Cast iron

• 1. Introduction

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  Cast iron snapped in May 1847, and five people died. A bridge carrying the Chester and Holyhead Railway across the River Dee in Chester collapsed under a passing train, less than a year after it opened. The metal at the heart of the wreck had served the world for nearly three thousand years. Its earliest artifacts date to the 8th century BC, unearthed by archaeologists in what is now Luhe County, Jiangsu, in China. Between that ancient beginning and that fatal afternoon lies a substance defined by a strange contradiction. Cast iron pours like water into a mold yet shatters under a sharp blow. It is strong when squeezed but weak when pulled. So how does a single class of alloy bridge ploughshares and pagodas, cannons and skyscrapers? What is happening inside it that makes one batch hard and brittle, another soft and tough? And why did engineers keep trusting it long after it began to fail?

• 2. The Shape Of Carbon

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  Carbon, ranging from 1.8 to 4 percent by weight, decides nearly everything about cast iron. It is the line that separates this family of alloys from steel, which holds less carbon. The carbon does not simply sit in the metal. It takes a physical form, and that form is the whole story. In white cast iron, the carbon combines into a compound called cementite, very hard but brittle, letting a crack pass straight through. In grey cast iron, the carbon appears as graphite flakes that deflect a passing crack and spawn countless new ones as the material breaks. In ductile cast iron, the carbon gathers into spherical graphite nodules that stop a crack from advancing. Silicon, present at 1 to 3 percent, is the lever that controls which form wins. A low percentage of silicon keeps carbon in solution, forming iron carbide and producing white cast iron. A high percentage forces carbon out of solution as graphite, producing grey cast iron. The same elements, rearranged, yield metals as different as glass and clay.

• 3. The Chemist's Toolbox

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  Manganese is added to molten cast iron to fight sulfur, a contaminant that forms iron sulfide, blocks graphite, and makes the melt viscous enough to cause defects. The required amount follows a precise rule: 1.7 times the sulfur content, plus 0.3 percent. The two combine into manganese sulfide, which is lighter than the melt and floats up into the slag. Add more than that, and manganese carbide forms, raising hardness and chilling. Nickel ranks among the most common alloying elements, refining the pearlite and graphite structures and evening out hardness between thick and thin sections. Chromium, a powerful carbide stabilizer, is added in small amounts to reduce free graphite and produce chill, often alongside nickel. Copper goes into the ladle or furnace at 0.5 to 2.5 percent to decrease chill, refine graphite, and increase fluidity. Molybdenum at 0.3 to 1 percent sharpens the chill and the pearlite structure. Titanium acts as a degasser and deoxidizer, vanadium at 0.15 to 0.5 percent stabilizes cementite, and zirconium at 0.1 to 0.3 percent helps form graphite. Even bismuth has a job: in malleable iron melts it is added at 0.002 to 0.01 percent to let more silicon dissolve.

• 4. Four Irons, Four Lives

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  Grey cast iron is the most widely used cast material on Earth by weight, and its name comes from the grey look of its fractured surface. Most cast irons hold 2.5 to 4 percent carbon, 1 to 3 percent silicon, and the rest iron. Grey iron's compressive strength rivals low- and medium-carbon steel, even as its graphite flakes leave it weak in tension. White cast iron takes the opposite path. Its cementite precipitate is so hard that the metal can reasonably be classed as a cermet, useful for the wear surfaces of slurry pumps and the balls and rings in coal pulverisers. To soften that brittleness, malleable iron begins as a white iron casting, then is heat treated for a day or two at about 950 degrees Celsius and cooled over a day or two. The slow process coaxes carbon into spheroidal particles rather than flakes, giving properties closer to mild steel. Ductile or nodular cast iron, developed in 1948, takes the idea further. Tiny amounts of magnesium, 0.02 to 0.1 percent, and cerium, 0.02 to 0.04 percent, bond to the edges of graphite planes and force the carbon into nodules as the metal solidifies. Unlike malleable iron, it can be cast in larger sections, with 3 to 4 percent carbon and 1.8 to 2.8 percent silicon.

• 5. From Ore To Mold

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  Pig iron, the product of melting iron ore in a blast furnace, is the raw beginning of all cast iron. The metal can be poured directly from molten pig iron or made by re-melting it, often with iron, steel, limestone, and coke, while contaminants are driven off. Burning phosphorus and sulfur out of the melt also burns out carbon, which then has to be replaced, and carbon and silicon are tuned to the job, anywhere from 2 to 3.5 percent and 1 to 3 percent. A special blast furnace called a cupola once did the melting, but modern foundries more often use electric induction or electric arc furnaces. Once melting finishes, the molten metal flows into a holding furnace or ladle. The casting that follows can take complex shapes that other metalworking methods cannot manage, though the brittleness rules out anything needing a sharp edge or flexibility. One clever trick exploits cooling speed itself. Thick castings are hard to cool fast enough to turn white all the way through, so a quick chill solidifies a white iron shell while the interior cools slowly into grey iron, producing a chilled casting with a hard surface and a tougher core. High-chromium white iron pushes scale even further, letting a 10-tonne impeller be sand cast, its chromium carbides reaching hardness of 1500 to 1800HV.

• 6. An Ancient Chinese Metal

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  Cast iron was invented in China in the 8th century BC, and the dating rests on analysis of an artifact's microstructures from the Warring States period. The Chinese poured it into molds to make ploughshares and pots, along with weapons and pagodas. Steel was more desirable, yet cast iron was cheaper, so it became the common choice for implements, while wrought iron or steel went into weapons. To tame its brittleness, Chinese metalworkers developed annealing: they held hot castings in an oxidizing atmosphere for a week or longer to burn carbon off the surface. Far away, deep in the Congo region of the Central African forest, blacksmiths built furnaces capable of temperatures over 1000 degrees more than a thousand years ago. They created cast iron in crucibles and poured it into molds, joining it with soft wrought iron interiors to make composite tools and weapons. Early European missionaries described the Luba people pouring cast iron into molds to make hoes, and metallographic analysis of Luba artifacts confirms the practice. The technology eventually traveled west from China. Al-Qazvini in the 13th century, along with later travellers, recorded an iron industry in the Alburz Mountains south of the Caspian Sea, close to the silk route.

• 7. Cannon And Coalbrookdale

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  Henry VIII, who reigned from 1509 to 1547, initiated the casting of cannon in England. English iron workers using blast furnaces soon learned to produce cast-iron cannons. They were heavier than the prevailing bronze guns but far cheaper, and they let England arm its navy better. The amounts of iron these cannons demanded forced large-scale production. The Weald's ironmasters kept making cast irons until the 1760s, with armament a main use after the Restoration. A quieter revolution came through cookware. In 1707, Abraham Darby patented a new way of making pots and kettles thinner and cheaper than traditional methods allowed. His Coalbrookdale furnaces became the dominant suppliers, joined in the 1720s and 1730s by a handful of other coke-fired blast furnaces. Power changed the equation next. Beginning in 1743 and growing through the 1750s, Britain applied the steam engine to pump water to a waterwheel and drive blast bellows. The hotter furnaces this produced allowed higher lime ratios and the switch from scarce charcoal to coke, and cast iron production surged in the decades that followed.

• 8. The Architecture That Broke

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  The Iron Bridge rose in Shropshire in the late 1770s, built by Abraham Darby III, the first cast-iron bridge and a structure that put the whole material in compression through its arches. Cast iron, like masonry, is very strong in compression, while wrought iron is strong in tension and tough against fracture. Thomas Telford grasped the difference, using the metal for his bridge at Buildwas, then for the Longdon-on-Tern Aqueduct on the Shrewsbury Canal, followed by the Chirk and Pontcysyllte Aqueducts, both still in use after recent restorations. Trouble came when builders ignored the rule. Cast-iron beam bridges spread on the early railways, like the Water Street Bridge of 1830 at the Liverpool and Manchester Railway's Manchester terminus, putting beam centres in bending and their lower edges in tension, where cast iron is very weak. The Dee bridge disaster followed, then the Tay Rail Bridge disaster of 1879, where lugs cast integral with the columns failed early and bolt holes were cast rather than drilled. After the Norwood Junction rail accident of 1891, thousands of cast-iron rail underbridges were replaced by steel by 1900. The metal found steadier ground above the rails. Cast-iron columns, pioneered in mill buildings, let architects raise multi-storey structures without massive masonry walls, opening factory floors and church sight lines. Textile mills, plagued by flammable fibres and a habit of burning down, were rebuilt with iron frames, the first at Ditherington in Shrewsbury. From those columns and beams grew the steel-framed skyscraper, and the decorative cast-iron facades still standing in the Soho district of New York.