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the chill thickness desired. Then again, some users prefer a sharply defined chill joining the gray body, while others prefer the chill and gray body to interlace or mingle with each other when combined. This feature is well displayed in the chilled section of car wheel seen at AB, Fig. 57, tendered the author by the Pennsylvania Car Wheel Co. of Pitts-burg, Pa. This factor is further treated in Chapter XXXVII., page 260.
Chilled rolls for hot rolling require different qualities than those used for cold rolling, and are a type of rolls subjected to the greatest abuse. This abuse lies in alternate expansion and contraction which takes place in the outer body of the rolls, being suddenly heated to about 500 degrees F. and then cooled to the atmosphere. The force of this power is often noticeable in remelting rolls in air furnaces, where from sudden heating of the outer body they will crack, in two or more pieces, with an explosion that can often be heard for quite a distance. Rolls for hot turning should not only be of such a character as to withstand the above alternate strains, but possess
FIG.  57.—SECTION  OF  CHILLED   CAST IRON  CAR  WHEEL.rolls. The founder is supposed to have such a control over mixtures that he can attain to within a % inch ofiven either, in producing the combined carbon. It is very important in assisting to prevent " cold shuts" or " chill cracks," when pouring a mould, to have the metal run freely, and hence the advantage of manganese over sulphur, as above stated.*