The Iron and steel magazine . dark, a phenomenon towhich Le Chatelier * has already called attention. In the last examples we have observed only reversibleprocesses. At the temperature of 7000 C, cementite dissolvesin ferrite to form martensite; if, instead of quenching in icewater, we had permitted the cooling to proceed slowly, as we * Le Chatelier: Laustenite, Revue de Metallurgie, 1504. 2o6 The Iron and Steel Magazine have seen, before the martensite would have become pearlite —cementite and ferrite. But if one heats iron, which above 7000 free cementite, a considerable time at
The Iron and steel magazine . dark, a phenomenon towhich Le Chatelier * has already called attention. In the last examples we have observed only reversibleprocesses. At the temperature of 7000 C, cementite dissolvesin ferrite to form martensite; if, instead of quenching in icewater, we had permitted the cooling to proceed slowly, as we * Le Chatelier: Laustenite, Revue de Metallurgie, 1504. 2o6 The Iron and Steel Magazine have seen, before the martensite would have become pearlite —cementite and ferrite. But if one heats iron, which above 7000 free cementite, a considerable time at a high tempera-ture (about 10000 C), a non-reversible process occurs: car-bide (cementite) = ferrite + (dissolved) carbon. This process is illustrated in photographs 22-24. Photo-graph 22 shows a quickly cooled white iron which exhibits brightcementite in relatively fine division, dotted with a dark groundmass which contains carbon in solid solution. This material,kept for 50 hours at 9800 C. in a vacuum, showed after slow. Fig. 23. Magnified 50 diameters cooling the structure of photograph 23, —magnified still more in24. Black, irregular knots are surrounded by light areas; betweenthe latter are streaks of pearlite. The material formerly ashard as glass, is easily filed, because of its composition, tempercarbon, ferrite, pearlite. If the temper carbon is removed insome way, perhaps by oxidation, an iron remains- which consistsonly of ferrite and pearlite, and so is malleable. This operation,which changes the white, brittle, non-malleable cast iron intoa soft metal, is called annealing; the final product, annealed ormalleable cast iron. This process of the division of the cementite into ferrite andtemper carbon has much bearing upon the theory. First of all, Iron ( cm bon . illoys 207 it musl be mentioned thai the materia] which served for theexperiments illustrated in 22 and 24 was almost free from Si, or at least did not contain more than per cent of 11 ; tin-total
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