Steel

• 1. Introduction

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  Steel begins with a feat that ancient people could not perform. Iron melts at about 1540 degrees Celsius, a temperature out of reach when the Iron Age began. Tin melts near 250 degrees, copper near 1100, and bronze below 1083, so those metals came first. Yet the oldest known pieces of steel were excavated in Anatolia, at a site called Kaman-Kalehoyuk, and they are nearly 4,000 years old, dating from 1800 BC. From that early ironware to a world that now pours more than 1.6 billion tons a year, steel has been a constant companion. It hides inside bridges, ships, trains, cars, and the steel skeletons of skyscrapers. This documentary asks how a simple alloy of iron and carbon became one of the most manufactured materials on Earth. How does carbon, added in tiny amounts, transform soft iron into something strong? Why did furnaces matter so much? And why is a metal so heavily made also one of the most recycled?

• 2. The Carbon Bargain

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  A carbon content not far exceeding 2% is what separates steel from ordinary iron. For plain carbon steel, that range runs between 0.02% and 2.14% by weight. Iron and carbon are always the main players, but the carbon does the decisive work. Introduced into the primarily-iron lattice, it inhibits deformation under mechanical stress, and that is the chief source of steel's strength.

  The bargain has a cost. The engineered mechanical properties come at the expense of the ductility and elongation of pure iron, both of which decrease as carbon is added. Push the carbon past 2.1% and the material is no longer steel at all. Plain iron-carbon alloys above that level are known as cast iron, which is not malleable even when hot, though its lower melting point gives it good castability.

  Wrought iron sits at the other extreme, now largely obsolete. It may carry a small amount of carbon, less than 0.1%, but it also holds large amounts of slag, around 1 to 2%. The whole craft of steelmaking is finding the narrow window between these neighbors, and other elements then tune what that window produces.

• 3. Crystals And Heat

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  At room temperature, the most stable form of pure iron is a body-centred cubic structure called alpha iron. It is fairly soft and can dissolve only a tiny amount of carbon, no more than 0.005% at 0 degrees Celsius. Carbon dissolved in alpha iron is called ferrite. At 910 degrees, iron rearranges into a face-centred cubic structure called gamma iron, and carbon within it is called austenite.

  The open face-centred structure of austenite holds far more carbon, as much as 2.1% at 1148 degrees, which is 38 times what ferrite can absorb. When carbon leaves solution, it forms a very hard but brittle material called cementite. In a steel with exactly 0.8% carbon, known as a eutectoid steel, slow cooling makes cementite and ferrite precipitate together in a layered structure called pearlite, named for its resemblance to mother of pearl.

  Cooling speed changes everything. Quench the steel fast enough and the carbon has no time to migrate, locking into the face-centred lattice as martensite, exceedingly hard but brittle. Below 0.2% carbon martensite takes a ferrite cubic form, and at higher carbon it takes a body-centred tetragonal structure. Because martensite expands as it forms, internal stresses build, and quench cracks commonly appear when steel is water quenched even when they are not visible.

• 4. Taming The Brittle Edge

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  Annealing is the gentle hand in the workshop. The steel is heated high enough to relieve local internal stresses, passing through three phases called recovery, recrystallization, and grain growth. It does not soften the whole product, only releases strains locked within the material.

  Quenching takes the opposite path, heating steel into the austenite phase and then plunging it into water or oil. The rapid cooling leaves a hard but brittle martensitic structure. To make that structure usable, the steel is tempered, which is a specialized form of annealing aimed at reducing brittleness.

  Tempering rewrites the microstructure. Some of the martensite transforms into cementite, or into spheroidite, easing internal stresses and defects. The result is a steel that is more ductile and more resistant to fracture, ready for tools that must keep a sharp, long-lasting edge.

• 5. The Elements Within

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  Chromium changes the surface of steel itself. Add at least 11%, and a hard oxide forms on the metal, giving stainless steel its resistance to corrosion. Stainless steels typically contain 18% chromium, and corrosion-resistant grades are abbreviated CRES. Some, the ferritic stainless steels, are magnetic, while the austenitic ones are nonmagnetic.

  Other additions write their own recipes. Nickel and manganese raise tensile strength and stabilize austenite, vanadium hardens steel and resists metal fatigue, and tungsten slows the formation of cementite to produce high-speed steel. Maraging steel is alloyed with nickel yet holds almost no carbon, just 0.01%, making it very strong but still malleable. Hadfield steel, named after Robert Hadfield, carries 12 to 14% manganese and hardens its skin when abraded, which suits tank tracks and the cutting blades on the jaws of life.

  Some elements arrive as troublemakers. Phosphorus, sulphur, nitrogen, and lead are largely treated as contaminants that make steel brittle and are removed during melting. Lead and sulphur are added on purpose only when easier machining matters more than toughness, as in nuts, bolts, and washers. Eglin steel takes the opposite road, combining over a dozen elements to make a low-cost steel for bunker buster weapons.

• 6. Furnaces Through Time

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  Bloomery furnaces and crucibles carried steelmaking through antiquity, long before any factory existed. Wootz steel was developed in Southern India and Sri Lanka in the 1st millennium BC, and large-scale crucible production in India arrived by the sixth century BC. In Sri Lanka, wind furnaces driven by the monsoon winds reached temperatures high enough to make high-carbon steel, and the ancient Sinhalese extracted a ton of steel for every two tons of soil.

  The reputation of this steel traveled the ancient world. The Greek alchemist Zosimos of Panopolis attested to its Indian origin, writing that Indians made high quality swords by melting soft iron in crucibles. In 327 BC, the defeated King Porus rewarded Alexander the Great not with gold or silver but with 30 pounds of steel. Wootz and Damascus steel earned fame for durability and the ability to hold an edge.

  China pursued its own paths. The Warring States period had quench-hardened steel, and by the 1st century AD the Han dynasty made a carbon-intermediate steel by melting wrought iron together with cast iron. Centuries later, crucible steel was produced in Merv by the 9th to 10th century AD, while in the Netherlands ultrahigh-carbon steel had already been made from the 2nd to 4th centuries AD.

• 7. The Industrial Leap

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  Henry Bessemer's process, introduced in 1855, opened the modern era of steelmaking. His method produced steel cheaply and in large quantities, so mild steel took over most of the work formerly done by wrought iron. The Gilchrist-Thomas process improved on it by lining the converter with a basic material to strip out phosphorus, and the Siemens-Martin process complemented Bessemer by co-melting wrought-iron scrap with pig iron.

  Earlier centuries had laid groundwork. The cementation process was described in a treatise published in Prague in 1574 and was in use in Nuremberg by 1601, then introduced to England around 1614 and used by Sir Basil Brooke at Coalbrookdale. The best raw material was oregrounds iron from a region north of Stockholm, Sweden. Crucible steel as a modern industry sprang from Benjamin Huntsman's invention in the 1740s.

  Then the old methods fell away. The Linz-Donawitz process of basic oxygen steelmaking, developed in 1952, made the same quantity of steel in one-twelfth the time of the open hearth process by limiting impurities such as nitrogen. Electric arc furnaces now reprocess scrap metal into new steel, though they draw about 440 kWh per metric ton and need cheap electricity to make sense. Today roughly 96% of steel is continuously cast, with only 4% poured as ingots.

• 8. A World Built And Rebuilt

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  China produced 54% of the world's steel in 2023, a striking shift from the era when American production centered on Pittsburgh, Bethlehem, Pennsylvania, and Cleveland, Ohio. The economic boom in China and India drove demand upward, and between 2000 and 2005 world steel demand rose by 6%. ArcelorMittal stands as the world's largest steel producer, while firms like Tata Steel, which bought Corus Group in 2007, expanded to meet the surge.

  The industry's weight on workers and the climate is heavy. In 1980 there were more than 500,000 U.S. steelworkers, but by 2000 the number had fallen to 224,000. Steelmaking accounts for around 8% of global emissions, and proposed remedies include replacing coke-based methods with hydrogen and applying carbon capture and storage.

  Steel answers part of that burden by coming back around. It is one of the world's most-recycled materials, with a global recycling rate over 60%. In the United States alone, more than 82,000,000 metric tons were recycled in 2008, for a rate of 83%. There is one curious exception to all this renewal. Steel made after World War II carries radionuclides from atmospheric nuclear testing, so low-background steel made before 1945 is still sought for Geiger counters and radiation shielding.