In ferrous materials the main alloying element is carbon (C). Depending on the amount of carbon present, alloys will have different properties, especially when the carbon content is either less/higher than 1.5%. This amount of carbon is specific as below this amount of carbon, material undergoes eutectoid transformation, while above that limit ferrous materials undergo eutectic transformation. Thus, the ferrous alloys with less than 1.5% C are termed as steels and the ferrous alloys with higher than 1.5% (2–4%) C are termed as cast irons.
On the basis of the percentage of carbon and their alloying elements present, these can be classified into the following groups.
Mild Steels: The percentage of carbon in iron ranges from 0.15% to 0.25%. These are moderately strong and have good weldability. The production cost of these materials is also low.
Medium Carbon Steels: These contain carbon between 0.3% and 0.6%. The strength of these materials is high but their weldability is comparatively less.
High Carbon Steels: These contain carbon varying from 0.65% to 1.5%. These materials get hard and tough by heat treatment and their weldability is poor. The steel formed in which carbon content is up to 1.5%, silica up to 0.5%, and manganese up to 1.5% along with traces of other elements is called plain carbon steel.
Cast Irons: The carbon content in these substances varies between 2% and 4%. The cost of production of these substances is quite low and these are used as ferrous casting alloys.
Grey Cast Iron: These alloys consist of carbon in the form of graphite flakes, which are surrounded by either ferrite or pearlite. Because of the presence of graphite, fractured surface of these alloys looks greyish, and so is the name for them. Alloying addition of Si (1–3 wt.%) is responsible for decomposition of cementite, and also high fluidity. Thus, castings of intricate shapes can be easily made. Due to graphite flakes, grey cast irons are weak and brittle. However, they possess good damping properties, and thus typical applications include base structures, bed for heavy machines, etc.; they also show high resistance to wear.
White Cast Iron: When Si content is low (< 1%) in combination with faster cooling rates, there is no time left for cementite to get decomposed, thus most of the brittle cementite retains. Because of presence of cementite, fractured surface appears white, hence the name. They are very brittle and extremely difficult to machine. Hence their use is limited to wear resistant applications such as rollers in rolling mills. Usually white cast iron is heat treated to produce malleable iron.
Nodular (or Ductile) Cast Iron: Alloying additions are of prime importance in producing these materials. Small additions of Mg/Cr to the grey cast iron melt before casting can result in graphite to form nodules or sphere-like particles. Matrix surrounding these particles can be either ferrite or pearlite depending on the heat treatment. These are stronger and ductile than grey cast irons. Typical applications include pump bodies, crank shafts, automotive components, etc.
Malleable Cast Iron: These are formed after heat treating white cast iron. Heat treatments involve heating the material up to 800–900°C, and keep it for long hours, before cooling it to room temperature. High temperature incubation causes cementite to decompose and form ferrite and graphite. Thus, these materials are stronger with appreciable amount of ductility. Typical applications include railroad, connecting rods, marine, and other heavy-duty services.
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