Spheroidal Graphite or Nodular Cast Iron (or Ductile Cast Iron)

 Spheroidal graphite (SG) cast iron is also known as 'nodular iron' or as 'ductile iron'.

SPHEROIDAL GRAPHITE OR NODULAR CAST IRON (OR DUCTILE CAST IRON)

1. What is Meant by Spheroidal Graphite Cast Iron? 

✓ Spheroidal graphite (SG) cast iron is also known as 'nodular iron' or as 'ductile iron'.

✓ Composition: The composition of a typical SG cast iron is given below:

Carbon - 3.2 to 4%

Silicon. - 1.8 to 3%

Manganese - 0.2 to 0.5%

Phosphorus - 0.08% max

Sulphur - 0.01% max

Remaining is iron.

✓ The SG iron is the cast iron with nodular or spheroidal graphite. The nodules, also called spheroids, are about the same as those in malleable cast iron (temper carbon), except that they are more perfect spheres.

2. Microstructure of SG Cast Iron

✓ The nodular cast iron is produced by adding magnesium and/or cerium to molten cast iron (i.e., the grey iron before casting). The magnesium converts the graphite of cast iron from flake form into spheroidal or nodular form. The resulting alloy is called spheroidal or nodular cast iron.

✓ The typical microstructure of a spheroidal cast iron is shown in Fig.3.8.

✓ The presence of spheroidal graphite improves the ductility, strength, fracture toughness, and other mechanical properties.

✓ Ductile cast iron derives its name from the fact that its ductility is increased (by 20).

✓ Addition of magnesium gives good results and hence it is widely used. Magnesium is usually added in the form of a master alloy such as ferro-silicon-magnesium or nickel-magnesium alloy.

3. Designation of SG Iron

✓ The ASTM has designated SG irons by a six or seven digit number. The first two or three digits stand for the minimum tensile strength in psi, the middle two digits for the minimum yield strength in psi, and the last two digits for percent elongation in a tensile test.

✓ For example, a grade 60-40-18 SG cast iron has a minimum tensile strength of 60 × 103 psi, a minimum yield strength of 40 × 103 psi, and 18% elongation.

✓ Table 3.14 presents the typical compositions, and mechanical properties of some malleable cast irons.

4. Characteristics of SG Cast Iron

The important properties of the SG cast iron are given below:

1. S.G. cast iron has excellent ductility, tensile, and yield strengths than grey and malleable cast irons.

2. It has good toughness than the grey cast iron.

3. It has good fatigue strength.

4. It exhibits good impact strength.

5. It possesses good hardness and high modulus of elasticity.

6. It has corrosion resistance similar to that of grey iron.

7. It possesses excellent castability and wear resistance.

8. It has ability to resist oxidation at high temperatures.

9. It has good machinability.

Thus, one can say that ductile iron has mechanical characteristics approaching those of steel and hence it behaves like steel.

5. Applications of SG Cast Iron

The typical applications of SG cast iron include valves, pump bodies, crank shafts, gears, pinions, rollers, rocker arms, flanges, pipe fittings, power transmission equipments, earth inoving machineries, and other machine components.