Iron and steel
by Chris Woodford. Last updated: May 2, 2022.
Think of the greatest structures of the
19th century—the Eiffel
Tower, the Capitol, the Statue of Liberty—and you'll be thinking of iron.
[1]
The fourth most common element in Earth's crust, iron has been in
widespread use now for about 6000
years.
[2]
Hugely versatile, and one of the strongest and cheapest
metals, it became an important building block of the Industrial
Revolution, but it's also an essential element in plant and animal
life. Combined with varying (but tiny) amounts of carbon, iron makes
a much stronger material called steel, used
in a huge range of
human-made objects, from cutlery to warships, skyscrapers, and space
rockets. Let's take a closer look at these two superb materials and find out what
makes them so popular!
Photo: The world's first cast-iron bridge, after which the village
of Ironbridge in Shropshire, England was named. It was built across the River Severn by Abraham Darby III in 1779
using some 384 tons of iron. You can read more about its history and construction
on the official Ironbridge website. Photo by Jason Smith courtesy of Wikimedia Commons.
What is iron like?
Photo: A sample of iron from a meteorite (next
to a pen for scale). From the mineral collection of Brigham Young
University Department of Geology, Provo, Utah. Photograph by Andrew
Silver courtesy of US Geological Survey Photographic Library.
You might think of iron as a hard, strong metal tough enough to
support bridges
and buildings, but that's not pure
iron. What we have there is alloys of iron
(iron combined with carbon and other elements), which we'll explain
in more detail in a moment. Pure iron is
a different matter altogether. Consider its physical properties (how
it behaves by itself) and its chemical properties (how it combines
and reacts with other elements and compounds).
Physical properties
Pure iron is a silvery-white metal that's easy to work and shape and
it's just soft enough to cut through (with quite a bit of difficulty)
using a
knife. You can hammer iron into sheets and draw it into wires. Like
most metals, iron conducts electricity
and heat very well and it's
very easy to magnetize.
Chemical properties
The reason we so rarely see pure iron is that it combines readily
with oxygen (from the air). Indeed, iron's major drawback as a construction material
is that it reacts with moist air (in a process called corrosion)
to form the flaky, reddish-brown oxide we call rust.
Iron reacts in lots of other ways too—with elements ranging from carbon, sulfur, and silicon
to halogens such as chlorine.
Broadly, iron's compounds can be divided into two groups known as
ferrous and
ferric (the old names) or iron (II) and iron (III); you
can always
substitute "iron(II)" for "ferrous" and "iron(III)" for
"ferric" in compound names.
Where does iron come from?
Photo: Iron is essential for a healthy diet, which is
why it's packed into many breakfast cereals. Here's a great little experiment from Scientific American to
extract the iron from your cornflakes.
Iron is the fourth most common element in Earth's crust
(after oxygen, silicon, and aluminum), and the second most common metal (after aluminum), but
because it reacts so readily with oxygen it's never mined in its
pure form (though meteorites are occasionally discovered that
contain samples of pure iron). Like aluminum, most iron "locked"
inside Earth exists in the form of oxides
(compounds of iron and oxygen). Iron oxides exist in seven main ores
(raw, rocky minerals mined from Earth):
- Hematite (the most plentiful)
- Limonite (also called brown ore or bog iron)
- Goethite
- Magnetite (black ore; the magnetic type of iron oxide, also called lodestone)
- Pyrite
- Siderite
- Taconite (a combination of hematite and magnetite).
Different ores contain different amounts of
iron. Hematite and magnetite have about 70 percent
iron, limonite has about 60 percent, pyrite and siderite have 50
percent, while taconite has only 30 percent. Using a combination of
both deep mining (under the ground) and opencast mining (on the
surface), the world produces approximately 1000 million tons of iron
ore each
year, with China responsible for just over half of it.
Which countries produce the world's iron? As you can see, China utterly dominates
as the source of about two thirds of the iron we use. Chart shows estimated figures for pig iron for 2021. In the United States, three companies currently produce pig iron in 11 different locations.
Source: US Geological Survey, Mineral Commodity Summaries, January 2022.
Types of iron
Pure iron is too soft and reactive to be of much real use, so most
of the "iron" we tend to use for everyday purposes is actually in the
form of iron alloys: iron mixed with other
elements (especially carbon) to make stronger, more resilient forms of the metal including
steel. Broadly speaking, steel is an alloy of iron that contains up
to about 2 percent carbon, while other forms of iron contain about
2–4 percent carbon. In fact, there are thousands of
different kinds of iron and steel, all containing slightly different
amounts of other alloying elements.
Pig iron
Basic raw iron is called pig iron because it's produced in the form of
chunky molded blocks known as pigs. Pig iron is made by
heating an iron ore (rich
in iron oxide) in a blast furnace: an enormous industrial fireplace,
shaped like a cylinder, into which huge drafts of hot air are
introduced in regular "blasts". Blast furnaces are often
spectacularly huge: some are 30–60m (100–200ft) high, hold dozens of
trucks worth of raw materials, and often operate continuously for years
at
a time without being switched off or cooled down. Inside the
furnace, the iron ore reacts chemically with coke (a carbon-rich form
of coal) and limestone. The coke "steals" the oxygen from the iron
oxide (in a chemical process called reduction), leaving behind a
relatively pure liquid iron, while the
limestone helps to remove the other parts of the rocky ore (including
clay, sand, and small stones), which form a waste slurry known as
slag. The iron made in a blast furnace is an alloy containing about
90–95 percent iron, 3–4 percent carbon, and traces of other elements
such as silicon, manganese, and phosphorus, depending on the ore
used. Pig iron is much harder than 100 percent pure iron, but still too
weak for most everyday purposes.
Cast iron
Photo: The cast-iron dome of the US Capitol. Credit: The George F. Landegger Collection of District of Columbia Photographs in Carol M. Highsmith's America,
Library of Congress, Prints and Photographs Division.
One of the world's most famous iron buildings, the Capitol in Washington, DC has a dome made of
4,041,146kg (8,909,200 pounds) of cast iron.
Cast iron is simply liquid iron that has been cast: poured into a
mold and allowed to cool and harden to form a finished structural shape, such as a pipe,
a gear, or a big girder for an iron bridge. Pig iron is actually a
very basic form of cast iron, but it's molded only very crudely
because it's typically melted down to make steel. The high carbon
content of cast iron (about the same as pig iron—roughly 2–4 percent) makes it
extremely hard and brittle: large crystals of carbon embedded
in cast iron stop the crystals of iron from moving about. Cast iron
has two big drawbacks: first, because it's hard and brittle, it's
virtually impossible to shape, even when heated; second, it rusts
relatively easily. It's worth noting that there are
actually several different types of cast iron, including white and
gray cast irons (named for the coloring of the finished product
caused by the way the carbon inside it behaves).
Wrought iron
Cast iron assumes its finished shape the moment the liquid iron
alloy cools
down in the mold. Wrought iron is a very different material made by
mixing liquid iron with some slag (leftover waste). The result is an iron alloy with a
much lower carbon content. Wrought iron is softer than cast iron and
much less tough, so you can heat it up to shape it relatively easily,
and it's also much less prone to rusting. However, relatively little
wrought iron is now produced commercially, since most of the objects
originally produced from it are now made from steel, which is
both cheaper and generally of more consistent quality. Wrought iron is
what people used to use before they really mastered
making steel in large quantities in the mid-19th century.
Photo: Three types of iron. Left: Pig iron is the raw material used to make
other forms of iron and steel. Each of these iron pieces is one pig. Middle: Cast iron was used for strong, structural components like bits of engines and bridges before steel became popular. Right: Wrought iron is a softer iron once widely used to make everyday things like street railings. Today, wrought iron is more of a marketing description for what is actually mild steel (low-carbon steel),
which is easily worked and shaped. Left photo by Alfred T. Palmer courtesy of US Library of Congress. Middle and right photos by explainthatstuff.com.
Types of steel
Strictly speaking, steel is just another type of iron alloy, but it
has a much lower carbon content than cast iron and roughly the
same (or sometimes slightly more) carbon than wrought iron,
and other metals are often added to give it extra properties.
[3]
Steel is such an amazingly useful material that we tend to talk about it as though it
were a metal in its own right—a kind of sleeker, more modern "son
of iron" that's taken over the family firm! It's important to
remember two things, however. First, steel is still essentially (and
overwhelmingly) made from iron. Second, there are literally thousands
of different types of steel, many of them precisely designed by
materials scientists to perform a particular job under very exacting
conditions. When we talk about "steel", we usually mean "steels";
broadly speaking, steels fall into four groups: carbon steels, alloy
steels, tool steels, and stainless steels. These names can be
confusing, because all alloy steels contain carbon (as do all
other
steels), all carbon steels are also alloys, and both tool steels and
stainless steels are alloys too.
Chart: Which countries produce the world's raw steel? Again, China utterly dominates.
Approximately 1.9 billion metric tons of steel are made worldwide each
year, and half of it comes from China. This chart shows estimated worldwide raw steel production figures for
the years 2018 (inner ring)–2021 (outer ring). In the United States, there were 101 "minimill" steel plants operating at the start of 2021 (down from 110 in 2018) making a total of about 106 million tons of steel (slightly down from 114 million tons in 2015). Indiana (27 percent), Ohio (11 percent), Pennsylvania (5 percent), Illinois and Texas (4 percent each) and Michigan (3 percent) together produce about half of all US steel. Source: US Geological Survey, Mineral Commodity Summaries, January 2022.
Carbon steels
The vast majority of steel produced each day (around 80–90 percent) is what we
call carbon steel, though it contains only a tiny amount of
carbon—sometimes much less than 1 percent.
In other words, carbon steel is just basic, ordinary steel. Steels with
about 1–2 percent carbon are called (not surprisingly)
high-carbon steels and, like cast-iron, they
tend to be hard and
brittle; steels with less than 1 percent carbon are known as
low-carbon steels ("mild steels") and like wrought iron, are
softer and easier to shape. A huge range of different everyday items are made with carbon
steels, from car bodies and warship hulls to steel cans and engine
parts.
Alloy steels
As well as iron and carbon, alloy steels contain one or more other
elements,
such as chromium, copper, manganese,
nickel, silicon, or vanadium.
In alloy steels, it's these extra elements that make the difference
and provide some important additional feature or improved property
compared to ordinary carbon steels. Alloy steels are generally
stronger, harder, tougher, and more durable than carbon steels.
Tool steels
Tool steels are especially hard alloy steels used to make tools,
dies, and
machine parts. They're made from iron and carbon with added elements
such as nickel, molybdenum, or tungsten to give extra hardness and
resistance to wear. Tool steels are also toughened up by a process
called tempering, in which steel is first
heated to a high
temperature, then cooled very quickly, then heated again to a lower
temperature.
Stainless steels
The steel you probably see most often is stainless steel—used in
household cutlery, scissors, and medical instruments. Stainless steels contain
a high proportion of chromium and nickel, are very resistant to corrosion and other chemical reactions, and are easy to clean, polish, and sterilize.
They're corrosion-proof because the chromium atoms react with oxygen in the air to
form a kind of protective outer skin that stops oxygen and water from attacking
the vulnerable iron atoms inside.
Making steel
There are three main stages involved in making a steel product. First,
you make
the steel from iron. Second, you treat the steel to improve its
properties (perhaps by tempering it or plating it with another metal).
Finally, you roll or otherwise shape the steel into the finished
product.
Making steel from iron
Photo: Making steel from iron with a Bessemer converter. It turns iron into steel with help from oxygen in the air. Photo by Alfred T. Palmer courtesy of US Library of Congress.
Most steel is made from pig iron (remember: that's an iron alloy
containing
up to 4 percent carbon) by one of several different processes
designed to remove some of the carbon and (optionally) substitute one
or more other elements. The three main steelmaking processes are:
- Basic oxygen process (BOP): The steel
is made in a giant egg-shaped container, open at the top, called a basic oxygen furnace, which is
similar to an ordinary blast furnace, only it can rotate to one side to pour off
the finished metal. The air draft used in a blast furnace is
replaced with an injection of pure oxygen through a pipe called a
lance. The basic idea is based on the Bessemer process developed by
Sir Henry Bessemer in the 1850s.
- Open-hearth process (also called the
regenerative open hearth): A bit like a giant fireplace in which pig iron, scrap steel, and iron ore are
burned with limestone until they fuse together. More pig iron is
added, the unwanted carbon combines with oxygen, the impurities are
removed as slag and the iron turns to molten steel. Skilled workers
sample the steel and continue the process until the iron has exactly
the right carbon content to make a particular type of steel.
- Electric-furnace process: You don't cook your dinner with an open fire, so why
make
steel in such a primitive way? That's the thinking behind the
electric furnace, which uses electric arcs (effectively giant
sparks) to melt pig iron or scrap steel. Since they're much more
controllable, electric furnaces are generally used to make
higher-specification alloy, carbon, and tool steels.
Photo: Making steel for weaponry with the three-ton
electric arc furnace at Rock Island Arsenal. Photo by Tony Lopez courtesy of Defense Imagery.
Making steel products
Liquid steel made by one of these processes is cast into huge bars
called ingots,
each of which weighs anything from a couple of tons (in typical steel
plants) to hundreds of tons (in really big plants making giant steel
objects). The ingots are
rolled and
pressed to make three types of basic steel "building blocks" known as blooms
(giant bars with
square ends), slabs (blooms with rectangular
ends), and billets
(longer than blooms but with smaller square ends).
These blocks are then shaped and worked to make all kinds of final
steel products. The
basic shaping process usually involves hot rolling
(for
example, reheating
blooms and then rolling them over and over again to make them
thinner). Girders are made by rolling steel then forcing it through
dies or milling machines to make such things as beams for buildings
and railroad tracks. Rollers that are very close together can be used
to squeeze steel into extremely thin sheets. Pipes are made by wrapping
sheets
round into circles then forcing the two edges together so they fuse
under pressure where they join.
Shaped steel can be further treated
in all kinds of ways. For example, "tins"
for food containers (which are mostly steel) are made by electroplating
steel sheets with molten tin
using the process of electrolysis (the reverse of the
electro-chemical
process that happens in batteries). Steel
that needs to be especially
resistant to weathering can be galvanized (dipped into a hot bath of
molten zinc so it acquires an overall protective coating).
