S.G. Cast Iron
Foundry is the most basic input industry and stringent demands of quality and quantity are being placed on it. With rapid industrialization and growth in other fields of production, up-to-date knowledge of materials and processes for casting is necessary in order to be able to produce sound castings economically.
In another article, we have talked about different types of cast irons. In this article, we will speak in detail about S.G. cast iron.
S.G. Cast Iron - Introduction
Also called as nodular cast iron or ductile cast iron, spheroidal graphite (S.G.) cast iron is a high strength cast iron. S.G. cast iron is produced by adding magnesium and / or cerium to the molten cast iron while it is being produced in foundry. Nodular / S.G. iron is a carbon-rich iron alloy. As S.G. iron contains high carbon content, the graphite is found as particles on the microstructure level embedded into metallic matrix. The term 'nodular' refers to the shape of the graphite particles which is achieved by adding magnesium / cerium. This is unlike the flakes like graphite structure of white or gray iron, and influences the properties of S.G. iron. Due to its mechanical properties nodular cast iron is also known as ductile cast iron. Compared to gray cast iron, the nodular particle shape increases strength and toughness becoming comparable to many grades of steel. Indeed, where cost is a concern and applications are not very critical, ductile cast iron is used instead of steel. A process called as inoculation further strengthens the S.G. castings. S.G. cast iron also undergoes heat treatment many times, and this makes a difference on the size, shape and distribution of graphite particles and the microstructure of the metallic matrix. The spheroidal graphite nodules distributed in the ferrite and/or pearlite matrix of S.G. iron is obtained through Magnesium inoculation; other alloying elements such as Molybdenum, Copper, Chromium, Vanadium and Nickel are also commonly added to the melt in order to obtain specific properties. The mechanical properties of ductile cast iron can therefore differ considerably especially in thick-walled casted components. This is because the local temperature evolutions at the surface and in the centre can differ considerably resulting in locally different microstructures. In general it is possible to produce different grades of S.G. cast iron, both in cast condition without heat treatment, and with heat treatment.
Mechanical Properties of S.G. Cast Iron
From material science point of view, the mechanical properties of alloys make an impact on the utility of the end product.
Toughness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Because the spheroids interrupt the matrix much less than graphite flakes, ductile iron has excellent toughness. As compared to gray iron, the tensile strength of S.G. iron is much more, comparable to that of steel. The strength and the hardening behaviour of S.G. cast iron are strongly influenced by the matrix material properties showing a ratio of yield stress to ultimate strength.
Yield strength: Yield strength is the maximum stress a material can endure beyond which it begins to permanently deform. It is the point at which a material ceases to be elastic and turns plastic. S.G. cast iron exhibits a tension-compression asymmetry with a larger compressive yield stress. They show a high internal stress under cyclic loading, mainly due to the interactions between matrix and graphite particles. However, ductile iron is generally superior to grey irons with respect to their yield strength. Gray iron is not very malleable or strong, and fractures rather easily.
Fatigue: In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. After a certain limit, it can initiate a crack due to the weakening of material caused by cyclic loading, and can lead to fracture. Fatigue can exacerbate structural damage in the presence of corrosive environments. There is a strong relation between graphite particle size and fatigue life. The fatigue life of nodular iron is governed by the size of a single graphite particle or casting defect, respectively, from where the fatigue crack initiates.
Ductility: Ductility is the ability of a material to be drawn or plastically deformed without fracture. It is therefore an indication of how pliable the material is. 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.
Castability: Since ductile cast iron can be moulded at a lower casting temperature than steel, this allows for a better surface finish. In addition, it also allows for casting of complex shapes.
The grades and the requirements of iron casting with spheroidal or nodular graphite classified on the basis of:
Production
S.G. iron is usually casted in foundries from pig iron, also called as crude iron. Pig iron is produced in a foundry from the blast furnace. Pig iron has a very high carbon content, along with silica and other trace elements. It is therefore very brittle. Pig iron is an intermediary product. S.G. iron is made primarily from the crude iron that’s produced by smelting ore in a blast furnace, usually with limestone acting as the flux. Producing ductile iron is relatively inexpensive in comparison to steel, while possessing excellent strength. There are many foundries in India that produce excellent quality S.G. cast iron, for further making machined components.
Uses of S.G. Cast Iron
In another article, we have talked about different types of cast irons. In this article, we will speak in detail about S.G. cast iron.
S.G. Cast Iron - Introduction
Also called as nodular cast iron or ductile cast iron, spheroidal graphite (S.G.) cast iron is a high strength cast iron. S.G. cast iron is produced by adding magnesium and / or cerium to the molten cast iron while it is being produced in foundry. Nodular / S.G. iron is a carbon-rich iron alloy. As S.G. iron contains high carbon content, the graphite is found as particles on the microstructure level embedded into metallic matrix. The term 'nodular' refers to the shape of the graphite particles which is achieved by adding magnesium / cerium. This is unlike the flakes like graphite structure of white or gray iron, and influences the properties of S.G. iron. Due to its mechanical properties nodular cast iron is also known as ductile cast iron. Compared to gray cast iron, the nodular particle shape increases strength and toughness becoming comparable to many grades of steel. Indeed, where cost is a concern and applications are not very critical, ductile cast iron is used instead of steel. A process called as inoculation further strengthens the S.G. castings. S.G. cast iron also undergoes heat treatment many times, and this makes a difference on the size, shape and distribution of graphite particles and the microstructure of the metallic matrix. The spheroidal graphite nodules distributed in the ferrite and/or pearlite matrix of S.G. iron is obtained through Magnesium inoculation; other alloying elements such as Molybdenum, Copper, Chromium, Vanadium and Nickel are also commonly added to the melt in order to obtain specific properties. The mechanical properties of ductile cast iron can therefore differ considerably especially in thick-walled casted components. This is because the local temperature evolutions at the surface and in the centre can differ considerably resulting in locally different microstructures. In general it is possible to produce different grades of S.G. cast iron, both in cast condition without heat treatment, and with heat treatment.
Mechanical Properties of S.G. Cast Iron
From material science point of view, the mechanical properties of alloys make an impact on the utility of the end product.
Toughness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Because the spheroids interrupt the matrix much less than graphite flakes, ductile iron has excellent toughness. As compared to gray iron, the tensile strength of S.G. iron is much more, comparable to that of steel. The strength and the hardening behaviour of S.G. cast iron are strongly influenced by the matrix material properties showing a ratio of yield stress to ultimate strength.
Yield strength: Yield strength is the maximum stress a material can endure beyond which it begins to permanently deform. It is the point at which a material ceases to be elastic and turns plastic. S.G. cast iron exhibits a tension-compression asymmetry with a larger compressive yield stress. They show a high internal stress under cyclic loading, mainly due to the interactions between matrix and graphite particles. However, ductile iron is generally superior to grey irons with respect to their yield strength. Gray iron is not very malleable or strong, and fractures rather easily.
Fatigue: In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. After a certain limit, it can initiate a crack due to the weakening of material caused by cyclic loading, and can lead to fracture. Fatigue can exacerbate structural damage in the presence of corrosive environments. There is a strong relation between graphite particle size and fatigue life. The fatigue life of nodular iron is governed by the size of a single graphite particle or casting defect, respectively, from where the fatigue crack initiates.
Ductility: Ductility is the ability of a material to be drawn or plastically deformed without fracture. It is therefore an indication of how pliable the material is. 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.
Castability: Since ductile cast iron can be moulded at a lower casting temperature than steel, this allows for a better surface finish. In addition, it also allows for casting of complex shapes.
The grades and the requirements of iron casting with spheroidal or nodular graphite classified on the basis of:
- mechancial properties measured on test pieces prepared from separately cast test samples, and
- mechanical properties measured on test pieces prepared from cast-on test samples
Production
S.G. iron is usually casted in foundries from pig iron, also called as crude iron. Pig iron is produced in a foundry from the blast furnace. Pig iron has a very high carbon content, along with silica and other trace elements. It is therefore very brittle. Pig iron is an intermediary product. S.G. iron is made primarily from the crude iron that’s produced by smelting ore in a blast furnace, usually with limestone acting as the flux. Producing ductile iron is relatively inexpensive in comparison to steel, while possessing excellent strength. There are many foundries in India that produce excellent quality S.G. cast iron, for further making machined components.
Uses of S.G. Cast Iron
S.G. cast iron is an engineering material displaying high ductility, elastic modulus, mechanical strength and corrosion resistance; in addition, it has low cost and is easy to produce and machine, and is thus widely used as a structural material. Apart from being cost effective, S.G. cast iron allows a high design flexibility, which is why it is widely used in industry, e.g. for gearboxes, crankshafts, pipes, water pump housings, valves and transportation casks.