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Lecture 6: Manufacture of Metals

Lecture 6: Manufacture of Metals. Prof. T.W. Eagar Fall 2002 MIT. Metals Manufacture. Casting first step in most metals manufacture all metals are cast except tungsten (the highest melting metal) which is sintered

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Lecture 6: Manufacture of Metals

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  1. Lecture 6:Manufacture of Metals Prof. T.W. Eagar Fall 2002 MIT

  2. Metals Manufacture • Casting • first step in most metals manufacture • all metals are cast except tungsten (the highest melting metal) which is sintered • steel wasn’t cast until 1860’s when Bessemer learned how to get high enough temperatures to melt steel

  3. Iron and Steel Production • previously, iron ore was reduced to porous sponge iron in pits with charcoal • at ~1000 deg. C • Blacksmiths then pounded the sponge iron into solid mass in forge • lost half of the iron to oxidation; very low yields • blast furnaces were then created to make high carbon cast iron • at 1100-1200 deg. C • is the highest temperature possible with carbon oxidation with room temperature air • It is poured out into sand as pig iron (4% carbon) • blacksmiths worked it into wrought iron (2% carbon) • still not steel • steel melts at 1500 deg. C • too high for carbon oxidation with room temperature air • Bessemer converter was made to pre-heat air in order to reach 1500 deg. C

  4. History of Steel Production • pig iron taken from blast furnaces and carbon is removed to make steel in Bessemer converter, open hearth furnace and now in basic oxygen furnace

  5. History of Steel Production • Bessemer Process • Open Hearth Furnace • Basic Oxygen Furnace (BOF) • Electric Arc Furnace (EAF)

  6. Economics of Steel Production • Integrated Steel Mill: • Blast Furnace - Pig Iron at ~$150 / ton • BOF/Rolling - Steel Sheet; adds ~$70 / ton • total: ~$220 / ton • $10 billion to build - only countries can • produces 12 million tons / year • Mini Mill • scrap steel costs $100 / ton • EAF recycles scrap into steel at $75 / ton • total: ~$175 / ton • costs $250 million to build • produces 1-2 million tons / year • steel has too many impurities to make quality sheet but still has good market • Overcapacity in world steel market causes low prices, losses to steel industry • US steel industry faced with extinction, went from 16 person hours/ton to 0.6 person hours/ton • US now has 10% of world market

  7. Ingots

  8. Ingot Casting • Ingots, large cylindrical or rectangular casting • needs to be cast in something with similar thermal mass unless you use chilled copper walls • Most metals shrink on solidification, want top to be the last to solidify, but also want to minimize shrinking zone • Silicon, germanium, bismuth, expand on solidification • Complex castings need to be fed properly • can reduce defects with vacuum arc remelting of casting surface • Castings tend to have defects, proportional to the size of the casting • Large heavy plates still have to be made by ingot casting, 1/3 of the weight in the “hot top” have to be returned for remelt • Plate yields are about 60%

  9. Ingot Casting • Largest casting is 700,000lb rotor forging (350 ton), for say 1000MW generators, could design larger generators, but can’t forge the shaft, • Only two places in the US can cast these pieces: US Steel & Bethlehem steel, built prior to WWI to make battleship gun barrels

  10. Continuous Casting • Continuously solidifying • 90-95% of all steel made uses this process • 10-12” sections are the thickest continuous casting processes • Ingot casting used for plates greater than 4” • In 1960’s this helped destroy the US Steel industry • Much higher yields, for plate (up to about 2” thick, can’t go much higher), go from 60% to about 90% • Sheet material yields go up to about 90% • By 1980’s US industry up to about 85% continuous casting • used to avoid large solidification zone • important for precious metals to avoid investment costs • important for large production (steel)

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