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Full text of "Metallurgy Of Cast Iron"

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FIG.   43.
-SAMPLES   OF  PIG IRON DIFFERING IN HARDNESS   UNDER  TEST, BUT   MAKING   CASTINGS   OF   LIKE   SOFTNESS., guided by hardness tests, would say that No.   i  would make a very soft casting while No.   2 would make a very hard one, when in fact each will give like softness in like   castings and treatment in cooling.    These samples were drilled with a press running at uniform speed and pressure.    It took  eight               ^ minutes to drill No. i and twenty-two minutes to drill" No. 2, a difference of fourteen minutes.    A half-inch twist drill was used and the method of drilling will be seen by the half holes on the back of the specimen seen in No. 3.    The difference in the hardness   of   these samples, it is to be remembered, is found in samples of like analysis, excepting in combined carbon and in iron, coming  from  the  same  tap   and  cast  in   sand moulds.  As long as uniformity in making iron cannot be achieved,  as is illustrated in Chapter XXIV., we may expect that the state of the carbon or hardness of pig iron will vary, and often not be in accordance with the  grade results  as  shown by  the  percentages  of silicon,  sulphur,   manganese,   and  phosphorus  which will be in the pig iron.    It will appear ridiculous to                 I those  who   know,   by  experience  and  research,   the                 [ deceptive nature of the appearance and hardness of                J sand-cast pigs that any one should now, at this day of                 / advancement in the metallurgy of cast iron,  try to                | introduce a. hardness test to define the grade of pig                | iron as now being generally cast.                                                       j! It is not to be understood that  every cast of pig               '! metal is deceptive to the eye, or hardness test.    It may               >-V be that three-fourths  of all the  iron  cast at  some furnaces may possess a true fracture of hardness or accord with the amount of silicon, sulphur, etc., an     "