for flake graphite cast iron.
This means that the strength of nodular graphite cast ironheavily depends on the matrix microstructure.
Iron castings can deliver excellent mechanical, physicaland chemical properties when their graphite and matrix
microstructures are properly controlled. High-strength,
high-function cast iron whose microstructure has been
suitably controlled can be expected to be applied to avariety of industrial applications. I hope this paper will
help manufacturers achieve this.

References
1) SOKEIZAI Annual Industrial Report: SOKEIZAI Vol.62 (2021)
8.
2) KYB Hydraulics Products Guide (Construction equipment,industrial vehicles, agricultural equipment, and other industrialequipment).
3) A.B. Everest: Trans. Amer. Foundrym. Soc., 70 (1962) 210.

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KAYABA TECHNICAL REVIEW No. 64 APR. 2022

4) NOGUCHI Toru: Journal of Japan Foundry Engineering Society,7) Japan Foundry Engineering Society (editor): Foundry Engineering

Volume 77 (2005) 794.

Handbook (Maruzen Publishing Co., Ltd.) (2002) 254.

5) NOGUCHI Toru: Journal of Japan Foundry Engineering Society,8) HARADA Shoji, NOGUCHI Toru: Evaluation of strength ofVolume 76 (2004) 333.

nodular graphite cast iron, AGNE Gijutsu Center Inc. (1999) 50.

6) HARADA Shoji, NOGUCHI Toru: Evaluation of strength of
nodular graphite cast iron, AGNE Gijutsu Center Inc. (1999) 52.

9) NAKAMURA Koukichi, SUMIMOTO Haruyoshi: Castings 39(1967) 480.

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