Iron Through The Ages

Topics Covered

Introduction

Iron makes up a large proportion of the Earth's crust and by volume is the 4th most abundant element on earth behind Oxygen, Silicon and Aluminum. But, how was it first discovered and why is it so important and intrinsically linked with the development of man?

The early name for iron literally means “stone from heaven” in several languages. This name probably resulted because the first iron used by ancient man was found in the form of Meteorites which have been falling on the earth since its beginning. The meteorites frequently contained large quantities of metallic iron, and the first iron used by ancient man was from these “stones from heaven.”

The first iron extracted from the naturally occurring iron oxide deposits could have resulted from primitive man building a wood (or charcoal) fire at the base of some windswept cliff. If the cliff outcropping happened to be rich in iron oxide, after days—or even months—of use, the fire cooled and the ashes contained a small amount of iron sponge. This sponge was the iron that remained after the oxygen had been removed. The iron had not been melted during its production. Like the iron in the stones from heaven, the sponge could be hammered into shapes and was relatively strong. Spears, arrow tips, daggers, and other tools and weapons could be fabricated.

Created from content provided by ASM International in the book "Metallurgy for the Non-Metallurgist, Second Edition Editor(s): Arthur C. Reardon"

The first iron to be developed was wrought iron. Wrought iron can readily be forged to almost any shape. In fact, the term wrought means “to shape by hammering or beating.” Wrought iron is very ductile for forging because it contains very little carbon (less than 0.05%). Carbon is an interstitial impurity that strengthens the iron lattice. In fact, iron alloys that contain between approximately 0.1 and 2.0 wt% C have a special name: steels.

Iron Smelting

As with the reduction of copper sulfide ores, the first reduction of iron oxide was probably accidental. It was the powers of observation that led these ancient metallurgists (who also served as the miners, chemists, and technologists of their day) to realize that iron could be produced in simple furnaces by direct carbon reduction of the oxide ore. The exact date of this first intentional reduction of iron cannot be established, but it is certain that the making and shaping of iron was widespread in Egypt by 1500 B.C. For the next 3000 years, techniques for the production of iron did not significantly change. Iron sponge was produced by carbon reduction of the oxides; iron products were made by pounding the sponge.

Iron oxide ores are present in many areas of the earth. Thus, roughly at the same time ancient man was reducing iron ores in Egypt, it also was being done in other areas. China, India, Africa, and Malaya served as sites for this initial development of ironmaking practices. It is perhaps significant that the furnaces developed in these countries were all quite similar. There were differences in shape and size, but the furnaces were functionally identical. The chemical reduction to iron occurred without melting, and the resulting metal was relatively pure and soft. It could be hammered into useful shapes and was termed wrought iron.

Unlike the reduction of copper sulfide ores, field smelting of iron is not practical because higher temperatures and a more carefully controlled environment are needed. Smelting of iron oxides requires some carbon or carbon monoxide to react with the iron oxide to produce iron and carbon dioxide. Iron smelting, therefore, must use carbon in the form of charcoal or coke, for two purposes. The carbon must react with oxygen or burn to supply the heat necessary for the smelting operation. In this case, the carbon is serving as a fuel. However, the carbon also serves as a reducing or reacting agent to free the iron from the iron oxide. The carbon reacts with the iron ore, forming carbon dioxide and iron. Early iron smelting operations were conducted in chimney furnaces, such as the one illustrated in Fig. 1. These furnaces could easily have been developed after the first accidental reduction of iron oxide.

Figure 1. An Early American Chimney / Blast Furnace

Blast Furnaces

Improved blast furnace designs enabled high-er temperatures to be reached until the temperature exceeded the melting point of the iron. The molten iron dissolved carbon and other impurities, which was not the case in the production of sponge and wrought iron. The product from the furnaces that melted the iron was called pig iron. Typical pig iron compositions are given in Table 1. The raw material used in the production of pig iron also differed from the raw materials used for wrought iron. Instead of iron ore and wood, iron ore, coke (made from coal), and a flux (usually limestone) were used. The limestone aided in the formation of slag that floated on top of the iron and removed impurities from the molten iron. Because of the blast of air that was forced through the furnace, the furnaces became known as blast furnaces.

Table. 1 Typical Chemical Composition of Pig Iron

The use of blast furnaces greatly increased the rate at which iron could be produced. A schematic of a blast furnace is shown in Fig. 2. In such a furnace approximately two units of iron ore, one unit of coke, one-half unit of limestone, and four units of air (the blast) are required for each unit of pig iron produced. The smelting process in a blast furnace begins when the charge of ore, coke, and limestone is loaded into the top.

The temperatures in a blast furnace are approximately 150 to 200 °C (300 to 400 °F) because of a rising stream of hot carbon monoxide that results from combustion (burning) of the coke. At this temperature, the carbon monoxide begins to react with the iron ore to free some of the iron. At the same time, some of the carbon monoxide is cooled by the charge and forms carbon dioxide and free carbon. This free carbon is the soot that darkened the skies over iron-producing cities from the 16th through the mid-20th century. Removal of this soot from the rising blast is difficult and represents an ever-present problem common to the production of metals from their ores.

Figure 2. Schematic of a Blast Furnace

The control of production practices to produce high-quality products without the contamination of the environment is a relatively modern innovation. Environmental contamination was not considered a problem until recently. However, the production of soot was well known, as illustrated in Fig. 3, which is a drawing of an early version of the blast furnace. This drawing appeared in Georgius Agricola’s book De Re Metallica, which was prepared in 1556 and translated from Latin into English by Herbert Hoover and his wife Lou Henry Hoover.

Figure 3. Sketch from De Re Metallica showing soot emissions from a medieval blast furnace

Strengthening - Wrought Iron

The pig iron produced in blast furnaces has the advantage of being rapidly produced, but pig iron is very hard and brittle due to high levels of carbon (Table 1). Wrought iron is ductile and tough but has little strength, while pig iron is strong but brittle and lacks the toughness to withstand sudden blows without breaking. In fact, the cast bar of a pig iron sword would shatter if it was hit against a tree. The reason for this difference in behavior is mainly due to the potent strengthening effects of carbon in iron.

Wrought iron is highly refined metallic iron that contains minute, relatively uniformly distributed insoluble nonmetallic particles. Typical distribution of these particles in a section of wrought iron is shown in Fig. 4. The individual particles are termed inclusions.

Figure 4. Micrograph showing typical slag inclusions in wrought iron. Original magnification: 500×.

When inclusions are aligned in the direction of metal flow during hammering as shown in Fig. 4, the groupings of inclusions are designated as stringers. Although approximately 1 to 4% of the volume of any wrought iron sample is inclusions, a typical chemical analysis of wrought iron shows the presence of very little carbon, silicon, and other elements found in pig iron (Table 1).

Unlike wrought irons, the lack of purity (i.e., high carbon content) in pig iron makes it hard and brittle. This brittleness makes pig iron difficult to shape by hammering or forging techniques. However, pig iron can be cast. In fact, carbon levels of pig iron (Table 1) approach the eutectic point of 4.30 wt% C in the iron-carbon system. In the iron-carbon system, the eutectic composition (4.30 wt% C) results in a melting point of 1140 °C. This is much lower than the melting point of pure iron. Eutectic compositions offer the advantage of complete melting at lower temperatures and rapid solidification (without a mushy zone).

The Development of Steel

Steels, which are irons containing between approximately 0.1 and 2% C, combine the toughness of wrought iron with the strength of pig iron. Steels can be made from wrought iron by heating the iron for a long period of time in a red hot charcoal fire. The hot iron absorbs carbon from the charcoal, so that carbon atoms diffuse into the iron lattice. The result is an iron-carbon solid-solution alloy that we know as steel. The increase in carbon also provides other advantages in the strengthening of steel when it is quenched from elevated temperatures. The Hittites played a major role in the early development of steel, and by the time of the writing of The Odyssey, heat treatment of steel was well known to the Greeks. The blinding of Polyphemus by Odysseus was described, “As when the smith plunges the hissing blade deep in cold water [whence the strength of steel], so hissed his eye around the olive wood.”

The development of steel into a material of common use was critical to history and societal development globally. For example, the Celtic invasion of Italy in 223 B.C. was unsuccessful partially because of the poor quality of Celtic weapons. The Romans had steel; the Celts had iron. The iron swords and daggers easily bent, and although one man might be killed or injured if the first blow was successful, it was difficult to fight with swords that had to be straightened between blows. The nation without steel was therefore not equipped to fight a nation with steel. Therefore, the possession of steel shaped the development of nations.

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This Article was created from Material provided by ASM International in the book "Metallurgy for the Non-Metallurgist, Second Edition" Edited by A.C. Reardon.

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