The presence of iron in everyday life began in about 1200 BCE, encompassing a wide range of uses from farming implements to weapons of war. Blacksmiths became a critical profession, working with iron to change its properties and shape the material into tools. Every village and town would have a blacksmith’s shop, where sickles, plowshares, nails, swords, candlestick holders, and more were produced.

The discovery of iron’s value led to what has become known as the Iron Age, due to the dominance of this material in social and military applications. Another milestone for metals would follow—the Industrial Revolution changed the way metals were produced and worked into products, including iron.

Types of Iron

There are two major types of iron produced: wrought iron & cast iron. Within those, cast iron includes its own family of metals.

Wrought Iron

The first type of iron produced and worked by blacksmiths was wrought iron. It is virtually pure elemental iron (Fe) that is heated in a furnace before being wrought (worked) with hammers on an anvil. Hammering iron expels most of the slag from the material and welds the iron particles together.

During the industrial revolution and the associated acceleration of construction activities, a new use for wrought iron was discovered. Its high tensile strength (resistance to breaking when under tension) made it ideal to use for beams in large construction projects such as bridges and high-rise buildings. However, the use of wrought iron for this purpose was largely abandoned in the early 20th century when steel products were developed with superior performance to iron for construction applications.

Cast Iron

Cast iron is produced by smelting iron-carbon alloys that have a carbon content greater than 2%. After smelting, the metal is poured into a mold. The primary difference in production between wrought iron and cast iron is that cast iron is not worked with hammers and tools. There are also differences in composition—cast iron contains 2–4% carbon and other alloys, and 1–3% of silicon, which improves the casting performance of the molten metal. Differences between wrought iron and cast iron can also be found in the details of chemical structure and physical properties.

Although both steel & cast iron contain traces of carbon and appear similar, there are significant differences between the two metals. Steel contains less than 2% carbon, which enables the final product to solidify in a single microcrystalline structure. The higher carbon content of cast iron means that it solidifies as a heterogeneous alloy, and therefore has more than one microcrystalline structure present in the material.

It is the combination of high carbon content, and the presence of silicon, that gives cast iron its excellent castability. Various types of cast irons are produced using different heat treatment and processing techniques, including gray iron, white iron, malleable iron, ductile iron, and compacted graphite iron.

Gray Iron

Gray iron is characterized by the flake shape of the graphite molecules in the metal. When the metal is fractured, the break occurs along the graphite flakes, which gives it the gray color on the fractured metal's surface. The name gray iron comes from this characteristic.

It is possible to control the size and matrix structure of the graphite flakes during production by adjusting the cooling rate and composition. Gray iron is not as ductile as other forms of cast iron and its tensile strength is also lower. However, it is a better thermal conductor and has a higher level of vibration damping. It has a damping capacity that is 20–25 times higher than steel and superior to all other cast irons. Gray iron is also easier to machine than other cast irons, and its wear resistance properties make it one of the highest volume cast iron products.

Vibration damping and wear resistance are properties that make this the right material for many street applications. Raw grey iron also produces a patina that keeps it safe from destructive corrosion even outdoors. 

White Iron

With the right carbon content and a high cooling rate, carbon atoms combine with iron to form iron carbide. This means that there are little to no free graphite molecules in the solidified material. When white iron is sheared, the fractured face appears white due to the absence of graphite. The cementite microcrystalline structure is hard and brittle with a high compressive strength and good wear resistance. In certain specialized applications, it is desirable to have white iron on the surface of the product. This can be achieved by using a good conductor of heat to make part of the mold. This will draw heat out of the molten metal quickly from that specific area, while the rest of the casting cools at a slower rate.

One of the most popular grades of white iron is Ni-Hard Iron. The addition of chromium and nickel alloys gives this product excellent properties for low impact, sliding abrasion applications.

White irons and ni-hard irons fall under a classification of alloys referred to as ASTM A532; the “Standard Specification for Abrasion-Resistant Cast Irons”.

Malleable Iron

White iron can be further processed into malleable iron through a process of heat treatment. An extended program of heating and cooling, results in the breakdown of the iron carbide molecules, releasing free graphite molecules into the iron. Different cooling rates, and the addition of alloys, produces a malleable iron with a microcrystalline structure.

Ductile Iron (Nodular Iron)

Ductile iron, or nodular iron, obtains its special properties through the addition of magnesium into the alloy. The presence of magnesium causes the graphite to form in a spheroid shape as opposed to the flakes of gray iron. Composition control is very important in the manufacturing process. Small amounts of impurities such as sulfur and oxygen react with the magnesium, affecting the shape of the graphite molecules. Different grades of ductile iron are formed by manipulating the microcrystalline structure around the graphite spheroid. This is achieved through the casting process, or through heat treatment, as a downstream processing step.

Compacted Graphite Iron

Compacted graphite iron has a graphite structure and associated properties that are a blend of gray and white iron. The microcrystalline structure is formed around blunt flakes of graphite which are interconnected. An alloy, such as titanium, is used to suppress the formation of spheroidal graphite. Compacted graphite iron has a higher tensile strength and improved ductility compared to gray iron. The microcrystalline structure and properties can be adjusted through heat treatment or the addition of other alloys.