Tutorial ihm

Introduction to Gray Cast Iron
Brake Rotor Metallurgy

Mark Ihm
TRW Automotive

TRW

Tutorial Outline

• Introduction
• Microstructures of Cast Irons
• Properties of Gray Cast Irons
• Influence of Casting Processes
• Future of Gray Cast Iron Rotors

Introduction to Gray Cast Iron Brake
Rotor Metallurgy

The properties of cast iron components are
controlled by the microstructure of the
material, which consequentially are
determined by the chemistry and processing
of the cast iron.

Equilibrium Iron-Carbon Binary
Phase Diagram

Liquid + Graphite

Austenite (γ) + Graphite

Ferrite (α) + Graphite

Equilibrium Solid Phases in the
Binary Iron-Carbon System

• Ferrite (α-Fe)
• Austenite (γ-Fe)
• Delta Iron (δ-Fe)
• Graphite (C)

Ferrite Iron Phase

• Body-center cubic crystal structure
• Stable up to 912°C in Fe-C system
• Density: 7.86 grams/cm 3 at 20°C
• Soft and very ductile phase

Austenite Iron Phase

• Face-center cubic crystal structure
• Stable from 740°C to 1493°C in Fe-C
system
• Strong, hard and tough phase

Delta Iron Phase

• Body-center cubic crystal structure
• Stable from 1394 °C to 1538°C
• Since temperature range is limited to very
high temperatures, very little is published
about mechanical and physical properties.

Graphite Carbon Phase
• Layered hexagonal structure with covalent
bonding of atoms in each layer
• Layers easily slide against each other and make
graphite a solid lubricant
• Soft and low strength

The “Lever Rule” in Equilibrium Binary
Phase Diagram

The LEVER RULE is used to determine the
compositions of phases and the relative
proportions of phases to each other in the two
phase regions of binary phase diagrams. The
LEVER RULE applies only to regions where
two phases exist together and only under
isothermal conditions.

The “Lever Rule”
Temperature

α Temp = T
β
α+β

A X" X X!
B
%B (weight %)
For an overall composition “X” at temperature “T”, the
composition of the "-phase is X" and the composition of
the !-phase is X!.

Temperature

α Temp = T
β
α+β

A X" X X!
B
%B (weight %)
For an overall composition “X” at temperature “T” the weight
fraction of the "-phase (F") is represented by the equation:
F" = (X! - X)/(X! - X")

Temperature

α Temp = T
β
α+β

A X" X X!
B
%B (weight %)
For an overall composition “X” at temperature “T” the weight
fraction of the !-phase (F!) is represented by the equation:
F! = (X - X")/(X! - X")

Practice Problem #1

Liquid + Graphite

~
Graphite
~
(Solubility of
iron in solid
graphite is
negligible)
~
100

For an overall composition of iron with 4 % carbon held at
741°C, what are the compositions of the two phases and weight
fractions of each phase?

Practice Problem #1

Liquid + Graphite

~
~


~
100
Xγ X
Xgraphite

The austenite phase contains 0.85% carbon at 741°C, and the
graphite phase contains 100% carbon.

Practice Problem #1

Liquid + Graphite

~
~


~
100
Xγ X
Xgraphite
The weight fraction of the austenite phase is calculated as:

Fγ = (Xgraphite – X)/(Xgraphite -Xγ) = (100 – 4)/(100 – 0.85) = 0.968 = 96.8%

Practice Problem #1

Liquid + Graphite

~
~


~
100
Xγ X
Xgraphite
The weight fraction of the graphite phase is calculated as:

Fgraphite = (X – Xγ)/(Xgraphite -Xγ) = (4 – 0.85)/(100 – 0.85) = 0.032 = 3.2%

Phase Transformations in Equilibrium
Binary Phase Systems
Single Phase Transformations (A B)
– Typically involve pure materials
– Compositions of phases are identical
– Do not involve diffusion of atoms
– Transformation occurs at a single temperature

Single Phase Transformations for Pure Iron
Liquid Delta Iron Austenite Ferrite
(non-crystalline) (bcc crystals) (fcc crystals) (bcc crystals)

2-Phase Transformations (A A + B)
– Occur over a range of temperatures and compositions
– Second phase precipitates and grows in first phase
– First phase is still stable
Liquid
Liquid + Graphite

~
C
γ Fe
~
C
Ferrite (α) + Graphite C Fe
Fe
~ 100

Second Phase Precipitate Microstructure
– The form of the second phase is dependent upon cooling rates
and diffusion rates of solute atoms in solution.
– The form is also dependent upon availability of nucleation
sites.
– The form may also be dependent upon the crystal structure of
two phases.
Solid dendrites
growing from a
rapidly cooled liquid

Graphite flakes
precipitated from
liquid iron

Phase Transformations in the Iron-Carbon
Equilibrium Binary Phase Systems
3-Phase Transformations Iron-Carbon Alloys
Peritectic Transformation
(Liquid + Solid A Solid B)
Liquid +
Graphite Eutectic Transformation
(Liquid Solid A + Solid B)
~

Austenite (γ) + Eutectoid Transformation
Graphite (Solid A Solid B + Solid C)
~

Ferrite (α) +
Graphite

~
100

• 3-Phase Transformations (A B + C)
– 3-phase transformations occur at a single temperature
and composition.
– Eutectic and eutectoid transformations involve
simultaneous nucleation and growth of two phases
together.
– Typical microstructure is a lamellar
structure with alternating layers of
the two phases.

Typical lamellar microstructure
from a eutectic transformation

Practice Problem #2

Liquid + Graphite

γ+L
~
Graphite
~
(Solubility of
iron in solid
graphite is
negligible)
~
100

For an overall composition of iron with 4 % carbon slowly
cooled from 1200°C to 1152°C, what are the weight fractions of
primary austenite and the eutectic microstructures?

Practice Problem #2

Liquid + Graphite

γ+L
~
~


~
100
Xγ X
Xeutectic
The weight fraction of the primary austenite microstructure is
calculated as:
Fγ = (Xeutectic – X)/(Xeutectic -Xγ) = (4.2 – 4)/(4.2 – 2.1) = 0.095 = 9.5%

Practice Problem #2

Liquid + Graphite

γ+L
~
~


~
100
Xγ X
Xeutectic
The weight fraction of the eutectic microstructure is calculated
as:
Feutectic = (X - Xγ)/(Xeutectic -Xγ) = (4 – 2.1)/(4.2 – 2.1) = 0.905 = 90.5%

Practice Problem #2

The microstructure consists of primary austenite and a
eutectic mixture of austenite and graphite:

Eutectic
Graphite
Primary
Flakes
Austenite
Regions

Eutectic
Austenite

Meta-Stable Iron-Carbon System

The formation of graphite in the equilibrium
iron-carbon system is dependent upon the
diffusion of carbon through the iron matrix to
form the graphite precipitates. If the cooling
rate is fast, then the carbon is not able to
segregate, and iron carbide (Fe3C) forms in
place of graphite. This meta-stable system is
commonly called the iron-iron carbide system.

Meta-Stable Iron-Iron Carbide
Binary Phase Diagram

Iron Carbide Phase
• Fe3C is the chemical composition, and it has
orthorhombic crystal structure.
• Iron carbide breaks down to iron and graphite with
sufficient time and temperature.
• For practical purposes it is considered stable
below 450°C.
• Very hard and brittle phase
• Commonly called “cementite”

Combination of Equilibrium and Meta-
Stable Phase Diagrams

Eutectoid Transformations in the Iron-
Carbon Binary Phase Systems
• The ferrite-graphite eutectoid transformation is extremely
uncommon due to the very slow cooling required.
• The ferrite-iron carbide transformation is the predominant
eutectoid transformation.
• The ferrite-iron carbide eutectic microstructure is commonly
called “pearlite”.
• Pearlite has a lamellar
microstructure.

Practice Problem #3

~

~ Graphite
(Solubility of
iron in solid
graphite is
~ negligible)
100

A cast iron with an overall composition of 4 % carbon is cooled slow enough through the
eutectic transformation temperature to produce austenite and graphite, but not cooled slow
enough through the eutectoid transformation temperature to produce ferrite and graphite.
At room temperature, what are the phases and microconstituents present and their
approximate weight fractions?

Practice Problem #3

L

~ <= L + γ
γ+C
~
Graphite
(Solubility of
iron in solid C + α + Fe3C
graphite is
negligible)
~
100

The phases present at room temperature will be graphite (C) , ferrite (α) and
cementite (Fe3C).

Practice Problem #3

Xγ X
From problem #1 we saw that the material consisted of 96.8% austenite and
3.2% graphite by weight at a temperature just above the eutectoid trans-
formation temperature. The existing graphite remains stable and the austenite
will transform to pearlite (the eutectic mixture of ferrite and cementite) through
this transformation.

Practice Problem #3

Xα X Xcementite
Using the LEVER RULE now with eutectoid austenite composition as X the weight
fractions ferrite and cementite that transform from the austenite are calculated as
follows:
Fα from γ = (Xcementite - X)/(Xcementite - Xα) = (6.7 – 0.8)/(6.7 – 0.02) = 0.883 = 88.3%
FFe C from γ = (X - Xα )/(Xcementite - Xα) = (0.8 – 0.02)/(6.7 – 0.02) = 0.905 = 11.7%
3

Practice Problem #3
The overall volume fractions of the ferrite and the cementite in the total
microstructure is calculated as follows:
Fferrite total = Fα from γ x Fγ = 0.883 x 0.968 = 0.855 = 85.5%
Fcementite total = FFe C from γ x Fγ = 0.117 x 0.968 = 0.113 = 11.3%
3

Phases Weight Percentages Microconstituents Weight
Percentages
Ferrite 85.5%
Pearlite 96.8%
Cementite 11.3%
Graphite Flakes 3.2%
Graphite 3.2%
100.0%
100.0%

Practice Problem #3
Microstructure consists of graphite flakes in a matrix of pearlite:

Eutectoid Eutectic
Pearlite Graphite Flakes
Matrix

Microstructure of Gray Cast Iron (200X and Nital Etch)

Other Microstructures in the
Iron-Carbon Binary System
• Martensite is an unstable phase that can form
when austenite is cooled below a critical
temperature too quickly for carbon to diffuse and
form iron carbide.
• Bainite is non-equilibrium microstructure of
acicular ferrite and fine carbides.
• Both martensite and bainite will revert to ferrite
and cementite particles with tempering at elevated
temperatures (typically greater than 150°C).

Martensite Phase
• Martensite has a body centered tetragonal crystal structure
• Carbon content must typically be greater than 0.1%
• Breaks down to iron and cementite (Fe3C) with sufficient
time and temperature (considered semi-stable below 150°C)
• Very hard and strong phase, but minimal ductility
• Martensite has a needle-like microstructure

Bainite Microstructure
• Bainite consists of acicular (needle-like) ferrite with very
small cementite particles dispersed throughout.
• The carbon content must typically be greater than 0.1%.
• Bainite transforms to iron and cementite with sufficient time
and temperature (considered semi-stable below 150°C).
• Bainite is a very hard and tough microstructure.

Lower Bainite Upper Bainite

Austenite Transformation Diagrams

• Austenite transformation diagrams are used to predict the
microstructures that will form from the austenite
depending upon time, temperature and cooling rate.
• Time-temperature transformation (TTT) diagrams measure
the extent of transformation with time at a constant
temperature.
• Continuous cooling transformation (CCT) diagrams
measure the extent of transformation as a function of time
for a continuously decreasing temperature.

Time-Temperature Transformation
Diagram for Cast Iron

Continuous Cooling Transformation
Diagram for Cast Iron

Tempering of Martensite and Bainite

Martensite and bainite will temper at temperatures above
150 °C to form ferrite and spheroidal iron carbides.

Martensite Tempered Martensite Heavily Tempered
Martensite

The Addition of Silicon to the Iron-
Carbon System
• Silicon is added to cast irons in the range of 1% to 4% in
order to increase the amount of under-cooling required for
the formation of cementite and promote the formation of
graphite during solidification.
• The range of silicon added is sufficient that the iron-carbon
binary phase diagram is insufficient to predict the phases
and microstructures that form.
• The iron-carbon-silicon ternary phase diagram and/or
sections of this diagram are needed to properly predict the
phases and microstructures that form.

Iron-Carbon-Silicon Ternary Phase Diagram

Temperature

Temperature
for y-axis

Si C

Pure Elements at Binary Phase Diagram
each Corner on the Three Faces
Fe

Effects of Silicon on the Eutectic and
Eutectoid Transformations
Iron-Carbon Iron-Carbon-Silicon(2%)
Phase Diagram Phase Diagram

Liquid Liquid
Austenite
+ Liquid Austenite Graphite
+ Liquid + Liquid

Austenite Austenite

Austenite + Austenite +
Carbide /Graphite Carbide /Graphite

Austenite Austenite
+ Ferrite + Ferrite
Ferrite +
Ferrite Ferrite Austenite +
Ferrite + Carbide /Graphite
Ferrite +
Carbide /Graphite Carbide /Graphite

Microstructural Effects of Silicon
Additions in Cast Irons
• Silicon strongly reduces the potential for eutectic carbides
during solidification and promotes the formation of primary
graphite.
• Silicon promotes the precipitation of secondary graphite on
the primary graphite during the eutectoid transformation,
which results in large areas of ferrite (commonly called “free
ferrite”) around the graphite particles.

Classifications of Cast Irons
• Classifications are determined by the eutectic
graphite/carbide forms present in the iron
microstructure.
• Classifications are controlled by alloying,
solidification rates and heat treatment.
• Classifications of cast irons
– White Irons
– Malleable Irons
– Gray Irons
Graphitic Cast Irons
– Ductile Irons
– Compacted Graphite Irons

White Cast Irons
• White cast irons form eutectic cementite during solidification.
• The white iron microstructure is due to fast solidification rates
and alloying that promotes eutectic carbide formation.
• White irons typically have low ductility, high hardness and
great wear resistance.
• White irons get their name
from the shininess of their
crystalline fractures in
comparison to the dull
gray fractures of graphite
irons.

Malleable Cast Irons

• Malleable cast irons are formed by annealing white irons to
transform the eutectic cementite to graphite.
• Malleable irons have good ductility and good strength.
• Matrix microstructure is dependent upon the cooling rate from
the graphitization annealing.
• Before the discovery of
nodular irons, malleable
irons were the only
ductile class of cast irons.

Gray Cast Irons
• Gray cast irons form graphite flakes during solidification.
• The gray iron microstructure is due to slow solidification rates
and silicon alloying that promotes graphite formation.
• Gray irons typically have low ductility and moderate strength,
but they have high thermal conductivity and excellent
vibration damping properties.
• Gray irons get their name
from their dull gray
fracture features.

Nodular Cast Irons
• Nodular cast irons form graphite spheres during solidification.
• The nodular iron microstructure is due to slow solidification
rates and magnesium or cerium alloying that promotes
spherical graphite formation.
• Nodular irons typically have high ductility and strength.
• Nodular irons were first
discovered in the 1940’s.
• Nodular irons are also
called “ductile irons” or
“spheroidal graphite
irons”.

Compacted Graphite Cast Irons
• Compacted graphite cast irons form graphite particles with a
shape between graphite flakes of gray cast iron and graphite
nodules of nodular cast iron.
• Compacted graphite cast irons have properties between those
of gray cast iron and nodular cast iron.
• Compacted graphite irons
require very tight control
of the nodularizing
alloying (magnesium or
cerium).

Graphite Flake Distributions per ISO 945

Graphite Morphologies in Gray Cast Iron
Castings
• Type A graphite flake structures are generally the
preferred structures.
• Type B graphite flake structures may result when
there is poor inoculation and nucleation.
• Type C graphite flake structures are typically found
in hypereutectic gray irons where the graphite flakes
are the first to precipitate from the melt.
• Types D and E graphite flake structures are
typically found where undercooling of melt is the
greatest (edges, parting lines, thin sections, etc…).

Graphite Morphology Variation in Gray
Cast Iron Castings
• Graphite morphology variation is affected by the
casting and/or mold design.
• Graphite morphology variation is increased with
poor inoculation practices.
• First areas to solidify may have types D and E
graphite morphologies if inoculation is insufficient.
• Types D and E graphite morphologies often have
free ferrite associated with them, which can affect
the machinability and other properties in the area
affected.

Properties of Gray Cast Irons
• Classifications and Mechanical Properties
• Elevated Temperature Properties
• Wear and Abrasion Resistance
• Heat Absorption Properties
• Thermal Conductivity
• Vibration Damping
• Corrosion Resistance
• Machinability

Gray Cast Iron Classifications and
Mechanical Properties
Casting Grade Theoretical Typical
Typical Minimum Brinnel
SAE J431
Class per Carbon Tensile Hardness
ASTM Content Strength Range
Current Previous A48M (%) (MPa) (BHN)
G7 G1800 20 3.50 - 3.70 124 163 – 223
G9 G2500 25 3.45 - 3.65 170 170 – 229
G10 G3000 30 3.35 - 3.60 198 187 – 241
G11 G3500 35 3.30 - 3.55 217 207 – 255
G12 G4000 40 3.25 - 3.50 272 217 – 259
G13 G4000 40 3.15 - 3.40 268 217 – 259

Mechanical Properties of Gray Cast Irons
Elastic modulus is lower than that of steels and
nodular iron, and it is non-linear. Elastic modulus
decreases with increasing graphite content.
Steels

Class 40 Gray Iron (comp)

Class 40 Gray Iron (tensile)

Class 20 Gray Iron (comp)

Class 20 Gray Iron (tensile)

Mechanical Properties of Gray Cast Irons

• Gray cast irons exhibit very little ductility. Typical
elongations in tensile testing are less than 0.5%.
• Impact strength and notch sensitivity are poor due to
the graphite flakes acting as stress risers.
• Fatigue strengths of gray cast irons are low due to
effects of the graphite flakes on crack initiation.

Elevated Temperature Mechanical
Gray cast irons maintain their mechanical properties
up to approximately 500°C. Above 500°C the
mechanical properties drop quickly.

Wear and Abrasion Resistance of Gray
Cast Irons
• The wear/abrasion resistance of gray cast irons are
dependent upon the microstructures.
• Increasing amounts of graphite and free ferrite reduce
wear/abrasion resistance.
• Increasing amounts of pearlite improves wear/abrasion
resistance.
• The higher grades tend to have greater wear/abrasion
resistance than lower grades.
• Gray cast irons have wear/abrasion resistance
comparable to non-heat treated medium carbon steels.

Heat Absorption Properties of
Gray Cast Irons
Heat Specific Melt
Material Density Capacity Heat Temperature
(g/cm3) (J/kg °K) (J/cm3 °K) (°C)
G1800 Gray Cast Iron 7.15 545 0.076 1150
G3000 Gray Cast Iron 7.2 545 0.076 1145
G4000 Gray Cast Iron 7.25 545 0.075 1145
1008 Plain Carbon Steel 7.86 481 0.061 1620
302 Austenitic Stainless Steel 7.93 500 0.063 1400
356 Cast Aluminum Alloy 2.69 963 0.358 675
Copper 8.94 494 0.055 1083
Brass 8.75 380 0.043 990
AZ63 Cast Magnesium 1.8 1005 0.558 455

Thermal Conductivity of Gray Cast Irons

G1800 Gray Cast Iron

G3500 Gray Cast Iron

Graphite

Vibration Damping Properties of Gray
Cast Irons
The composite nature of gray cast irons (steel plus
graphite flakes) along with crystal and bonding
structure of graphite makes gray cast irons one of the
best vibration damping metals.

Corrosion Resistance of Gray Cast Irons
The difference in electrode potentials between the ferrite/iron
carbide matrix and the graphite flakes is very large. This results
in mini-galvanic cells with graphite as the cathode and the
ferrite/iron carbide matrix as the sacrificial anode, which is the
primary cause of the very poor corrosion resistance of gray cast
iron. Rusts Formation
Fe+3 + 3(OH-) ⇒ Fe(OH)3
(precipitated onto surfaces)
Water

Anode Reaction
Cathode Reaction
Fe ⇒ Fe+3 +3 e-
3 e- + 3/2H2O + 3/4O2 ⇒ 3(OH-)

Manufacturability of Gray Cast Irons
• Excellent Castability
– Low melting temperatures of near eutectic compositions
minimize oxidation of the molten iron.
– Small solidification temperature range of near eutectic
compositions helps minimize shrinkage porosity.
– Low density of graphite reduces the volumetric shrinkage
during the solidification of the eutectic material.
• Excellent Machinability
– Graphite flakes make gray cast irons chip well, which
reduces stress on machining tools.
– Graphite flakes also act as solid lubricants.
– High thermal conductivity minimizes heat build-up in tool.

Properties Desired for Brake Rotors
• High strength and durability to sustain torque loads from braking
• Stable mechanical and frictional properties through range of
expected service temperatures
• High wear resistance through range of expected service
temperatures
• High heat absorption capability to absorb braking energy
• High thermal conductivity to transport frictional heat away from
braking surfaces
• High vibration damping capacity to minimize NVH issues
• Minimal thermal expansion to minimize performance variability
• High degree of corrosion resistance
• Excellent machinability
• Inexpensive material and processing costs

Sample Questions
1. Which grade of gray cast iron (G7 or G10)
would provide greater wear resistance for rotor
applications?
would provide greater resistance to thermal
cracking for rotor applications?
would provide reduced brake noise for rotor
applications?

Sample Questions
1. Which grade of gray cast iron (G7 or G10) would provide
greater wear resistance for rotor applications?
Answer: Grade G10 has a lower graphite content and greater
pearlite content, and therefore, it has greater wear resistance.

2. Which grade of gray cast iron (G7 or G10) would provide
greater resistance to thermal cracking for rotor applications?
Answer: Grade G7 has a higher graphite content and higher
thermal conductivity, and therefore, it has greater thermal
cracking resistance.

Sample Questions
1. Which grade of gray cast iron (G7 or G10) would
provide reduced brake noise for rotor
applications?
Answer: Neither grade will guarantee reduced
brake noise. While Grade 7 has a higher graphite
content and better damping properties than grade
G11, the effects of the change in resonant
frequencies and frictional behavior could actually
increase brake noise. Brake noise analyses must
take into account all of the effects that a material
change can cause.

Typical Gray Cast Iron Rotor
Casting Process
Core Molding Raw Sand Molding Raw Melt Raw Materials
Materials Materials

Melting Process
Core Making Process Sand Mixing Process

Core Setting Sand Molding Process Melt Treatments

Metal Pouring Process

Casting Cooling and
Shakeout

Casting Cleaning
Processes

Melting Processes Used for Production
of Gray Cast Iron Castings

• Cupola Melting
• Electric Melting
– Electric Induction Melting
– Electric Arc Melting
• Combustion Fired Reverberatory Melting

Raw Materials Used in the Melting of
Gray Cast Irons
• Iron Sources
– Scrap Iron
• Internal Returns
• Machined Chip Briquettes
• External Purchased Scrap
– Scrap Steel
– Pig Iron
• Coke
• Graphite and Silicon Carbide
• Ferro-silicon and Ferro-manganese

Cupola Melting of Gray Cast Iron
• It’s a continuous melting process.
• Exothermic reaction between
coke and air provides the thermal
energy for melting.
• Coke and its reaction products
provide the carbon in the melt.
• Uses greatest variety of charge
materials.
• Varying melt content is very
difficult.
• It is more difficult to meet
environmental standards.

Electric Melting of Gray Cast Iron
• It’s a batch melting process.
• Induced electric current in charge or
electric arc provides the thermal
energy for melting.
• Graphite and silicon carbide are
added to provide carbon in the melt.
• Restricted to specific charge
materials.
• Each batch can have a different
target composition.
• Less environmental concerns.
• More sensitive to impurities.

Inoculation of Molten Gray Cast Iron
• Inoculant is added to the liquid metal to help prevent the
formation of eutectic carbides and aid graphite
nucleation.
• Inoculants are primarily ferrro-silicon often with small
amounts of varying elements to aid in nucleation.
• Inoculants are typically added in the pouring/transfer
ladles and in-stream during pouring.
• Inoculation also helps to prevent dendritic graphite (types
D and E per ISO 945).
• The effects of inoculation are reduced with time after
they are introduced to the liquid metal. This is commonly
called “inoculant fade.”

Molding Processes Used for Production
of Gray Cast Iron Castings

• Permanent Mold Processes
• Investment Mold Processes
• Sand Mold Processes
– Cope and Drag Sand Molding
– Disamatic Flaskless Sand Molding
– Lost Foam Sand Molding

Cope & Drag Sand Molding Process
• Mold parting is horizontal.
• Molded sand and casting are contained in a steel flask.
• There are minimal casting size and weight limitations.
• There is greater flexibility for gating and riser design.
• In some cases this process can provide a more uniform
casting microstructure and soundness.

Disamatic Flaskless Sand Molding Process
• Mold parting is vertical.
• All metal in gates, risers and casting cavities are
contained within the flaskless sand molds.
• There are casting size and weight limitations due to
the hydrostatic pressure built up within the mold.
• There is reduced flexibility for gating and risers.

Molding Sand Properties
• The properties of green sand and core sand have a
significant impact on the dimensional consistency and
metallurgical quality of the castings.
• Typical green sand properties controlled in the mixing
and molding process:
– Green sand compactability – Wet tensile strength
– Green sand strength – Volatile material composition
– Sand moisture – Percent Active Clay
– Sand temperature – Weight Loss on Ignition
– Green Sand Permeability – Green Sand Granularity
– Green Sand Plasticity

Casting Cooling in the Sand Mold
• The time the casting spends in the sand mold is
typically 20 to 60 minutes. This time is often referred to
as the “shake-out” time.
• The time the casting spends in the sand provides slow
uniform cooling of the castings.
– A long shake-out time can help minimize residual
stresses that can lead to rotor warpage in service.
– A long shake-out time could result in free ferrite
formation and soften the castings. This is especially true
for high carbon gray cast irons.
– Too short of a shake-out time may lead to possible
austenite transformations to martensite or bainite.

Removal of Castings from the Sand Molds
• In the Disamatic flaskless molding process the sand molds
and castings are pushed off the end of the molding lines
onto shaker tables.
• In the cope and drag molding process the sand molds and
castings are pressed out of the flasks.
• Didion rotary drums and/or shaker tables are used to
separate the sand from the castings and break off gates and
risers.

Casting Cleaning
• Any remaining gates, risers or sprues still attached
to the castings are manually hammered off.
• Castings are shot blasted to remove any remaining
sand and clean off minor flash. Either tumble or
rack blasting are used.
• Castings are ground (manually or semi-
automatically) to removed excessive flash or gate
material that may interfere with machining.

Heat-Treatment of Rotor Castings

Gray cast iron can be heat treated with many of the
same processes that are used for steels and nodular
cast irons.

Rotor Casting Heat-Treatments
• Stress Relieving Heat-Treatments
• Annealing Heat-Treatments
• Other Heat-Treatments

Stress Relieving Heat-Treatment of
Gray Cast Iron Rotors
• Stress relieving of gray cast iron rotors can be
performed to minimize rotor warpage that can occur
under extreme service conditions.
• Stress relieving of semi-finished rotors is done in
Europe for many rotors designed for high performance
vehicles.
• Stress relieving is typically performed in the
temperature range of 500°C and 650°C for periods up
to 24 hours.
• Stress relieving has no significant effect on
microstructure or mechanical properties.

Annealing Heat-Treatment of Gray
Cast Iron Rotor Castings
• Annealing of gray cast iron rotor castings was performed in
the past to soften hard castings.
• Annealing is typically performed in the temperature range
of 600°C and 700°C for periods up to 24 hours.
• Annealing will reduce the hardness of the castings.
• Annealing will begin to spherodize the iron carbide in the
pearlite microstructure.
• Annealing may increase the amount of free ferrite through
graphitization of the iron carbide in the pearlite matrix.
• Annealing is typically not allowed for gray cast iron rotors
today.

Other Heat-Treatments Considered for
Gray Cast Iron Rotor Castings
Many other heat treatments have been considered to
improve specific properties of the gray cast iron, but
the costs and/or the technical disadvantages have
outweighed the benefits.

• Austempering Heat Treatments
• Induction Hardening Heat Treatments
• Carburizing and Nitrocarburizing Heat Treatments

Cast Iron Rotor Casting Process
Sample Questions
1. Which melting process (cupola or electric) would
likely be best if you desired a slightly lower silicon
content than is used in the standard gray cast iron
grades?
2. Which molding process (cope and drag or Disamatic)
would likely provide a more uniform casting integrity
and balance for a integral hub and rotor design with a
large offset?
3. Which sand separation process (Didion drum or
shaker tables) would be more likely to cause damage
to thin and/or sensitive sections of a casting?

Sample Questions

Which melting process (cupola or electric) would likely
be best if you desired a slightly lower silicon content
than is used in the standard gray cast iron grades?

Answer: It is difficult to adjust chemistries with a
cupola melting process because of the continuous
melting. Electric batch melting provides the ability to
tailor chemistries specific to designs.

Sample Questions
Which molding process (cope and drag or Disamatic)
would likely provide a more uniform casting integrity and
casting balance for a integral hub and rotor design with a
large offset?
Answer: The Disamatic molding process typically
gates from the bottom and has risers at the top, which can
result in varying degrees of shrinkage porosity with more
complex casting designs. The cope and drag process can
better utilize a riser and gating pattern that minimizes
shrinkage porosity.

Sample Questions
Which sand separation process (Didion drum or shaker
tables) would be more likely to cause damage to thin
and/or sensitive sections of a casting?

Answer: The casting in a Didion drum can fall and strike
other casting from distances as much as 2 to 3 feet.
Therefore, shaker tables may provide less damage to
castings with thin or sensitive sections than Didion
drums.

Areas for Future Material Developments
for Gray Cast Iron Rotors

• Coatings and surface treatments to prevent or minimize
brake surface corrosion
• Alloying to improve thermal conductivity and/or wear
resistance
• Alloying or heat treatments to modify the
microstructure for improved vibration damping
• Composites of gray iron and other metals or ceramics

Alternative Materials to Gray Cast Iron
for Brake Rotor Applications
• Aluminum Metal-Matrix Composite Materials
– Operational brake surface temperatures are limited to
approximately 450°C maximum.
– Cost is approximately 2 to 3 times the cost of gray cast iron
rotors.
• Graphite/Graphite and Graphite/SiC Composite
Materials
– Operational temperatures not limited by rotor material.
– Frictional properties are better at higher temperatures.
– Cost is nearly a hundred times the cost of gray cast iron
rotors.

Concluding Remarks

Gray cast iron has nearly all the properties
that are desired for brake rotor applications.
This combined with the very low costs of
materials and processing makes gray cast iron
a potentially unbeatable material value in
brake rotor applications.

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