We take many of the things for granted these days, but as a materials engineer I have always
found it incredible how much we take materials for granted. Everything we build is dependant
on these materials. They are so significant that we have named entire periods of human
history after them. From the stone age to the information (space) age. They have all
been made possible by the materials we have at our disposal and our mastery of their properties.
I have spoken about Aluminium and Silicon before. But today we are going to talk about
one of the most influential periods in human history. The Iron Age.
Some of the earliest evidence of iron being used as a material goes back as far as 3500
BC in Egypt where beads of iron taken from a meteor were found. Meteoric iron was a highly
prized material due to it’s heavenly association. Tutankhamun was buried with a dagger made
of the material, but meteoric iron was the only naturally occuring source of iron at
the time, because Iron reacts readily with oxygen to form Iron ore. There is no oxygen
in space so meteors delivered this material to earth in a form humans could use without
having the technology to extract it from it’s ore.
The Iron age began at various points across the world as humans started to learn how to
extract Iron from its ore and it’s end date varies between regions too, in Britain the
Iron age began around 800 BC and ended when the Roman’s invaded in 43 AD, marking the
start of the Roman Age. If we continued to define human history by the materials being
mastered at that time, I would argue that the Iron age lasted right up until a little
over 150 years ago, when steel was first mass produced. Now while this era is called the
Iron age, the best weapons at the time were made from steel. You may not have known, but
Iron and Steel are are mostly the same material.
The main difference between the Iron and Steel is the amount of carbon they contain. Anything
with a carbon content above 2% is cast iron. In general, a higher carbon content results
in a harder and less ductile material. Cast iron has a very high carbon content, which
makes it very hard, but also very brittle. As iron started to become more popular more
and more of the early bronze cannons were replaced with cast iron, as it was cheap to
manufacture and could be fired more often without being damaged, but these material
properties meant that cast iron cannons had a tendency to explode with no warning making
them dangerous to operate.
Cast Iron is not suited for structural use either. In fact it’s use in bridges in the
mid 19th century led to a series bridge collapses. Later these bridges were rebuilt using wrought
iron. Wrought iron contains less that 0.08% carbon, which makes it a much better material
for applications like this. As it is ductile, allowing it to bend under loads without breaking,
but it has a low carbon content, which makes it a lot softer than cast iron. Steel is between
the two with a carbon content between 0.2 and 2 percent. Giving it an ideal balance
between hardness and ductility. The history of Iron is defined by our ability to control
that carbon content.
Iron is the 4th most common metal on earth, just below aluminium, but it reacts with oxygen
readily to form iron oxide ores.
Rust is one form of iron oxide and preventing it is a constant struggle in structural maintenance.
The eiffel tower has been painted 17 times since it’s construction to protect it from
that corrosion. Every 7 years about 60 tonnes of paint is applied to the Eiffel Tower and
the colour of paint has changed over the years. The tower was originally a venetian red and
has changed a few times from a more yellowish brown to a chestnut brown until the adoption
of the current, specially mixed “Eiffel Tower Brown” in 1968.
Because Iron reacts so readily with oxygen to form iron oxide. Iron does not exist on
the surface of the planet in a usable form. The first step to process iron is to remove
that oxygen.
In the mid bronze age the first signs of production of Iron are seen. Most of this early iron
was smelted in these furnace called bloomeries. One of my favourite channels on YouTube, primitive
technologies actually created a miniature version in one of his videos.
These bloomeries heat the iron ore using charcoal as a heat source. The burning of charcoal
produces carbon monoxide, which reacts with the iron oxide in the ore to form carbon dioxide
and iron. The bloomery is heated above the melting point of the impurities, but below
the melting point of iron. And so as the fire rages, material falls to the bottom of the
bloomery and the heavier iron consolidates at the bottom, while the impurities form a
molten pool called slag, which can be drained away. When the iron is removed it is in the
form of this porous mixture of impurities and iron. It needs to be worked with a hammer
to consolidate the iron, while the waste material is beaten off. The material left over is wrought
iron, which as we discussed before has a very low carbon content. These bloomeries produced
very small quantities of iron especially before the waterwheel was introduced to drive the
bellows, which allowed the bloomery to grow in size while keeping the temperature high
enough.
Despite the small quantities it produced the bloomery revolutionised human life, even beyond
the obvious military advantages of iron weapons. Iron ore is much more common than the copper
and tin that spurred the bronze age, allowing iron to be produced in many areas. These communities
could make their own tools and weapons without having to import the material from abroad.
Iron plows were stronger and heavier allowing farmers to plow their land quicker and thus
grow more food. Likewise iron scythes could cut more hay. A single farmer could feed more
people, allowing more people to dedicate their lives to different trades. Society was becoming
more stratified and trade was increasing and things began to accelerate even more as we
discovered better ways of extracting iron, like the blast furnace.
Blast furnaces increased the production of iron dramatically. Blast furnaces do heat
the iron above it’s melting point along with flux materials. A flux is a chemical
that will combine with the impurities allowing them to be extracted easily, in this case
the iron ore is mixed with limestone and coke.
The furnace gets its name from the method that is used to heat it. Pre-heated air at
about 1000oC is blasted into the furnace through nozzles near its base.
The largest Blast Furnaces in the UK produce around 60 000 tonnes of iron per week. The
blast furnace at Redcar, which is one of the largest in Europe, has produced up to 11 000
tonnes per day (77 000 tonnes per week) but is currently running at 8000 tonnes per day.
This is equivalent to all the iron needed for about 5 cars every minute.
Coke is a refined form of coal with very little impurities and it works similar to the charcoal
in the bloomeries by producing carbon monoxide when burned, which in turn reacts with the
oxygen in the iron ore to remove it, as shown before.The heat from the process decomposes
the limestone into calcium oxide and carbon dioxide.The calcium oxide then reacts with
the silica impurities in the ore to form calcium silicate. This along with other impurities
form a liquid slag layer that floats on top of the heavy molten iron, which can be drained
away.
This method allowed vast quantities of ore to be converted to iron quickly, but it has
a drawback. At higher temperatures iron readily absorbs carbon. So the iron created in blast
furnaces has a very high carbon content, making it cast iron. So an extra step is needed to
decrease the carbon content to produce iron or steel.
This can be done in a number of ways. Refineries heat the iron back up to oxidise the carbon.
It would then be beaten with a hammer to knock the oxidised carbon out of the material, to
produce wrought iron once again.
There were methods of producing it, but the small yield and time needed made it expensive.
One way, which small quantities were being produced by was to mix wrought iron and cast
iron in a sealed crucible, which prevented atmospheric carbon from entering the material.
One of Awe Me’s videos demonstrated this technique. The primary method for producing
steel at the time involved heating wrought iron with charcoal and leaving it for up to
a week to allow it to absorb the carbon. The time and fuel needed to do this was prohibitive,
making steel expensive and not suitable for general industrial use.
Wrought iron was now being produced at an industrial scale, but a method for mass producing
steel was still not available.
With the expansion of the railroads in the early 19th century the pressure to develop
a faster and cheaper method was growing. All our modern rail tracks are made from high
strength steel, it’s superior hardness over wrought iron allows it to resist wear. This
is the difference between a worn steel rail and a new one, this kind of wear happened
so quickly with wrought iron that certain sections of popular lines needed to be replaced
every 6 to 8 weeks. Steel also has a superior strength over wrought iron, allowing it to
carry more load, if you watched my last video, you will know why this I shape helps the rail
carry even more load.
If you watched my last video you will know why this shape was used for the rail blah
blah.
So you can see why finding a method of mass production was so important. And this is where
the British Metallurgist Sir Henry Bessermer came in. Bessemer designed a converter that
looked liked this. Molten iron was poured in here from a blast furnace and hot air is
passed through the bottom. This oxygen in the air oxidises the impurities in the iron.
The carbon reacts to form carbon monoxide which is expelled as a gas. While the silicon
and manganese, oxidise to form a layer of slag. This process was very fast, in fact
early on it was a victim of it’s own efficiency, as it it removed too much carbon and left
too much oxygen in the iron. To combat this another alloy, that I am definitely about
to pronounce wrong, containing iron, carbon and manganese called spiegeleisen was added.
The manganese would react with the oxygen to remove it and the carbon increased the
carbon content as needed.
But it had another problem in the early days. The process did not remove phosphorus from
the iron and high concentrations of phosphorus make the steel brittle. So initially the bessemer
converter could only be used with iron obtained from ores with low phosphorus concentrations,
which were scare and expensive. This problem was later solved by Welshman Sidney Gilchrist
Thomas, who discovered that adding a chemically basic material like limestone to the process
would draw the phosphorus into the slag.
This availability of cheap steel caused an explosion in growth in the rail industry.
Steel is so vital to our daily lives, that it is often considered a measure of economic
success of a country. A high production of steel means a high demand for steel, a high
demand means your country is building infrastructure. For example this a graph showing China’s
steel production from the 1990s to present shows the rapid rise of China as a global
superpower during their economic reform.
Without steel our buildings could never have grown to the heights we see today, bridges
like the famous golden gate bridge would have been impossible. There is even more to learn
about steel’s fascinating history like how the expert blacksmiths of Japan managed to
create the Katana. They learn how to carefully control the crystalline structure of their
steel to forge the perfect blade, but we will talk about that in another video.