Iron, Steel, and Invar
Iron
An ancient ax and spearhead made of iron.
A strong, hard metal that’s easy to shape, iron came into common use beginning around 1400 B.C. Iron working was a major advance in technology: People were able to make better tools such as farming implements relatively inexpensively, which greatly improved their lives. Iron was also used to make weapons.
Iron is even more useful today. It can be cast, or poured into a mold, to make things such as engine blocks and other automotive parts. Wrought iron (which means "worked" iron) can be shaped into lacy-looking furniture and decorative gates. Iron’s most significant claim to fame, though, is that it can be combined with other elements to make steel, which is used in countless products we rely on every day.
Steel
A modern car made of steel.
Steel is stronger than iron and considerably more versatile. From the thinnest surgical needles to immense ships, steel is the material of choice. The automotive industry uses huge quantities. And at home, you’ll find steel in your washing machine, refrigerator, and vacuum cleaner, as well as in your forks, knives, and spoons.
How can one material serve so many different needs? The answer is that there are actually thousands of different steels. Each steel is an alloy (a mixture) of iron, up to 2 percent carbon, and often other elements. About twenty different elements can be used in various combinations, and each different alloy "recipe" will give the resulting steel different properties. In addition, various ways that a steel is processed, such as heating and cooling techniques and mechanical treatments, can enhance certain properties. Do you need a material that’s exceptionally strong? Highly corrosion resistant? Easy to weld? Chances are excellent that there’s a steel just right for you.
Stainless Steel
Stainless steel flatware.
Let’s look closely at one familiar example: the stainless steel flatware that you may eat your meals with every day. Introduced to the home market in the 1930s, this flatware offered an inexpensive alternative to silverware and was also easier to care for.
The stainless steel for your flatware consists primarily of iron, nickel, and chromium, although tiny amounts of other elements may be present. The carbon content is low, 0.08 percent or less, which makes the steel easy to form. Chromium, which makes up about 18 percent of this alloy, is the key to the steel’s corrosion resistance: It reacts with oxygen in the air to form a very thin protective layer of chromium oxide on the surface of the steel. Nickel, at about 8 percent, adds to the corrosion resistance and also helps make the steel easy to shape.
One of the six stainless steel cars built in 1936.
There are many other stainless steel recipes. To make knife blades, for example, you’ll want an alloy with a higher carbon content than you’d use for forks and spoons. That’s because more carbon will make the blade easier to sharpen, although shaping the steel is more difficult.
Six stainless steel cars were built in 1936. They were driven throughout the United States to introduce stainless steel to potential customers of the new stainless steel flatware and kitchen sinks. Four of these cars have survived until today in all their corrosion-resistant glory.
Making iron
Iron is abundant in the earth’s crust, but it’s trapped in iron ores—minerals in which iron has combined with oxygen to form an iron oxide. There can also be various other materials in the ore, which are known as impurities.
To separate the iron from the oxygen, the ore is mixed with coke (a form of carbon) and limestone and dropped into the top of a blast furnace. A continuous blast of extremely hot air is injected into this gigantic furnace, which could be as tall as a 15-story building. The coke is ignited and burns. The temperature in the furnace can reach about 1600 degrees Celsius, which melts the raw materials. The burning coke also produces carbon monoxide, and this gas flows up through the melted materials, attracting oxygen and reducing the iron oxide to iron. At the same time, the limestone reacts with some of the impurities, forming a liquid called slag. Slag floats on top of the denser molten iron that collects at the bottom of the furnace, so each can be "tapped off," or removed, separately.
The iron that emerges from the blast furnace, called "pig iron," may be used to make cast iron or perhaps wrought iron. Most of it, however, goes directly into making steel.
Making steel
Cross section of a blast furnace.
Pig iron contains about 4 or 5 percent carbon that it picked up in the blast furnace, and the carbon makes it brittle. That’s fine for products where strength isn’t an issue, but to make steel, much of that carbon has to go. And there are still some remaining impurities that a steelmaker would like to get rid of as well.
The most popular refining method uses a basic oxygen furnace. Molten pig iron is combined with scrap iron or scrap steel in the furnace, and oxygen is blown onto the melted mixture at an extremely high speed. The oxygen burns off some of the carbon and other impurities. When the carbon is at the desired percentage, as much as 2 percent, but often a lot less, you have what’s called carbon steel: steel with properties that are primarily determined by the amount of carbon.
If you add other elements to achieve different properties—manganese to increase strength and resistance to wear, for example, or molybdenum to improve strength and resistance to heat—you have what’s known as an alloy steel. To make an alloy steel, the basic steel is put into a huge container called a ladle, and the other elements are added.
The molten steel is cast into basic shapes of different sizes and cooled. Next up is a process called rolling. The steel is passed between two rollers that flatten and lengthen it, much as pie dough is shaped by a rolling pin. If the steel is "hot rolled," heated to a temperature of about 1200 degrees Celsius, it’s much more able to withstand stress without cracking or breaking. If the steel is "cold rolled," rolled at room temperature, it makes the steel thinner and smoother, and gives it a shiny finish. Sometimes both processes are used.
Rolling can produce many final shapes, as well. The most common are flat sheets and the narrower flat strips, but this process can also turn out structural beams and a variety of bars. Wire needs more work: It’s made by drawing a round bar through a series of smaller and smaller holes called dies. For some intended uses, there’s additional processing, but basically this is it. The finished steel can be sent off to a customer who will build a bridge, manufacture cookware, or make millions of paper clips—whatever the particular steel was designed to do best.
Charles Edouard Guillaume and Invar
Charles Edouard Guillaume
Most metals and alloys, including steel, expand significantly when they’re heated. Engineers work around this; for example, they build expansion joints into bridges. But there are many situations in which it is so much better if the material expands hardly at all. Charles Edouard Guillaume discovered such a material.
Swiss-born physicist Guillaume spent most of his working life at the International Bureau of Weights and Measures in Sèvres, France. One project he was involved with had to do with trying to establish an international standard for the meter, the metric unit of length. Today, the meter is defined as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second. During Guillaume’s career in the late nineteenth century, however, a standard was a particular metal bar known as a prototype. Replicas were calibrated to the prototype and distributed to where they were needed.
In 1889, an international convention adopted a new prototype for the meter made of platinum and iridium. It was hard and durable, and the most precise prototype yet—but it was outrageously expensive. This troubled Guillaume, because the price of a replica put it out of reach of most scientists, and he resolved to seek a less costly solution. His work led to the discovery of a fairly inexpensive iron-nickel alloy—a steelike material—that expands very little when heated. He named the alloy Invar because it was almost unchanging or "invariable." It has the lowest thermal expansion of metals and alloys at temperatures ranging from room temperature to more than 200 degrees Celsius.
Although Invar was never used as a meter prototype, it’s been put to work in standards for length measurement used by surveyors. It’s also used in a vast number of temperature-sensitive devices. In today’s home, for instance, you’ll find Invar in toasters, irons, and gas stoves, as well as in CRT computer monitors and TV sets.
In 1920, Charles Edouard Guillaume was awarded the Nobel Prize in physics "in recognition of the service he has rendered to precision measurements in physics by his discovery of anomalies in nickel steel alloys."
CRT monitors and toasters are examples of Invar use today.
First published 2 January 2007
