Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1 % of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. This nodular graphite structure inhibits the creation of linear cracks hence the ability to withstand distortion. Fig 1 shows typical micro structure of ductile iron.

Fig 1 Typical micro structure of ductile iron

With ductile iron, the safety and reliability of process equipment is improved. The improved mechanical properties increase its resistance to breakage from physical load, or mechanical and thermal shock far above that of gray iron. The corrosion resistance of ductile iron is equal or superior to gray cast iron and to cast steel in many corrosives. Its wear resistance is comparable to some of the best grades of steel and superior to gray iron in heavy load or impact load situations. Since it can be cast with the same low cost procedures used for gray cast iron, it is considerably less expensive than cast steel and only moderately more expensive than gray iron. The substantial advantages obtained from its high yield strength and ductility make it an economical choice for many applications.

Common Ductile Iron Grades

While there are many different ductile iron specifications, foundries routinely offer 3 common grades;

Physical and Mechanical Properties ASTM A536, Grade 60-40-18 ASTM A536, Grade 65-45-12 ASTM A536, Grade 80-55-06
Tensile Strength, min, psi 60,000 65,000 80,000
Tensile Strength, min, MPa 414 448 552
Yield Strength, min, psi 40,000 45,000 55,000
Yield Strength, min, MPa 276 310 379
Elongation in 2 in. or 50 mm, min % 18 12 6
Density lb/in3 0.256 0.256 0.256
Density g/cm3 7.1 7.1 7.1
Melting Temperature (Degrees F) 2,100 – 2,190 2,100 – 2,190 2,100 – 2,190
Melting Temperature (Degrees C) 1,150 - 1,200 1,150 - 1,200 1,150 - 1,200
Compressive Strength Ksi 429 429 429
Compressive Strength MPa 2960 2960 2960
UNS F32800 F33100 F33800

The family of ductile iron

Ductile iron is not a single material, but a family of versatile cast irons exhibiting a wide range of properties which are 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 give ductile iron ductility and toughness which is superior to all other cast irons, and equal to many cast and forged steels. This feature is essential to the quality and consistency of ductile iron. With a high percentage of graphite nodules present in the structure, mechanical properties are determined by the ductile iron matrix. The importance of matrix in controlling mechanical properties is emphasized by the use of matrix names to designate the following types of ductile iron.

• Ferritic ductile iron – Graphite spheroids in a matrix of ferrite provides an iron with good ductility and impact resistance and with a tensile and yield strength equivalent to low carbon steel. Ferritic ductile iron can be produced ‘as-cast’ but may be given an annealing heat treatment to assure maximum ductility and low temperature toughness.

• Ferritic pearlitic ductile iron – These are the most common grade of ductile irons and are normally produced in the ‘as cast’ condition. The graphite spheroids are in a matrix containing both ferrite and pearlite. Properties are intermediate between ferritic and pearlitic grades, with good machinability and low production costs.

• Pearlitic ductile iron – Graphite spheroids in a matrix of pearlite result in an iron with high strength, good wear resistance, and moderate ductility and impact resistance. Machinability is also superior to steels of comparable physical properties.

The above three types of ductile iron are the most common and are usually used in the ‘as cast’ condition, but ductile iron can be also be alloyed and/or heat treated to provide the following grades for a wide variety of additional applications.

• Martensitic ductile iron – Using sufficient alloy additions to prevent pearlite formation, and a quench-and-temper heat treatment produces this type of ductile iron. The resultant tempered martensite matrix develops very high strength and wear resistance but with lower levels of ductility and toughness.

• Bainitic ductile iron – This grade can be obtained through alloying and/or by heat treatment to produce a hard, wear resistant material.

• Austenitic ductile iron – Alloyed to produce an austenitic matrix, this ductile iron offers good corrosion and oxidation resistance, good magnetic properties, and good strength and dimensional stability at elevated temperatures.

• 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.

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