152 8·Iron and Steel (a) (b) FIGURE 8.8.Photomicrographs of (a)graphite flakes in gray cast iron(as polished,not etched, 100x),and (b)nodular or ductile cast iron (annealed for 6 hr at 788C and furnace cooled, 100x 3%nitel etch).Reprinted with permission from Metals Handbook,8th Edition,Vol.7 (1972),ASM International,Materials Park,OH,Figures 647 and 709,respectively,pages 82 and 89,respectively. Nodular Cast The ductility and the strength of cast iron can be improved con- Iron siderably (approaching properties known for steel)by adding only a small amount (less than 1%!)of magnesium and/or ce- sium.This causes the graphite to precipitate in the form of spher- ical particles (nodules)that are imbedded in pearlite [Figure 8.8 (b)].Moreover,if this material,called nodular or ductile cast iron,is heated for several hours near 700C,or is slowly cooled, a ferrite matrix will result which increases the ductility.The re- sulting material is heavily used in the automotive and machine industries,such as for crankshafts,gears,and valves. White Cast Another type of cast iron,called white cast iron,contains less Iron than 1%Si and forms by rapid cooling.As we know from above, this causes the retention of cementite as a matrix and thus a very hard and brittle material.If thick pieces of white cast iron are appropriately quenched,only the surface contains the hard phase whereas the interior,because of the slower cooling rate,trans- forms into gray cast iron.This combination which contains a hard and therefore wear-resistant surface is used,for example, for rollers in rolling mills.152 8 • Iron and Steel The ductility and the strength of cast iron can be improved con￾siderably (approaching properties known for steel) by adding only a small amount (less than 1%!) of magnesium and/or ce￾sium. This causes the graphite to precipitate in the form of spher￾ical particles (nodules) that are imbedded in pearlite [Figure 8.8 (b)]. Moreover, if this material, called nodular or ductile cast iron, is heated for several hours near 700°C, or is slowly cooled, a ferrite matrix will result which increases the ductility. The re￾sulting material is heavily used in the automotive and machine industries, such as for crankshafts, gears, and valves. Another type of cast iron, called white cast iron, contains less than 1% Si and forms by rapid cooling. As we know from above, this causes the retention of cementite as a matrix and thus a very hard and brittle material. If thick pieces of white cast iron are appropriately quenched, only the surface contains the hard phase whereas the interior, because of the slower cooling rate, trans￾forms into gray cast iron. This combination which contains a hard and therefore wear-resistant surface is used, for example, for rollers in rolling mills. FIGURE 8.8. Photomicrographs of (a) graphite flakes in gray cast iron (as polished, not etched, 100 ), and (b) nodular or ductile cast iron (annealed for 6 hr at 788°C and furnace cooled, 100 3% nitel etch). Reprinted with permission from Metals Handbook, 8th Edition, Vol. 7 (1972), ASM International, Materials Park, OH, Figures 647 and 709, respectively, pages 82 and 89, respectively. White Cast Iron Nodular Cast Iron (a) (b)