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Facts of the Matter Richard Brill |
Steel yourself
for the ‘fortuitous
coincidence’ of ironIron frames the structure of civilization the way it forms the skeletons of skyscrapers. It is the stuff from which weapons are made, whether they be primitive swords, guns or high-tech tanks. Rails made of iron allowed cities to expand outward, and brought people and supplies to new regions. Iron plows revolutionized farming, just as iron tools revolutionized woodworking. We cook in iron pots, eat with iron utensils, ride in iron automobiles, busses, trains and ships powered by iron engines as we occupy steel-framed buildings that rise into the clouds.
The discovery of how to smelt iron was one of the most important in history. The transition from bronze to iron weapons gave the iron-age warriors dominance to overcome their Bronze Age opponents.
How lucky are we live on a planet that has such copious amounts of iron. What other substance could have played the role that iron played in the growth and development of civilization?
Iron is the fourth most abundant element comprising planet Earth -- overall and in the earth's crust -- behind oxygen, silicon, and either magnesium (overall) or aluminum (crust).
Iron is a very reactive metal and combines easily with oxygen, as is the case when iron rusts. For this reason iron is never found uncombined on earth, except in the occasional meteorite. Iron's reactivity is also the reason why geological processes can concentrate iron oxide in massive ore bodies.
The smelting of iron ore consists of removing the oxygen from the iron oxide, which requires breaking the enthusiastic bond between iron and oxygen atoms. This is done by heating the iron oxide in the presence of carbon, which steals the oxygen away from the iron.
Without the addition of carbon, pure iron metal is too soft to be of much use. The irony of iron and the coincidence of history are that it is impossible to smelt iron without carbon being added to it.
No one knows how iron smelting was discovered, although it is an extension of the process used to extract the other metals of antiquity from their ores. Metallic iron came to be used only gradually, having been introduced around 1900 BC at the height of the Bronze Age but not consolidated as a material of primary use for a thousand years.
At first the metallic iron produced was soft and inferior to bronze, which had held the high-tech metal distinction for the previous 3,500 years. Over the intervening three millennia, we have learned how to optimize treatment of iron as it cools, and figured out how carbon affects its properties to take advantage of the range of properties that the iron-carbon marriage produces.
Pure iron, without the added carbon, is soft and tarnishes readily. Too much carbon and it becomes brittle like graphite. Just the right amount and it becomes steel, the driving force behind the industrial age and the growth of the United States as an economic superpower.
Metals, among them iron, aluminum, copper, and magnesium, consist of crystals. Like mineral crystals the atoms in solid metal are arranged in an orderly array that repeats symmetrically in three dimensions. Crystal structure is one of two aspects that control the properties of any substance. The other is composition: what kind of atoms are in the array.
With iron there are two different crystal structures present at different temperatures, and only the high temperature form can accommodate carbon atoms. At high temperatures the marriage keeps carbon dissolved in solid solution, but at low temperatures there is a split; the split that makes iron the center of the structure of civilization that it supports.
In soft metals such as sodium and calcium, the electrons are only loosely attached to the atoms, compared to the strongly confined electrons in the structures of nonmetallic crystals, which consist of covalent or ionic bonds. Instead of being localized as in nonmetallic crystals, the negatively charged electrons are free to swarm around, their thermal energy great enough to escape the individual atoms, but too small to escape the substance as a whole. The individual atoms are best visualized as ions floating in a sea of electrons, a form of solid plasma.
The soft metals are soft because there is little resistance to rearrangement of the ions, similar to a bowl of marbles. The macro properties of metals, including electrical and heat conductivity, reflectivity, and malleability, derive from the nature of the metallic bond.
In hard transition metals such as iron, there is a small amount of covalent bonding that provides the metallic crystals with extra strength, but still the metallic bonds dominate. Lighter metals, those with small atomic numbers, tend to be less metallic and are less brittle, while heavier atoms such as lead, gold, and uranium tend to be more malleable.
Iron falls midway in atomic number, and has an ideal balance between brittleness and malleability.
There are two things that give iron its "personality." Foremost is that iron has two decidedly different crystal structures. Upon cooling, molten iron crystallizes first into a high temperature crystal structure (austentite) around 2,800 degrees Fahrenheit. When the solid iron cools below 1,650 degrees Fahrenheit a transformation takes place as the atoms rearrange into the low temperature structure (ferrite).
Carbon affects and is effected by the transformation. It lowers the temperature at which the transformation take place, and the resulting low temperature structure can not contain the carbon.
If the cooling rate is slow enough the carbon precipitates during the transformation and becomes intergrown with the iron crystals. This adds strength and brittleness to the iron, but also enriches the remaining high temperature crystals with carbon.
Eventually, as the iron cools below a critical temperature of 1,330 degrees Fahrenheit, the critical concentration of carbon is reached, the excess carbon is rejected and precipitates as iron carbide, resulting in a composite material with hard, brittle iron carbide interfingering with soft, malleable iron.
It is known as steel.
The ideal concentration of carbon is exactly 0.8%, which works out to 1 carbon atom for every 125 iron atoms. Less than this and the steel is too soft; more than this it is too brittle.
The present industrial era, which might be dubbed the "steel age," began in 1855 when the Bessemer process that combined smelting and doping iron with controlled amounts of carbon was introduced. Soon steel mills were engineered that could process huge quantities of iron ore and turn it into finished steel at low cost.
Steel optimizes the competing factors of strength and brittleness that carbon (in the form of iron carbide) adds to the iron. But steel still corrodes. The transformation from high to low temperature structure is complete by the time it reaches room temperature. Steel is all ferrite, the low temperature form, but austentite, the high temperature form, is much less subject to the corrosive action of oxygen.
Adding other metals, mostly chromium, causes the transformation to occur at lower temperatures. The right amount added to the steel allows it to cool all the way to room temperature without the transformation from austentite to ferrite occurring, to form what is called "stainless steel." Manganese, nickel, copper, and carbon in various proportions make stainless steels with slightly different properties. Along with its corrosion resistance, stainless steel is nonmagnetic, so don''t buy that trendy stainless refrigerator if displaying fridge magnets is your thing.
Various amounts of silicon also produce steel with slightly different properties such as density, rigidity, melting point, thermal expansion or resistance to chemicals. Vanadium and tungsten add both hardness and temperature resistance and are used to make high-speed drill bits and in other tooling applications.
The irony of iron is that carbon is the both the key to its release from the ore and the key to the physical properties of the metal. Its properties are unlocked when mixed with the very carbon without which it would forever remain locked away in the form preferred by Mother Nature.
If, as some literary types might protest, the term "irony" cannot be applied to the actions of nature, then I would settle for "fortuitous coincidence." Whether irony, coincidence, or design, as we move at an ever increasing rate into the post-industrial, information-driven world, let us not forget that we still live in the iron age, or a refined version of it. Just try to build a computer without it.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
