3 Types of Iron Used in Metallurgy and Engineering

Iron is a transition element having all the properties and characteristics of this group of elements in the periodic table. It has colored ions, exists in various oxidation states and takes part in coordinate covalent bonding. Iron is a component of haemoglobin where it takes part in reversible bonding with the molecules of oxygen making it easier for oxygen to be transported to all parts of the body via haemoglobin.  Furthermore, iron-sulphur proteins found in the mitochondria and in the chloroplast act as electron carriers which further gives credence to the ability of iron to change its oxidation states. Before we delve into the various types of iron, we shall first look at physical properties of this very special metal. 

Physical Properties of Iron

Appearance—————————– It is silvery and solid with luster

Malleability—————————– Very malleable

Relative density———————–  7.9

Conductivity—————————- Good conductor of heat and electricity

Ductility———————————- Ductile

Melting point————————— 1500*C

Tensile strength———————— High tensile strength

Magnetism——————————- Can be easily magnetized.

Types of Iron

Iron can exist in several forms according to their percentage purity, intended use or carbon content. There are basically three types of iron and they are:- Pig iron, Cast iron and wrought iron. We shall describe these types of iron and give their uses in industrial and manufacturing processes.

1  Pig Iron:– This type of iron is obtained directly from the blast furnace and is impure. It contains about 5%carbon which is present both as iron carbide and graphite as well as other impurities such as phosphorus, silicon, sulphur and manganese which all occur in varying proportions depending on the type of ore used and the operating temperatures of the extraction system. These impurities lowers the melting point of this iron from about 1500*C to about 1200*C. Pig iron is hard and quite brittle and has very limited industrial use.

2  Cast Iron:– This is gotten from pig iron that has been melted with some scrap iron and then cooled in moulds to give the required shapes. This metal cannot be welded and cannot be forged. It is brittle and contains impurities too but much lower than that found in pig iron. It is used in the manufacture of objects that do not require high tensile strength such as radiators, cookers, stoves, railings, base of bunsen burners and lamp posts. Cast iron is easy to melt and expands slightly when subjected to a cooling.

3  Wrought Iron:– This can be described as is the purest form of iron and contains only about 0.1% carbon. Wrought iron is obtained by heating cast iron in a furnace with iron ore, haematite. During this process, sulphur and carbon are oxidized and then removed as their gaseous oxides. Phosphorus and silicon(IV)oxide present in the process are oxidized and also removed from the semi-molten mass of iron as slag. This metal is soft and malleable but very tough. It can be shaped by hammering it at a temperature of 1000*C which is about 500*C below its melting point. It can also be welded quite easily and forged. It is used for making rods and sheets used in construction activities; it is also used in making nails, chains, agricultural implements, horse shoes and the core of electromagnets

Let us now take an indepth look at the different types of iron and their uses in engineering and metallurgical works.

Iron is one of the most widely used materials on the planet. It is found in a wide variety of structures, appliances and devices, and has many different grades for different properties and applications.

It is also a key component in hemoglobin, an erythrocyte protein that delivers oxygen to the body’s tissues. It is magnetic, and can be manipulated into various forms.

Cast Iron

Cast iron is a type of iron with traces of carbon in its molecular structure. This makes it an excellent material for a variety of uses, particularly in applications requiring high levels of wear resistance or low corrosion rates. The casting process allows for large variations in microstructure, which gives a wide range of properties to different types of cast iron. This is due to the extent to which impurities and alloys are added to the iron, the cooling rate during and after casting, as well as the heat treatment applied afterward.

The most common traditional form of cast iron is gray, which has flakes of graphite and is harder than other forms of cast iron. It is not as ductile or as resistant to abrasion as other types of cast iron, but it is very easy to machine and has good thermal conductivity. It is commonly used in engine blocks, cylinder heads and manifolds, furnaces and burner parts, gear blanks and housings, and hydraulic components.

Ductile cast iron is a more durable version of gray cast iron with improved wear resistance. It is made through a more rigorous process, which changes the iron carbides into nodules of graphite. This allows for better machinability and lower impact, sliding abrasion wear. It is used in heavy duty bearing surfaces, shafts, sprockets and drives for railroad rolling stock, farm and construction machinery and engines, and automobile door hinges.

Malleable iron is another variant on cast iron, with a microcrystalline structure that has a much higher level of ductility than other forms of cast iron. It is hard to machine, but its ductility means that it can be used for bearing surfaces, chains and sprockets, connecting rods, crankshafts and drive train axle components. It is also used in automobile and truck suspension components, steering knuckles and plow shares.

Ductile and grey cast irons are susceptible to corrosion in acidic environments, such as concentrated sulfuric and phosphoric acids at elevated temperatures, and mildly oxidizing acids such as acetic, oleic and stearic. However, they can be protected from these acids by the addition of chromium to the metal or by the use of a nickel-rich cast iron known as Ni-resist.

Wrought Iron

Iron is the most abundant of all metals in nature and is a vital element of hemoglobin, the protein that transports oxygen throughout the body. Most people know that it is important to get enough iron in their diets; however, the amount needed changes with age and differs between men and women. It is most commonly found in animal products and vegetables, with a typical daily requirement of 8 mg for adults.

The most familiar type of iron is wrought iron, which contains less than 0.1 percent carbon and about 1 to 2 percent slag (a byproduct of smelting). It is soft and ductile, able to be bent and shaped into various forms. Its low carbon content makes it superior for many purposes to cast iron, which is hard and brittle. It is malleable, meaning it can be heated and reheated repeatedly while being worked, becoming stronger the more it is deformed. Wrought iron is also resistant to fatigue, meaning it can be flexed repeatedly without failing.

Unlike cast iron, wrought iron is made by melting pig iron and then forging it with tools such as a hammer. The process was developed in the middle ages and continued to be used up through the 19th century, when it was eventually superseded by steel. Its comparatively low cost, high tensile strength, and good corrosion resistance made it popular for railroads, shipbuilding, and decorative architectural uses, such as iron fences and gates.

In the modern industrial world, wrought iron is used for making tools and machinery because of its relatively low price, high tensile strength, good corrosion resistance, and excellent workability. It is often used in construction because it can be welded and forged easily, though it is not as strong as steel in compression.

Wrought iron can be prone to corrosion in damp conditions. It can be susceptible to galvanic corrosion, which occurs when dissimilar metals are in direct contact and an electrolyte is present. This is most common when wrought iron comes into contact with copper or zinc, and can also occur between steel and wrought iron.

Steel

Steel is iron alloyed with a wide variety of elements, primarily carbon. The amount of carbon used in a specific batch of steel will vary, but it can be varied within the range of less than two percent to more than eighteen percent. The addition of other elements, such as chromium and manganese, can also alter the alloy’s properties.

Steel’s strength and ductility make it the most useful material in construction. It can be shaped, machined and welded to form many different structures. It is also relatively easy to transport, allowing it to be brought close to the site of construction for installation. Steel is also an excellent conductor of electricity, which makes it a vital component of power systems and transmission lines.

The tensile strength of steel can vary considerably between types. The lowest value is approximately 290 N/m2, and the highest is about 870 N/m2. Carbon contributes to this variation by preventing the iron atoms from sliding past each other, as they do in pure iron.

In modern times, steel is made from raw materials primarily by the Bessemer Process (a refinement of the original 19th century method) or the Gilchrist-Thomas Process. In both processes, iron is melted in a blast furnace and then mixed with other metals, such as scrap steel or pig iron.

A large percentage of modern vehicles are made from steel, including cars, trucks, and motorcycles. Steel is also important in the production of many other consumer goods, such as home appliances, tools, and machinery.

The agricultural industry also relies heavily on steel. Farmers have used cast iron plows for centuries, but modern farming would not be possible without the invention of steel-constructed farm machinery. This equipment includes barns and silos, harvesting equipment such as tractors and cotton pickers, livestock management devices like feeders and watering troughs, and more. In addition, many people use galvanized steel pipes to deliver fuel, water, and power to their homes. This is due to the fact that these pipes are strong enough to withstand a significant amount of pressure. They are also inexpensive to produce and install.

Ductile Iron

Ductile iron, also called Nodular Graphite Iron and Spheroidal Graphite Iron, is stronger than gray iron but softer, making it easier to cast complex parts and less expensive to make than steel. It has a wide range of engineering properties and is now being used in place of steel castings, weldments and forgings in many applications. It is also a good choice for continuous casting because it can be formed into shapes without the need for rework.

Like gray and wrought iron, ductile iron has a microstructure consisting of spherical nodules of graphite embedded in a hard ferrite matrix. The nodules are shaped differently in ductile iron and this shape influences the mechanical properties of the metal. Unlike the flake graphite shape in gray iron, which creates fracture planes and limits strength, spheroidal graphite nodules increase ductility. In addition, the spherical shape of the nodules helps to absorb impact energy.

In order to produce different grades of ductile iron, the foundry must manipulate the matrix structure around the graphite nodules. This can be accomplished by controlling the cooling rate, inoculation practice and the addition of alloy elements. For example, increasing the ratio of pearlite to ferrite decreases strength while improving machinability and wear resistance.

Unlike the relatively pure, flexible ferrite found in grey and wrought irons, ductile iron contains both ferrite and pearlite. When the ratio of pearlite to ferrite is higher, the iron has lower tensile and yield strength but greater ductility. The spherical shape of the graphite also contributes to the superior ductility and toughness of ductile iron.

In order to produce different grades of ductile, there must be a balance between the size of the eutectic cells and the amount of combined carbon in the matrix. Larger eutectic cell sizes contribute to increased tensile and yield strengths but at the expense of ductility. To control the graphite morphology, the addition of nodulizers, such as magnesium, ferrosilicon, cerium, sulfur or manganese, is essential. Also, the chemistry of the molten iron must be closely monitored in order to maintain the desired graphite nodularity and nodule density.