PART 1 : TYPES OF CAST IRON

General

Cast irons are essentially iron and carbon alloys that also contain some other elements such as silicon, manganese, sulfure and phosphorus. They are sometimes alloyed with chromium, nickel, copper, molybdenum, vanadium or other elements to enhance their resistance to wear, corrosion or high temperatures.

Unlike most metals, cast irons are not classified according to their chemical composition but instead by their microstructure. Cast iron’s microstructure appears in the form of graphite or carbide particles surrounded by a matrix of steel which can be ferritic, perlitic austenitic or martensitic, depending on the alloy elements, rate of cooling, and heat treatment received. We can therefore distinguish several types of cast irons :

White cast iron

Thus named for the white appearance of its breakage surface, its carbon remains bound to the iron to form carbides during solidification. The presence of these carbides make it very hard, but also very brittle. Its hardness makes it an ideal material for applications requiring high wear resistance such as crusher balls, drawing dies and extrusion nozzles.

Grey cast iron

So named for the grey appearance of its breakage surface. Carbon precipitates in the form of graphite flakes (see cast iron diagram, below). These graphite flakes act as weaknesses that promote crack propagation inside the material, thereby reducing its tensile strength.

However, these same flakes give it excellent vibration absorption capacity and good thermal conductivity. This cast iron is very common; it is easy to cast and machine since the graphite flakes act as a lubricant. It is also used to manufacture transmission parts, fire hydrants, housing, engine blocks, counterweights or machinery bases.

Malleable cast iron

This cast iron is obtained by annealing white cast iron through a fixed temperature and time at temperature cycle. Annealing breaks down the carbides into nodules of serrated graphite, also called temper carbon (see cast iron diagram). These nodules are less compact, however, than those found in ductile cast iron. There are two types of malleable cast iron. The most common is called ”blackheart” and is obtained by the process described above. The other type is found primarily in Europe and is called ”whiteheart”; it is obtained through decarburization of white cast iron.

Ductile cast iron

When a limited amount of special elements like magnesium or cerium are added, the carbon precipitates in the form of compact graphite nodules without having to undergo heat treatment (see cast iron diagram, below). This cast iron normally contains much less sulfur than those previously mentionned. Its mechanical properties are generally slightly better than those of malleable iron. It is used for the same applications as malleable cast iron, and also for large gears, forming dies, rollers and roll housings.

Compacted graphite cast iron

This cast iron too is obtained by adding minimal amounts of special elements such as magnesium, calcium, titanium, or aluminum. The carbon precipitates in the form of compacted flakes (see cast iron diagram, below). Its mechanical properties fall somewhere between those of grey cast iron and ductile cast iron. It is used to make brake discs, cylinder heads, sprocket gears, manifolds, housings or pulleys.

Alloy cast iron

Alloy elements can be added to white, grey, malleable, ductile and compacted graphite cast iron to enhance their resistance to wear or corrosion, their high temperature properties, or their mechanical properties. The most common additions are chromium (up to 35%), nickel (up to 45%), molybdenum (up to 5%), copper (up to 10%) and silicon (up to 18%). They are most often added in combination since the effect of one element will reinforce or enhance the effect of another.

Typical chemical compositions for the cast irons listed above are given in the following table :

Physical properties of cast iron

Thermal conductivity

Cast irons with graphite in flake form conduct heat better than those containing nodular graphite, and much better than white cast iron. Generally, any given steel will not conduct heat as well as grey cast iron with the same matrix, because the thermal conductivity of graphite is very high. It is five times greater than a ferritic matrix, eight times greater than a perlitic matrix and 50 times that of iron carbide. In addition, the presence of alloy elements in a matrix reduces its conductivity.

Electrical conductivity

The presence of alloy elements in a steel or cast iron matrix reduces the material’s electrical conductivity. Likewise, the presence of graphite reduces cast iron conductivity, and even more so when the graphite is in flake form, since there are more obstacles to the flow of current in this case.

Thermal expansion coefficient

Cast iron has a coefficient of thermal expansion similar to that of carbon steel, except for highly alloyed cast irons which can have lower or higher coefficients, depending on their alloy elements.

Next part : cast iron Weldability (2)