Ductile Iron

Ductile Iron is not a single material, but a family of materials offering a wide range of properties obtained through microstructure control. The common feature that all Ductile Irons share is the roughly spherical shape of the graphite nodules. These nodules act as “crack-arresters” and make Ductile Iron “ductile.” This feature is essential to the quality and consistency of Ductile Iron, and is measured and controlled with a high degree of assurance by competent Ductile Iron foundries.

The ASTM has five standards covering Ductile Iron castings. ASTM A 536 is the most frequently used, covering the general engineering grades of Ductile Iron. The other standards cover austenitic and special Ductile Iron applications.

Specialty Castings produces ductile iron according to ASTM A 536 with the following characteristics:

Grade 60-40-18 65-45-12 80-55-06 100-70-03
Tensile Strength, psi 60,000 65,000 80,000 100,000
Yield Strength, psi 40,000 40,000 55,000 70,000
Elongation, % 18 12 6 3

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Austempered Ductile Iron (ADI)

ADI, the most recent addition to the Ductile Iron family, is a sub-group of Ductile Irons produced by giving conventional Ductile Iron a special austempering heat treatment. Nearly twice as strong as pearlitic Ductile Iron, ADI still retains high elongation and toughness. This combination provides a material with superior wear resistance and fatigue strength. ADI offers this superior combination of properties because it can be cast like any other member of the Ductile Iron family, thus offering all the production advantages of a conventional Ductile Iron casting.

It is subjected to the austempering process to produce mechanical properties that are superior to conventional ductile iron, cast and forged aluminum and many cast and forged steels. .The yield strength of ADI is over three times that of the best cast or forged aluminum. In addition ADI weighs only 2.4 times more than aluminum and is 2.3 times stiffer. ADI is also 10% less dense than steel. For a typical component, ADI costs 20% less per unit weight than steel and half of that aluminum.

A comparison of forged steel, pearlitic ductile iron and ADI is shown below. Specialty Castings produces ADI according to the widely accepted ASTM specification 897 Grades 1 through 5.

Mechanical Property MATERIAL
Forged Steel Pearlitic Ductile Iron Grade 150/100/7 ADI
Tensile strength, psi 115,000 100,000 160,000
Yield strength, psi 75,000 70,000 120,000
Elongation, % 10 3 10
Brinnel Hardness 262 262 286
Impact strength, ft-lb (joules) 130 40 120

High Silicon Ductile Iron

Alloy Ductile Irons containing 2-4% silicon were developed to meet the increasing demands for high strength Ductile Irons capable of operating at high temperatures in applications such as exhaust manifolds or turbocharger casings. The primary properties required for such applications are oxidation resistance, structural stability, strength, and resistance to thermal cycling.

These unalloyed grades retain their strength to moderate temperatures, perform well under low to moderate severity thermal cycling and exhibit resistance to growth and oxidation that is superior to that of unalloyed Gray Iron.

Material Tensile Strength
psi
Stress Rupture
psi
800F 1000F 1200F 1000h @ 1000F
Gray Iron 37,000 25,000 12,000 5,900
60-40-18 D.I. 40,000 25,000 13,000 8,300
4% Si D.I. 56,000 36,000 13,000 10,000

Gray Iron: Unalloyed, stress-relieved
Ductile Irons: Sub-critically annealed at 1450 F