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Ductile Iron vs Steel

Posted by admin on November 5, 2015

Which Alloy is Better? Ductile Iron vs Steel

As we’ve already compared ductile iron castings to gray iron castings we thought we’d compare ductile iron and cast steel across a variety of properties to help determine which alloy is better for a given application.  This blog is a simple, generic overview.  We’ll take a look at tensile strength, ability to absorb shock, weldability, abrasion resistance and corrosion resistance.

Note: 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

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

Ductile Iron (ASTM A536) vs Cast Steel (ASTM A27)
 
As the strength of ductile iron increases ductility decreases.

Cast ASTM A536 Ductile Iron

Cast Steel ASTM A27

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

 

 

 

 

 

 


Shock Absorption and Weldability

Ductile iron has superior shock absorption to 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:

  • Cable drums
  • Frames
  • Gear boxes
  • Pumps

Benefits of ductile iron are as follows:

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

Characteristic

Cast Steel

Ductile Iron

Castability

 

Preferred

Ease of Machining

 

Preferred

Vibration Dampening

 

Preferred

Compressive Yield Strength

 

Much Superior

Surface Hardenability

 

Better for Sliding Wear

Modulus of Elasticity

Slightly higher

 

Impact Resistance

Preferred

 

Corrosion Resistance

 

Preferred

Wear Resistance

Depends on wear mechanism

Depends on the wear mechanism - superior performance in friction based applications

Cost of Manufacture

 

Preferred

 

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