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  CAST IRON VS. CARBON STEEL  
 

We will compare cast iron and carbon steel from mechanical and physical properties.

Cast iron includes gray iron and ductile iron. Gray iron is cheaper than ductile iron, but it has much lower tensile strength and ductility than ductile iron, so gray iron is not replaceable material for carbon steel, unless you do not have mechanical requirements, such as counter weights and stove parts etc.

The ductile iron, also called as nodular iron, is the replaceable material for carbon steel, because ductile iron also has high tensile strength and elongation.

Ductile iron with its carbon content of 3.0% - 3.9% has a higher carbon content than cast steel which normally has a carbon content of 0.08% - 0.60%.

TENSILE STRENGTH

Material specification ASTM A536 is for the normal ductile iron in USA, the ASTM A27 is for the normal carbon steel in USA, so let us compare them.

ASTM A536 Ductile Iron

ASTM A27 Cast Steel

Ultimate Tensile Strength (ksi)

Yield Strength (ksi)

Elongation (%)

Ultimate Tensile Strength (ksi)

Yield Strength (ksi)

Elongation (%)

60

40

18

60

30

22

65

45

12

65

35

24

80

55

6

70

36

22

You can see that there is no large difference when it comes to tensile strength, ductile iron has the greater yield strength (40 ksi). Carbon steel, on the other hand, can only reach 36 ksi yield strength.

So, ductile iron has enough tensile strength versus carbon steel.

SHOCK ABSORPTION AND WELDABILITY

Ductile iron has superior shock absorption to carbon steel. The average damping capacity for ductile iron is 6.6 times greater than SAE 1018 steel (reference - P. 435 ASM Cast Iron hand book).

Cast steel has greater weldability. To properly weld ductile iron, it's important to follow specialized weld procedures.

ABRASION RESISTANCE

Ductile iron has higher abrasion than cast steel and is typically used in friction wear mechanisms e.g. engine crankshafts. Its superior abrasion resistance is due, in large part, to the high volume percentage of graphite which acts as a graphitic lubricant.

In abrasive wear mechanisms, austenitic ductile iron grades offer great wear resistance as well as improved strength. ADI grades of ductile iron are made by alloying the metal and heat treating.

CORROSION RESISTANCE

Ductile iron's corrosion resistance is superior to unalloyed steel, and even highly alloyed steels in certain environments. Ductile iron's corrosion resistance can be improved by understanding the corrosion mechanism and alloying the material appropriately.

IMPACT RESISTANCE

Ductile iron's impact properties are microstructure dependant. Ductile irons, in general, have good impact resistance. Type A395 ductile iron is fully ferritic. This is important as impact resistance depends on the degree of ferritization in the microstructure. Cast steel would be more consistent being a homogeneous microstructure.

Ductile iron, because of the increased strength and ductility is used in more engineered applications, such as Cable drums, Frames, Gear boxes and Pumps etc.

BENEFITS OF DUCTILE IRON AND CARBON STEEL

The Benefits of ductile iron are as follows:

  • Improved castability
  • Lower prices
  • Improved vibration dampening
  • Improved corrosion resistance
  • Superior compressive yield strength

The Benefits of carbon steel are as follows:

  • By silica sol lost wax investment casting process, the surface quality could be perfect
  • Higher elongation and low temperature impact property, this is the key benefit
  • Get more special physical properties though proper heat treatment, such as higher hardness

All in all, if ductile iron could meet your requirement, then it will be the best choice, otherwise, you may have to choose carbon steel, if carbon steel can not meet you special requirement, then alloy steel.

 

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