Cast iron reaches an elastic modulus of about 164–169 GPa in ductile grades, versus approximately 207 GPa for A36 steel, a stiffness difference of roughly 38–43 GPa [S1].
The comparison depends on cast-iron microstructure, not the label “cast iron”: listed gray iron ranges from 66 to 138 GPa, ductile iron clusters near 164–169 GPa, and steel grades such as A36 and 304–316 stainless are listed at 193–207 GPa [S1].
Elastic Modulus by Cast-Iron Type
Gray iron grade G1800 has a modulus of 66–97 GPa, grade G4000 has 110–138 GPa, and their Poisson’s ratio is 0.26 [S1]. These grades are not interchangeable stiffness assumptions because the modulus range of the higher grade can be more than twice the lower grade’s upper value.
Ductile iron grades 60-40-18, 80-55-06, and 120-90-02 have moduli of 169, 168, and 164 GPa respectively, with Poisson’s ratios of 0.29, 0.31, and 0.28 [S1]. A broader cast-iron property listing places gray iron ASTM Class 20 at 83,000 MPa, Class 35 at 114,000 MPa, Class 60 at 152,000 MPa, and ductile grades 60-40-18, 65-45-12, 120-90-02, and 80-55-06 at 169,000, 168,000, 164,000, and 168,000 MPa respectively [S4].
The engineering distinction is important: a flange, housing, or machine base designed as gray iron may require a different stiffness model from a ductile-iron component. The material families are covered separately in the cast iron and alloy steel reference material.
Steel as the Stiffer Reference
Carbon steel A36 is listed at 207 GPa, with a Poisson’s ratio of 0.30 [S1]. The same engineering table lists stainless alloy 304 at 193 GPa, 316 and 316L at 193 GPa, 440 A at 200 GPa, and 17-7PH at 204 GPa; every listed stainless grade has a Poisson’s ratio of 0.30 [S1].
That places steel close to 200 GPa, while common ductile iron sits near 164–169 GPa. For the same geometry, material and loading, an elastic model based on E = 207 GPa will predict less deformation than one based on E = 169 GPa, all else equal. The comparison with carbon steel becomes clearer when stiffness is kept separate from strength, toughness, and casting capability.
Higher elastic modulus does not automatically mean a better material. Steel is generally more ductile and impact resistant, while cast iron provides wear resistance, damping, and efficient casting of complex shapes [S1][S5].
Selection by Stiffness, Damping, and Load
Gray iron is suited to stiffness-critical castings where vibration damping matters; its listed applications include motor blocks and pistons for G4000, while G1800 is associated with applications where resistance is not critical [S1]. Ductile iron grades 60-40-18, 80-55-06, and 120-90-02 are associated with pressure-containing parts such as valves and pumps, gears, and high-resistance components [S1].
Use this practical comparison for preliminary design:
Gray iron belongs in vibration and damping applications, not automatic impact service. Ductile iron is the appropriate cast-iron choice where pressure, toughness, or higher resistance matters. Steel is the stronger option for severe tensile or impact loading [S5][S7].
Temperature and Test-Condition Limits
Cast-iron elastic modulus changes during heating, and the variation depends on the alloy’s position in the phase diagram and its extent of eutectic character [S3]. The greatest reported reductions occur for hypoeutectic compacted-graphite iron and hypereutectic spheroidal-graphite iron, while the modulus increase after one heating-and-cooling cycle becomes greater as eutectic extent increases [S3].
The tabulated 66–207 GPa values are room-temperature reference values, and processing history and material quality can cause significant variation within a grade [S1]. Elevated-temperature analysis, thermal cycling, and welded or cast component geometry can therefore change the elastic response, so a room-temperature number should not be treated as a temperature-independent constant.
The quoted definition of Young’s modulus is “the ratio of stress (force per unit area) along an axis to strain (ratio of deformation over initial length) along that axis” [S2]. It predicts elastic elongation or compression only while stress remains below the material’s yield strength [S2].
Specification and Sourcing Practice
Specify cast iron by the applicable grade rather than by a generic elastic modulus. Public grade tables include ASTM A48 material, with a 4.5% carbon cast iron entry associated with ASTM A-48 and a listed ultimate strength of 170 MPa, while its Young’s modulus entry is not supplied in that table [S2][S6].
Use the specific ASTM or EN grade, casting condition, and applicable material standard in the procurement document. The available data show that a single cast-iron stiffness value can conceal a 103 GPa spread, from 66 GPa for G1800 to 169 GPa for 60-40-18 [S1].
For components where fatigue, impact, corrosion, or temperature materially affect performance, request test data for the actual casting rather than relying only on handbook values. Cast iron, stainless steel, and other ferrous grades can then be assessed on modulus, strength, damping, and environmental performance rather than on one stiffness number.
The next traceable engineering step is to identify the exact cast-iron or steel grade, calculate elastic deflection using its specified modulus, and verify the selected value against the component’s actual service temperature and casting condition.