Ductile Iron Grades
Cast iron is a versatile industrial material. Indeed, it has played a leading role in the evolution of manufacturing industry. As per evidences found, humans knew about the casting process at least 2500 years back. Evidences of objects casted from iron have been found in China in this period, a testimony to foundries being present in China at that time. From China, foundry technology reached Europe and then to other parts of the world, including India. Today, China and India are major producers of gray cast iron, ductile / S.G. cast iron, and many other types of alloys. India is the second largest producer of cast iron, and there are approximately 4500 foundry units in India. Of these, 80% can be classified as small scale units & 10% each as medium & large scale units. Approximately 500 foundry units have some kind of international quality accreditation. The large foundries are modern & globally competitive & are working at nearly full capacity. Most foundries use cupolas using LAM Coke. There is growing awareness about environment & many foundries are switching over to induction furnaces & some units in Agra are changing over to cokeless cupolas.
[source: http://www.foundryinfo-india.org/About_Us/profile_of_indian.aspx]
Today, there exist a variety of casting processes, and each process offers unique advantages and disadvantages.
Ductile Cast / S.G. Iron
Cast iron essentially is an alloy of iron, typically using smelting iron ore - called pig iron - that contains 2 to 4 percent carbon. It may also contain various percentages of silicon, manganese, molybdenum, cerium, nickel and traces of sulphur and phosphorus. The permutation and combination of these elements, along with further processing like heat treatment, annealing, etc. imparts different properties to cast iron. There are different types of cast iron, depending upon the constituents and the percentage of elements mentioned above. These include gray cast iron, ductile cast iron (also called S.G. cast iron or nodular iron), white cast iron, etc. When people say 'cast iron', they usually mean gray cast iron, so called because of the presence of graphite flakes that imparts gray colour to the castings. However, it is brittle and has low tensile strength. Ductile cast iron, a type of casting characterized by carbon precipitation that occurs in the form of spheres, offers better tensile strength than simple cast iron. It is also called as nodular cast iron or S.G. iron. The graphite in ductile iron is present in spheroidal form, and is the reason why it is also called as nodular iron. Magnesium or Cerium needs to be added to ductile iron while producing it in foundry in order to make it nodular. As the name implies, ductile cast iron is malleable, has strength similar to steel, and is highly machinable. Indeed, it only has a slightly lesser percentage of carbon than steel, and is economical to manufacture than steel. Ductile cast iron from foundries find many end uses, including pipes, automobile sector, and agricultural equipment.
Grades of Ductile Cast Iron
Ductile or S.G cast iron is popular as it has very homogeneous structure, no hard spots or inclusions. Machined components that are made from ductile / S.G. cast iron offer higher spindle speeds, and longer component life. Like mentioned above, addition of certain elements or pre and post casting processes affects the physical and mechanical properties of the ductile cast iron. It is therefore necessary to standardize its classification in order to achieve uniform properties. This is done by assigning different grades to the ductile / S.G. cast iron.
There are various ways of grading ductile cast iron. One of the ways in which it is graded depends upon the microstructure of the metal around the graphite, called as the matrix. The characteristic of this microstructure depends upon the composition of the alloy and the cooling rate of the casting. The common grades of ductile iron differ primarily in the matrix structure that contains the spherical graphite. These differences are the result of differences in composition, in the cooling rate of the casting after it is cast, or as the result of heat treatment. Minor differences in composition or the addition of alloys may be used to enhance the desired microstructure. The most important point to note is that the properties of ductile cast iron can be changed with post production processing treatments. The higher graded ductile irons are usually annealed so that the matrix structure is entirely carbon-free ferrite. The intermediate grades are often used in the as-cast condition without heat treatment and have a matrix structure of ferrite and pearlite. The high strength grades are usually given a normalizing heat treatment to make the matrix all pearlite, or they are quenched and tempered to form a matrix of tempered martensite. The grades of cast iron therefore depend upon the treatment it has been given. Used as-cast, annealed, quenched and tempered, normalized, or heat treated - ductile cast iron can be graded on the process it has undergone. All said and done, however, the grades of cast iron is defined by the mechanical properties it offers - the chief parameters being tensile strength and elongation. Again, these properties are dependent upon the percentage of carbon / manganese / sulphur / nickel and other elements present in the cast iron, and the process it has undergone.
[source: http://www.foundryinfo-india.org/About_Us/profile_of_indian.aspx]
Today, there exist a variety of casting processes, and each process offers unique advantages and disadvantages.
Ductile Cast / S.G. Iron
Cast iron essentially is an alloy of iron, typically using smelting iron ore - called pig iron - that contains 2 to 4 percent carbon. It may also contain various percentages of silicon, manganese, molybdenum, cerium, nickel and traces of sulphur and phosphorus. The permutation and combination of these elements, along with further processing like heat treatment, annealing, etc. imparts different properties to cast iron. There are different types of cast iron, depending upon the constituents and the percentage of elements mentioned above. These include gray cast iron, ductile cast iron (also called S.G. cast iron or nodular iron), white cast iron, etc. When people say 'cast iron', they usually mean gray cast iron, so called because of the presence of graphite flakes that imparts gray colour to the castings. However, it is brittle and has low tensile strength. Ductile cast iron, a type of casting characterized by carbon precipitation that occurs in the form of spheres, offers better tensile strength than simple cast iron. It is also called as nodular cast iron or S.G. iron. The graphite in ductile iron is present in spheroidal form, and is the reason why it is also called as nodular iron. Magnesium or Cerium needs to be added to ductile iron while producing it in foundry in order to make it nodular. As the name implies, ductile cast iron is malleable, has strength similar to steel, and is highly machinable. Indeed, it only has a slightly lesser percentage of carbon than steel, and is economical to manufacture than steel. Ductile cast iron from foundries find many end uses, including pipes, automobile sector, and agricultural equipment.
Grades of Ductile Cast Iron
Ductile or S.G cast iron is popular as it has very homogeneous structure, no hard spots or inclusions. Machined components that are made from ductile / S.G. cast iron offer higher spindle speeds, and longer component life. Like mentioned above, addition of certain elements or pre and post casting processes affects the physical and mechanical properties of the ductile cast iron. It is therefore necessary to standardize its classification in order to achieve uniform properties. This is done by assigning different grades to the ductile / S.G. cast iron.
There are various ways of grading ductile cast iron. One of the ways in which it is graded depends upon the microstructure of the metal around the graphite, called as the matrix. The characteristic of this microstructure depends upon the composition of the alloy and the cooling rate of the casting. The common grades of ductile iron differ primarily in the matrix structure that contains the spherical graphite. These differences are the result of differences in composition, in the cooling rate of the casting after it is cast, or as the result of heat treatment. Minor differences in composition or the addition of alloys may be used to enhance the desired microstructure. The most important point to note is that the properties of ductile cast iron can be changed with post production processing treatments. The higher graded ductile irons are usually annealed so that the matrix structure is entirely carbon-free ferrite. The intermediate grades are often used in the as-cast condition without heat treatment and have a matrix structure of ferrite and pearlite. The high strength grades are usually given a normalizing heat treatment to make the matrix all pearlite, or they are quenched and tempered to form a matrix of tempered martensite. The grades of cast iron therefore depend upon the treatment it has been given. Used as-cast, annealed, quenched and tempered, normalized, or heat treated - ductile cast iron can be graded on the process it has undergone. All said and done, however, the grades of cast iron is defined by the mechanical properties it offers - the chief parameters being tensile strength and elongation. Again, these properties are dependent upon the percentage of carbon / manganese / sulphur / nickel and other elements present in the cast iron, and the process it has undergone.
Each country has its own standard and methodology for grading ductile cast iron, and metals in general. There is no universal acceptance of grades of ductile cast iron. In India, for example, the Bureau of Indian Standards was established in 1986 to promote harmonious development of the activities of standardization, marking and quality certification of goods and attending to connected matters. It has set its own standards for ductile iron, and defines grades based on these norms. The institution revises or modifies its norms from time to time, so customers should make sure they are referring the latest standards. Chinese foundries have their own grades; and so do countries like Germany, France, U.S., UK and others. Overseas customers should hence ensure that the foundries in India that they are outsourcing casting to know which grades to adhere to. That said, most foundry owners in India are well versed with different grades, especially for countries like UK, Germany, France and USA.