字幕列表 影片播放 列印英文字幕 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.