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  • 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 mixedEiffel Tower Brownin 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.

We take many of the things for granted these days, but as a materials engineer I have always

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The History of Iron and Steel

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    joey joey に公開 2021 年 06 月 08 日
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