Gray Cast Iron properties
Gray cast iron is the most common cast iron alloy, with a minimum tensile strength ranging from 140 MPa (20 ksi) to over 400 MPa (58 ksi) depending on the grade. It is distinguished by the presence of graphite in the form of flakes (lamellae) that provide high machinability, good damping capacity, excellent thermal conductivity, and a dark gray fracture surface. It is widely used in components where rigidity, vibration absorption, and low cost are critical, such as engine blocks, brake discs, pump housings, and machine tool beds.
| Typical property (middle grade ASTM A48 CL 30) | SI Value | Imperial Value |
|---|---|---|
| Tensile strength | 225 MPa | 33 ksi |
| Brinell hardness (BHN) | 175 | 175 |
| Density | 7.2 g/cm³ | 0.26 lb/in³ |
Chemical composition
Section titled “Chemical composition”Carbon content ranges between 2.5 % and 4.0 % by weight, while silicon content lies between 1 % and 3 %. These two elements are primarily responsible for the graphitic nature of the alloy; silicon acts as a graphite stabilizer and, at 3 % Si, virtually all carbon precipitates as free graphite instead of forming iron carbide (cementite). The exact composition is often left to the manufacturer’s discretion, as specified in standard EN 1561.
| Element | Typical range (% by weight) | Observations |
|---|---|---|
| Carbon (C) | 2.5 – 4.0 | High carbon favors the formation of lamellar graphite. |
| Silicon (Si) | 1.0 – 3.0 | Graphite stabilizer; affects strength and hardness. |
| Manganese (Mn) | 0.2 – 1.0 | Neutralizes sulfur and strengthens pearlite. |
| Phosphorus (P) | ≤ 0.3 | Controlled amounts improve fluidity in casting. |
| Sulfur (S) | ≤ 0.15 | Kept low to avoid brittleness. |
The microstructure contains between 6 % and 10 % by volume of graphite, which takes the form of three-dimensional flakes and appears as fine lines on a polished section. The matrix can be ferritic, pearlitic, or a mixture of both, depending on cooling rate and heat treatment.
Mechanical properties by treatment condition
Section titled “Mechanical properties by treatment condition”As-cast gray iron exhibits tensile strength ranging from 138 MPa (20 ksi) in ASTM A48-20B grade to approximately 400 MPa (58 ksi) in higher alloyed grades. Elongation at break is extremely low, typically < 1 %; therefore, it is characterized not by ductility but by its brittle behavior and high compressive strength, which can reach 3 to 5 times the tensile strength.
| Grade (ASTM A48 standard) | Minimum tensile strength (MPa / ksi) | Approximate Brinell hardness (BHN) | Typical Condition/Matrix |
|---|---|---|---|
| CL 20 | 138 / 20 | 160 | Ferritic, high carbon equivalent. |
| CL 25 | 172 / 25 | 170 | Ferritic-pearlitic. |
| CL 30 | 207 / 30 | 175 | Pearlitic. |
| CL 35 | 241 / 35 | 180 | Pearlitic. |
| CL 40 | 276 / 40 | 180 – 210 | Pearlitic. |
| CL 45 – CL 60 | 310 – 414 / 45 – 60 | 200 – 250 | Alloyed and/or heat treated, pearlitic or bainitic. |
Higher grades (above CL 40) require additions of alloying elements (Cr, Ni, Mo, Cu) and/or heat treatments to achieve higher strength without losing castability. Quenching and tempering or austempering (ADI) are not usually applied to lamellar gray iron; however, a softening annealing can be used to obtain a fully ferritic matrix and reduce hardness. Fatigue and toughness properties are inherently low due to the notch effect of the graphite flakes.
Physical properties
Section titled “Physical properties”The density of gray cast iron is approximately 7.20 g/cm³ (7200 kg/m³), equivalent to 0.260 lb/in³. The proportion of graphite and exact composition cause slight variations, but most commercial alloys fall between 7.1 and 7.3 g/cm³.
| Physical property | Typical SI value | Imperial Value |
|---|---|---|
| Density | 7.20 g/cm³ | 0.260 lb/in³ |
| Melting point (range) | 1140 – 1200 °C | 2085 – 2190 °F |
| Velocity of sound propagation (longitudinal) | ~ 4500 m/s | – |
| Poisson’s ratio | 0.26 – 0.29 | – |
Thermal properties
Section titled “Thermal properties”Thermal conductivity of gray cast iron is 45 – 55 W/(m·K) (26 – 32 BTU/(hr·ft·°F)) at room temperature, depending on graphite content. This high conductivity, together with a specific heat of 460 J/(kg·K) (0.11 BTU/(lb·°F)), makes it ideal for applications requiring rapid heat dissipation, such as brake discs and cookware.
| Thermal property | Typical SI value | Imperial Value |
|---|---|---|
| Thermal conductivity (at 100 °C) | 50 W/(m·K) | 29 BTU/(hr·ft·°F) |
| Specific heat | 460 J/(kg·K) | 0.11 BTU/(lb·°F) |
| Coefficient of thermal expansion (20 – 200 °C) | 10.5 × 10⁻⁶ K⁻¹ | 5.8 × 10⁻⁶ in/in·°F |
| Maximum continuous service temperature (no load) | Approx. 500 °C | 930 °F |
Vibration damping capacity averages 0.01 to 0.02 logarithmic decrement, between 5 and 30 times higher than steel, thanks to the graphite/matrix interfaces.
Electrical properties
Section titled “Electrical properties”Electrical resistivity of gray cast iron is 0.06 to 0.12 × 10⁻⁶ Ω·m (6 – 12 µΩ·cm) at 20 °C, a value that increases with silicon content and the presence of graphite. This resistivity is approximately double that of carbon steel, limiting its use in applications requiring high electrical conductivity.
| Electrical property | Typical value |
|---|---|
| Electrical resistivity (20 °C) | 80 × 10⁻⁹ Ω·m (8 µΩ·cm) |
| IACS conductivity | ≈ 8 – 12 % |
Magnetic properties
Section titled “Magnetic properties”Gray cast iron is ferromagnetic with an initial relative magnetic permeability of around 100 – 300. It exhibits low coercivity (50 – 200 A/m) and saturation induction reaching approximately 1.5 – 1.8 T. The high magnetic damping is due to the heterogeneous microstructure, making it useful in low-frequency electrical machine cores, although its use has been displaced by silicon steels in transformers.
| Magnetic property | Typical value |
|---|---|
| Initial relative permeability | 150 |
| Saturation induction | 1.6 T (16 kG) |
| Coercivity (Hc) | 100 A/m |
Welding
Section titled “Welding”Gray cast iron has very limited weldability: a successful joint requires preheating to 400 – 600 °C (750 – 1100 °F) and extremely slow cooling to avoid the formation of hard and brittle martensite in the heat-affected zone. Cold welding procedures (with nickel electrodes) or, preferably, brazing or metallizing are more commonly used. Repairs of large parts such as engine blocks or machine beds are often performed using the “metalock” method (cold stitching). In any case, the joint strength never equals that of the base material and the welded area remains prone to cracking.
Applications by industry
Section titled “Applications by industry”The most representative applications of gray cast iron include:
| Industry | Typical components | Property exploited |
|---|---|---|
| Automotive and transportation | Engine blocks, brake discs and drums, flywheels, exhaust manifolds | Thermal conductivity, damping, dimensional stability |
| Machinery and capital equipment | Machine beds, housings, low-speed gears, pulleys, machine tool heads | Vibration damping, rigidity, low cost |
| Hydraulics and pneumatics | Valve bodies, pump and compressor housings, cylinders | Compressive strength, machinability, tightness |
| Power generation and railway | Blocks for stationary diesel engines, cylinder heads, counterweights, brake shoes (in decline due to noise) | Thermal stability, controlled wear |
| Home appliances and cookware | Cookware (frying pans, pots), irons, stove and fireplace bodies | High heat capacity, thermal shock resistance, heat retention |
| Construction and architecture | Manhole covers, grates, ornamental posts, urban furniture | Low cost, durability, castability |
Comparison with similar materials
Section titled “Comparison with similar materials”Gray cast iron is usually compared with ductile iron (nodular) and malleable iron. The following table summarizes the most significant differences with tensile strength values of representative grades.
| Material and grade (standard) | Tensile strength (MPa / ksi) | Elongation (%) | Hardness BHN | Main characteristic |
|---|---|---|---|---|
| Gray cast iron ASTM A48 CL 30 | 207 / 30 | < 0.5 | 175 | Excellent damping, low cost, brittle. |
| Ductile iron ASTM A536 60-40-18 | 430 / 62 | 20 | 170 | High ductility and toughness, spheroidal graphite. |
| Ductile iron ASTM A536 80-55-06 | 575 / 83 | 7 | 190 | Higher strength, moderate ductility. |
| Ferritic malleable iron | 310 / 45 | 10 | 120 – 150 | Good ductility, but more costly process. |
| White iron (no graphite) | 250 – 500 / 36 – 73 | ~0 | 350 – 450 | Extreme wear resistance, impossible to machine. |
Compared to ductile iron, gray iron is notably more brittle but offers better damping capacity, higher thermal conductivity, and superior castability. With malleable iron, gray iron shares the pearlitic matrix but malleable iron, lacking lamellar graphite, absorbs energy in a ductile manner. Compared to cast carbon steels, gray iron sacrifices strength and toughness in favor of moldability and low cost.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical tensile strength of gray cast iron?
Section titled “What is the typical tensile strength of gray cast iron?”Minimum tensile strength ranges from 140 MPa (20 ksi) in ASTM A48 CL 20 to 400 MPa (58 ksi) in alloyed grades such as CL 60.
What Brinell hardness can be expected from a class 30 gray cast iron?
Section titled “What Brinell hardness can be expected from a class 30 gray cast iron?”ASTM A48 CL 30 typically exhibits a Brinell hardness of 175 BHN, within a usual range of 160 to 210 BHN for commercial unalloyed grades.
What is the thermal conductivity of gray cast iron at room temperature?
Section titled “What is the thermal conductivity of gray cast iron at room temperature?”Thermal conductivity is around 50 W/(m·K) (29 BTU/(hr·ft·°F)), approximately double that of many carbon steels.
Up to what temperature can a gray cast iron part operate continuously?
Section titled “Up to what temperature can a gray cast iron part operate continuously?”It can be used without structural load up to about 500 °C (930 °F); above that temperature oxidation and microstructural changes limit service life.
Can gray cast iron be welded to repair a crack?
Section titled “Can gray cast iron be welded to repair a crack?”Yes, but the chances of success are modest; it requires preheating to 400 – 600 °C and very slow cooling, or using special nickel electrodes and cold welding procedures.
How much graphite does a typical gray cast iron contain by volume?
Section titled “How much graphite does a typical gray cast iron contain by volume?”Graphite occupies between 6 % and 10 % of the total volume, giving the fracture its characteristic gray color and facilitating machinability.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=3864
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/cast-iron-d_1163.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=1505