Why do Iron Casting Foundries prefer high quality Ductile (SG) Iron for casting?
August 10,2023
Spheroidal Graphite Iron
The Spheroidal Graphite (SG) cast iron is also known as nodular cast Iron, Ductile Iron.
Microstructural Reveal
In an ordinary grey cast iron graphite is present as 'flakes' which tend to have sharp-edged rims. The flakes have a bit of strength, and they act as broad-faced discontinuities in the structure whereas the sharp and edged rims become the root regions of stress concentration.
In SG cast iron the graphite flakes are replaced by spherical particles of graphite, so that the metallic matrix is much less broken up, and the sharp stress raisers are eliminated and it prevents cracks from occurring, thereby enhancing the capability to withstand cracking and distortion.
Special Treatment
The formation of this Spheroidal graphite is affected by adding small amounts of cerium or magnesium to the molten iron just before casting. Since both the elements have strong carbide-forming tendencies, the silicon content of the iron must be high enough (at least 2.5%) to prevent the formation of white iron (by chilling) in thin sections
Magnesium is the more widely used and is usually added (as a nickel-magnesium alloy) in amounts sufficient to give a residual magnesium content of 0.1% in iron.
Characteristics of SG Iron
Due to its remarkable mechanical properties, fracture toughness, design freedom, castability, energy efficiency due to low melting and pouring temperatures, machinability and cost-effectiveness, there is a continuously growing demand for complex, frequently heavy-sectioned SG Iron castings.
Those irons consisting of graphite nodules in a ferrite matrix will have high ductility and toughness whilst those consisting of graphite nodules in a pearlite matrix will be characterized by high strength.
The SG Iron has high withstanding ability to distortion without causing any permanent damages like the Crack initiation, propagation and failure during actual applications.
| Property | Grey Iron | Ductile Iron |
|---|---|---|
| Tensile Strength(MPa) | 250 | 750 |
| Elastic Modules(GPa) | 105 | 160 |
| Elongation(%) | 0 | 2.5 |
| Endurance Ratios -Rotating Bending -Three Point Bending -Tension-Compression |
0.35-0.50 0.55-0.65 0.20-0.30 |
0.45-0.50 0.65-0.75 0.65-0.75 |
| Thermal Conductivity(W/m-K) | 48 | 32 |
Advantage over other Cast Irons
The several-decade advancement of the process of treatment of liquid melt of high-strength cast iron with spheroidal graphite provides today with a perfect and steady spheroidal morphology and required content of graphite inclusions in the metal.
This exhibits an enhanced combination of mechanical, process and operating properties in the large-scale production of quality castings.
These features make Spheroidal Graphite Iron Casting the most important Cast Iron for various industries.
