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Classes of Gray Iron

Gray irons are commonly classified by their minimum tensile strength. A class 30 gray iron indicates that it has a nominal tensile strength of 30,000 psi. In the International Standard or Sl System a similar iron would be grade 220 with a tensile strength of 220 MPa (megapascals) or 220 newtons per square millimeter. A class designation may be used to indicate a grade of iron even when tensile strength is not an important consideration and may not be specified or tested. However, when the class designation is used in conjunction with a standard specification that requires a minimum tensile strength, then actual tensile tests are made to determine if the metal meets this requirement.

In a particular size or type of casting, gray irons can also be satisfactorily designated by their Brinell hardness, (see Table 2). This designation has the advantage of using a nondestructive test that can be applied in routine inspection. These methods of designation are satisfactory for most applications because the common engineering properties of gray iron can be related to its strength and hardness.

The chemical composition of gray iron is not commonly specified because it does not assure obtaining specific mechanical properties. However, for special applications some aspect of chemical composition may be specified to assure the suitability of the iron for a specific need. For example, an alloy content range may be specified to assure an adequate response to heat treatment or to provide strength or oxidization resistance in service at a red heat. A minimum carbon content may be specified to provide adequate thermal shock resistance.

Table 2. Grades of Automotive Gray Iron Castings
Designated by Brinell Hardness as in Specification SAE J431

SAE Grade

Specified Hardness BHN*

Minimum Tensile Strength for Design Purposes

Other Requirements

G1800 187 max 18,000 psi 124 MPa   G2500 170-229 25,000 psi 173 MPa   G2500** 170-221 25,000 psi 173 MPa 3.4% min C and microstructure specified G3000 187-241 30,000 psi 207 MPa   G3500 207-255 35,000 psi 241 MPa   G3500b** 207-255 35,000 psi 241 MPa 3.4% min C and microstructure specified G3500c** 207-255 35,000 psi 241 MPa 3.5% min C and microstructure specified G4000 217-269 40,000 psi 276 MPa  

• * Hardness at a designated location on the castings.
• **For applications such as brake drums, disc and clutch plates to resist the thermal shock

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ASTM A48 Class 30 (Related Standards – DIN GG20, BS 1452 Grade 220) is a gray cast iron. Gray iron consists of graphite flakes in a metallic matrix. When fractured, it is grayish in colour – hence the name (specifically, a fracture will follow along the graphite flakes which are gray).  The low cost of production and attractive properties make gray iron one of the most commonly used ferrous alloys.  Gray iron class 30 is one grade in a family of many.  Chemistry ranges are not specified in the ASTM A48 standard. Instead, chemistry and hardness values listed in this data sheet would be typical for a class 30. 

Read: If you’re interested in comparing gray iron and ductile iron read “Ductile Iron Versus Gray Iron Castings.”

Composition

 

C

Mn

Si

P

S

Min%

3.1

0.5

2.1

.02

.05

For more oem casting component supplierinformation, please contact us. We will provide professional answers.

Max%

3.3

0.9

2.3

0.1

0.12

 

Physical and Mechanical Properties

UTS (ultimate tensile strength)

30,000 psi (207MPa)

Hardness

174 – 210 BHN

Density lb/in3 (g/cm3)

0.258 (7.15)

Thermal Conductivity   Btu/hr·ft·F (W/m·K)

30 (53) at 212°F (100°C).  Will drop as temperature increases.

Specific Heat at 70F  Btu/lb·F (J/Kg·k)

0.14 (586)

Coefficient of Thermal Expansion Ɛ/F(Ɛ/C)X106 average between 68-212F

5.8 (10.5)

Melting Temperature

2050 – 2120 F

Compressive Strength Ksi (MPa)

109 (752)

Gray Iron Properties and Advantages

• Gray iron has great vibration and sound damping properties.  For this reason, it is still one of the best materials for machine bases
• Great thermal conductivity because of its network of graphite flakes
• Good strength in compression – typically 3-4 times that of its ultimate tensile strength
• Excellent machinability due to the presence of the graphite flakes 
• Fluidity during pouring means that it can be cast into thin sections and complex shapes

Applications

• Automotive (brake drums, clutch plates, heads, cylinder liners, pistons)
• Ingot moulds
• Machine-tool parts
• Heavy duty diesel engine blocks
• Gearboxes
• Flywheels
• Counterweights
• Municipal-street castings
• Pipe
• Valves
• Pumps (and pump parts)

Read: To learn more about gray iron, please check out our gray iron webpage.

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