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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

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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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