The Iron and steel magazine . eived the name of martensite.* If we fix the formof these crystals by sudden cooling (chilling) we obtain a regularstructure which has the character of crystal clusters (places ofcleavage, crystalline granulation). Photograph 17 shows a whitecast iron chilled at 9900 C. The white fields are unalteredcementite, broken through by a ground mass whose structure is * Austenite (?). — Editor. Iron-t arbon . \llo 203 given m photograph [8. We see thai it consists of a light ground mass wliicli is dotted with dark needles crossing each oilier m sharp corners or in triangl
The Iron and steel magazine . eived the name of martensite.* If we fix the formof these crystals by sudden cooling (chilling) we obtain a regularstructure which has the character of crystal clusters (places ofcleavage, crystalline granulation). Photograph 17 shows a whitecast iron chilled at 9900 C. The white fields are unalteredcementite, broken through by a ground mass whose structure is * Austenite (?). — Editor. Iron-t arbon . \llo 203 given m photograph [8. We see thai it consists of a light ground mass wliicli is dotted with dark needles crossing each oilier m sharp corners or in triangles; and also of Mack irregular areaswithout recognizable form. In order to understand the meaning of these three differentparts, one must understand the process of solution taking placewhen heating to 7000 C. and the possible changes later on quench-ing. It has been proved by H. Le Chatelier that when thecementite dissolves in the ferrite, there takes place a suddendecrease of volume followed by a sudden increase of volume,. Fig. 20. Magnified 750 diameters after which the volume corresponding to the rising temperatureslowly increases. This sudden decrease in volume correspondsto the formation of a structureless, easily stained constituentwhich Osmond has recognized and named troostite. Thustroostite will always appear when cementite is about to dissolve,and this will happen until the ground mass is saturated withcementite. The amount of troostite will be the smaller thehigher the quenching temperature, because the crystals withincreasing temperature are more quickly saturated with cemen-tite, and also the longer the heating at a given temperature hascontinued, whereby time is given to the troostite to change intothe crystalline martensite. 204 The Iron and Steel Magazine On heating we have thus at 9000 C. the constituents cement-ite, martensite and troostite. If the condition could be keptcompletely unchanged through the cooling, only these threeconstituents would occur. But
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