Introduction
Cast iron is relatively inexpensive, powerful and durable, making it ideal for many casting applications. While other cast materials such as aluminium and bronze have greater tensile strength, cast iron is among the easiest to cast and is used across many different industries.
In particular, spheroidal graphite iron (SGI), also known as ductile iron, is a form of cast iron which contains nodular graphite inclusions. This type of cast iron is low cost but high strength, offering significantly improved ductility, tensile strength and dynamic load capability when compared to standard cast iron. SGI is also far more resistant to cracking and vibration. As a result, it has become increasingly popular for use in industries such as automotive, construction, marine and oil & gas, where components are exposed to high stress and vibration.
This article will examine the properties, advantages and disadvantages of SGI, as well as the importance of the processes used to create it. Finally, we will look at some of the common applications for SGI, including automotive, energy and pipeline production.
Properties of Spheroidal Graphite Iron (SGI)
SGI is a type of cast iron that has been thermally treated in order to generate nodular graphite inclusions, which increases its strength. The spheroidization of the graphite results in increased ductility and improved tensile strength compared to other types of cast iron.
The addition of magnesium to the molten cast iron is used to encourage the formation of the nodular graphite inclusions. After casting, the components are cooled in a furnace and the magnesium atoms oxidize, leading to the formation of graphite flakes. These flakes rearrange themselves into spherical shapes, forming the spheroidal structure characteristic of SGI.
This structure gives SGI a high degree of structural strength and improved shock resistance, making it an ideal material for applications such as engine components, transmission parts and structural castings.
Advantages and Disadvantages of SGI
When compared to grey cast iron, SGI offers numerous advantages. It has better mechanical properties, including improved tensile strength, ductility and machinability. It also has a higher fatigue strength and resistance to wear, making it ideal for components that experience high dynamic loads and vibration.
SGI also offers improved radiation resistance, due to its ability to absorb gamma and X-ray radiation. This makes it a useful material for components that may be exposed to radiation, such as reactor vessel internals.
The main disadvantages of SGI are its higher cost and greater variability compared to grey iron. SGI parts are also more difficult to machine and require special cutting tools.
Processes Used to Create SGI
SGI is created using a combination of thermomechanical and chemical processes. As mentioned previously, the addition of magnesium to the cast iron is an important part of the process. This magnesium helps to encourage the formation of the spheroidal graphite inclusions.
The cast iron must then be heated to a high temperature (around 1300°C) in order to allow the graphite inclusions to form. It is then quickly cooled, before being heat treated again at a lower temperature.
This process is essential to ensure that the graphite inclusions are not destroyed by the heat treatment process. Without it, the graphite would dissolve and the material would become brittle and weak.
Common Applications of SGI
SGI has become increasingly popular in recent years due to its cost-effectiveness and high strength. It is used in a variety of industries for components such as engine blocks and engine heads, cylinder heads, car suspension parts, engine valves and crankshafts.
SGI is also used for components in energy production and pipeline production, such as drill components, heat exchangers and flow control valves.
Conclusion
Spheroidal graphite iron (SGI) is an increasingly popular form of cast iron which offers improved strength, ductility and machinability compared to standard cast iron.
The processes used to create SGI involve adding magnesium to the cast iron, followed by a combination of thermomechanical and chemical treatment. Without these processes, the graphite inclusions cannot form and the material cannot reach its full strength potential.
SGI is employed in a variety of industries and applications, such as automotive, energy production and pipeline production, due to its cost-effectiveness and improved properties.
