Lecture 6: Manufacture of Metals

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)