Key Mechanical Properties of Ductile Iron (Tensile Strength, Yield Strength, Elongation)

Ductile Iron (also known as Nodular Cast Iron or Spheroidal Graphite Iron) is a widely used engineering material, favored for its excellent combination of strength, toughness, and ductility. Its mechanical properties are closely related to the spheroidization of graphite particles, among which tensile strength, yield strength, and elongation are the three core indicators that determine its application scope and service performance. Below is a detailed explanation of these key mechanical properties.

Tensile Strength

Tensile strength refers to the maximum stress that ductile iron can withstand before fracture under tensile load, which is an important indicator reflecting the material’s ability to resist breaking. The tensile strength of ductile iron is mainly affected by the matrix structure (ferrite, pearlite, bainite, etc.) and the spheroidization rate of graphite.

In general, the tensile strength of common ductile iron grades ranges from 400 MPa to 1200 MPa. For example, the tensile strength of ferritic ductile iron (such as grade EN-GJS-400-18) is about 400 MPa, while that of pearlitic ductile iron (such as grade EN-GJS-700-2) can reach 700 MPa. High-strength ductile iron with bainite matrix can even achieve a tensile strength of more than 1000 MPa, which is suitable for heavy-duty load-bearing components.

Yield Strength

Yield strength is the stress at which ductile iron produces a specified permanent plastic deformation (usually 0.2% plastic deformation) during tensile testing. It reflects the material’s ability to resist plastic deformation and is an important basis for designing load-bearing components to avoid excessive deformation.

The yield strength of ductile iron is positively correlated with its tensile strength. For ferritic ductile iron, the yield strength is generally between 250 MPa and 350 MPa; for pearlitic ductile iron, it ranges from 400 MPa to 600 MPa; and for high-strength ductile iron, the yield strength can exceed 700 MPa. The yield strength can also be adjusted by heat treatment processes (such as quenching and tempering) to meet different engineering requirements.

Elongation

Elongation is the percentage of the total elongation of the sample after fracture to its original length, which is a key indicator of the ductility and toughness of ductile iron. A higher elongation means the material has better plastic deformation ability and is not easy to break under impact or sudden load.

The elongation of ductile iron varies greatly with the matrix structure. Ferritic ductile iron has the best ductility, with an elongation of 10% to 25% (e.g., EN-GJS-400-18 has an elongation of ≥18%). Pearlitic ductile iron has moderate ductility, with an elongation of 2% to 10%. High-strength ductile iron with bainite or martensite matrix has lower elongation (usually ≤5%), but it has higher strength and hardness.

In summary, the tensile strength, yield strength, and elongation of ductile iron are mutually restrictive and complementary. By adjusting the matrix structure, spheroidization rate, and heat treatment process, the mechanical properties of ductile iron can be customized to adapt to different application scenarios, such as automotive parts, engineering machinery, and pipeline components, balancing strength and ductility.