The base metal:

iron

Study the production and structural forms of iron from ferrite and austenite to the alloy steel

Iron ore is one of the most abundant elements on Earth, and one of its primary uses is in the production of steel. When combined with carbon, iron changes character completely and becomes the alloy steel.

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The major component of steel is iron, a metal that in its pure state is not much harder than copper. Omitting very extreme cases, iron in its solid state is, like all other metals, polycrystalline—that is, it consists of many crystals that join one another on their boundaries. A crystal is a well-ordered arrangement of atoms that can best be pictured as spheres touching one another. They are ordered in planes, called lattices, which penetrate one another in specific ways. For iron, the lattice arrangement can best be visualized by a unit cube with eight iron atoms at its corners. Important for the uniqueness of steel is the allotropy of iron—that is, its existence in two crystalline forms. In the body-centred cubic (bcc) arrangement, there is an additional iron atom in the centre of each cube. In the face-centred cubic (fcc) arrangement, there is one additional iron atom at the centre of each of the six faces of the unit cube. It is significant that the sides of the face-centred cube, or the distances between neighbouring lattices in the fcc arrangement, are about 25 percent larger than in the bcc arrangement; this means that there is more space in the fcc than in the bcc structure to keep foreign (i.e., alloying) atoms in solid solution.

Iron has its bcc allotropy below 912° C (1,674° F) and from 1,394° C (2,541° F) up to its melting point of 1,538° C (2,800° F). Referred to as ferrite, iron in its bcc formation is also called alpha iron in the lower temperature range and delta iron in the higher temperature zone. Between 912° and 1,394° C iron is in its fcc order, which is called austenite or gamma iron. The allotropic behaviour of iron is retained with few exceptions in steel, even when the alloy contains considerable amounts of other elements.

There is also the term beta iron, which refers not to mechanical properties but rather to the strong magnetic characteristics of iron. Below 770° C (1,420° F), iron is ferromagnetic; the temperature above which it loses this property is often called the Curie point.

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Building Blocks of Everyday Objects

25.4.3: Steel Production

Before 1860, steel was expensive and produced in small quantities, but the development of crucible steel technique by Benjamin Huntsman in the 1740s,the Bessemer process in the 1850s, and the Siemens-Martin process in the 1850s-1860s resulted in the mass production of steel, one of the key advancements behind the Second Industrial Revolution.

Learning Objective

Postulate the effects of improved steel production on the progression of industry.

Key Points

• Steel is an alloy of iron and other elements, primarily carbon, that is widely used in construction and other applications because of its high tensile strength and low cost. Steel’s base metal is iron. It was first produced in antiquity, but two decades before the Industrial Revolution an improvement was made in the production of steel, which at the time was an expensive commodity used only where iron would not do.
• Benjamin Huntsman developed his crucible steel technique in the 1740s. He was able to make satisfactory cast steel in clay pot crucibles, each holding about 34 pounds of blister steel. A flux was added, and they were covered and heated by coke for about three hours. The molten steel was then poured into molds and the crucibles reused. For a long time Huntsman exported his whole output to France as local producers refused to work with steel harder than they were already using.
• Steel is often cited as the first of several new areas for industrial mass-production that characterize the Second Industrial Revolution. Before about 1860, steel was still an expensive product. The problem of mass-producing cheap steel was solved in 1855 by Henry Bessemer with the introduction of the Bessemer converter at his steelworks in Sheffield, England. Further experiments by Göran Fredrik Göransson and Robert Forester Mushet allowed Bessemer to perfect what would be known as the Bessemer process.
• Although initially Bessemer met with rebuffs and was forced to undertake the exploitation of his process himself, eventually licences were applied for in such numbers that Bessemer received royalties exceeding a million pounds sterling. By 1870, Bessemer steel was widely used for ship plate. The Bessemer process also made steel railways competitive in price. Experience quickly proved steel had much greater strength and durability and could handle the heavier and faster engines and cars.
• After 1890, the Bessemer process was gradually supplanted by open-hearth steel making. Carl Wilhelm Siemens developed the Siemens regenerative furnace in the 1850s. This furnace operated at a high temperature by using regenerative preheating of fuel and air for combustion. In 1865, Pierre-Émile Martin took out a license from Siemens and applied his regenerative furnace for making steel. The Siemens-Martin process was slower and thus easier to control. It also permitted the melting and refining of large amounts of scrap steel, further lowering steel production costs and recycling an otherwise troublesome waste material.
• The Siemens-Martin process became the leading steel-making process by the early 20th century. The availability of cheap steel allowed larger bridges, railroads, skyscrapers, and ships. Other important steel products were steel cable, steel rod, and sheet steel, which enabled large, high-pressure boilers and high-tensile strength steel for machinery. Military equipment also improved significantly.

Key Terms

Second Industrial Revolution

A phase of rapid industrialization in the final third of the 19th century and the beginning of the 20th, also known as the Technological Revolution. Although a number of its characteristic events can be traced to earlier innovations in manufacturing, such as the establishment of a machine tool industry, the development of methods for manufacturing interchangeable parts, and the invention of the Bessemer Process, it is generally dated between 1870 and 1914 up to the start of World War I.

Bessemer process

The first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air blown through the molten iron. The oxidation also raises the temperature of the iron mass and keeps it molten.

crucible steel

A term that applies to steel made by two different methods in the modern era and produced in varying locales throughout history. It is made by melting iron and other materials. It was produced in South and Central Asia during the medieval era but techniques for production of high-quality steel were developed by Benjamin Huntsman in England in the 18th century. However, Huntsman’s process used iron and steel as raw materials rather than direct conversion from cast iron as in the later Bessemer process. The homogeneous crystal structure of this cast steel improved its strength and hardness compared to preceding forms of steel.

cementation

An obsolete technology for making steel by carburization of iron. Unlike modern steel making, it increased the amount of carbon in the iron. It was apparently developed before the 17th century. Derwentcote Steel Furnace, built in 1720, is the earliest surviving example of a furnace using this technology.

carburization

A heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder. Unlike modern steel making, the process increased the amount of carbon in the iron.

 

 

Steel and the Industrial Revolution

Steel is an alloy of iron and other elements, primarily carbon, that is widely used in construction and other applications because of its high tensile strength and low cost. Steel’s base metal is iron, which is able to take on two crystalline forms, body-centered cubic (BCC) and face-centered cubic (FCC), depending on its temperature. It is the interaction of those allotropes with the alloying elements, primarily carbon, that gives steel and cast iron their range of unique properties. In the BCC arrangement, there is an iron atom in the center of each cube, and in the FCC, there is one at the center of each of the six faces of the cube. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that otherwise occur in the crystal lattices of iron atoms.

Steel (with lower carbon content than pig iron but higher than wrought iron) was first produced in antiquity, but two decades before the Industrial Revolution an improvement was made in the production of steel, which at the time was an expensive commodity used only where iron would not do, such as for cutting-edge tools and for springs. Benjamin Huntsman developed his crucible steel technique in the 1740s. After many experiments, Huntsman was able to make satisfactory cast steel in clay pot crucibles, each holding about 34 pounds of blister steel. A flux was added, and they were covered and heated by coke for about three hours. The molten steel was then poured into molds and the crucibles reused. The local cutlery manufacturers refused to buy Huntsman’s cast steel, as it was harder than the German steel they were accustomed to using. For a long time Huntsman exported his whole output to France. Blister steel used by Huntsman as raw material was made by the cementation process or by carburization of iron. Carburization is a heat treatment process, in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder. Unlike modern steel making, the process increased the amount of carbon in the iron.

 

Second Industrial Revolution

Steel is often cited as the first of several new areas for industrial mass-production that characterize the Second Industrial Revolution beginning around 1850, although a method for mass manufacture of steel was not invented until the 1860s and became widely available in the 1870s after the process was modified to produce more uniform quality.

 

Before about 1860, steel was an expensive product, made in small quantities and used mostly for swords, tools, and cutlery. All large metal structures were made of wrought or cast iron. The problem of mass-producing cheap steel was solved in 1855 by Henry Bessemer with the introduction of the Bessemer converter at his steelworks in Sheffield, England. In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and air was blown through the molten iron from below, igniting the dissolved carbon from the coke. As the carbon burned off, the melting point of the mixture increased, but the heat from the burning carbon provided the extra energy needed to keep the mixture molten. After the carbon content in the melt dropped to the desired level, the air draft was cut off. A typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour. Bessemer demonstrated the process in 1856 and had a successful operation going by 1864.

Alhough the Bessemer process is no longer commercially used, at the time of its invention it was of enormous industrial importance because it lowered the cost of production steel, leading to steel being widely substituted for cast iron.Bessemer’s attention was drawn to the problem of steel manufacture in an attempt to improve the construction of guns.

Bessemer licensed the patent for his process to five ironmasters, but from the outset, the companies had great difficulty producing good quality steel. Göran Fredrik Göransson, a Swedish ironmaster, using the purer charcoal pig iron of that country, was the first to make good steel by the process, but only after many attempts. His results prompted Bessemer to try a purer iron obtained from Cumberland hematite, but had only limited success because the quantity of carbon was difficult to control. Robert Forester Mushet, after thousands of experiments at Darkhill Ironworks, had shown that the quantity of carbon could be controlled by removing almost all of it from the iron and then adding an exact amount of carbon and manganese in the form of spiegeleisen (a ferromanganese alloy). This improved the quality of the finished product and increased its malleability.

When Bessemer tried to induce makers to take up his improved system, he met with general rebuffs and was eventually driven to undertake the exploitation of the process himself. He erected steelworks in Sheffield in a business partnership with others, such as W & J Galloway & Sons, and began to manufacture steel. At first the output was insignificant, but gradually the magnitude of the operation was enlarged until the competition became effective and steel traders became aware that the firm of Henry Bessemer & Co. was underselling them to the extent of UK£10-£15 a ton. This argument to the pocket quickly had its effect, and licenses were applied for in such numbers that, in royalties for the use of his process, Bessemer received a sum considerably exceeding a million pounds sterling. By 1870, Bessemer steel was widely used for ship plate. By the 1850s, the speed, weight, and quantity of railway traffic was limited by the strength of the wrought-iron rails in use. The solution was to turn to steel rails, which the Bessemer process made competitive in price. Experience quickly proved steel had much greater strength and durability and could handle the heavier and faster engines and cars.

However, Mushet received nothing and by 1866 was destitute and in ill health. In that year his 16-year-old daughter, Mary, traveled to London alone to confront Bessemer at his offices, arguing that his success was based on the results of her father’s work. Bessemer decided to pay Mushet an annual pension of £300, a very considerable sum, which he did for over 20 years, possibly to keep the Mushets from legal action.

After 1890, the Bessemer process was gradually supplanted by open-hearth steel making. Sir Carl Wilhelm Siemens developed the Siemens regenerative furnace in the 1850s and claimed in 1857 to be recovering enough heat to save 70–80% of the fuel. This furnace operated at a high temperature by using regenerative preheating of fuel and air for combustion. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, but Siemens did not initially use it for that. In 1865, the French engineer Pierre-Émile Martin took out a license from Siemens and first applied his regenerative furnace for making steel. The most appealing characteristic of the Siemens regenerative furnace is the rapid production of large quantities of basic steel, used for example to construct high-rise buildings.

The most appealing characteristic of the Siemens regenerative furnace was the rapid production of large quantities of basic steel, used for example to construct high-rise buildings. Through Siemens’ method, an open-hearth furnace could reach temperatures high enough to melt steel, but Siemens did not initially use it for that. It was Martin who first applied the regenerative furnace for making steel.

The Siemens-Martin process complemented rather than replaced the Bessemer process. It was slower and thus easier to control. It also permitted the melting and refining of large amounts of scrap steel, further lowering steel production costs and recycling an otherwise troublesome waste material. Its worst drawback was and remains the fact that melting and refining a charge takes several hours. Furthermore, the work environment around an open hearth furnace was and remains extremely dangerous.

The Siemens-Martin process became the leading steel making process by the early 20th century. The availability of cheap steel allowed larger bridges, railroads, skyscrapers, and ships. Other important steel products—also made using the open hearth process—were steel cable, steel rod, and sheet steel which enabled large, high-pressure boilers and high-tensile strength steel for machinery, creating much more powerful engines, gears, and axles than were previously possible. With large amounts of steel, it also became possible to build much more powerful guns and carriages, tanks, armored fighting vehicles, and naval ships.

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