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2<)0                        METALLURGY   OF   CAST   IRON.
of a chill or that maybe wholly " white iron " and still find a difference in the hardness of iron. A good article on testing hardness, etc., appears on page 434.
The success of chilled work is as dependent upon the degree of hardness of the chill as upon its depth. One set of conditions may exact a harder chill than another, and what may prove best in one line of work may be a failure in another; as, for example, the same kind of chill would not answer as well for paper or calender purposes as for steel or iron rolling. Variations in sulphur, manganese and phosphorus are chiefly potential in giving a special character to the hardness of a chill.
For "friction wear," as with car wheel, high sulphur will give better life than high manganese combined with low silicon, to cause chill. For "heat wear,'' hardness or chill is best obtained by high manganese in preference to sulphur combined with low silicon. Chilled iron is rarely, in any case, a homogeneous mass, and sulphur, more than any other element, retards the union of the molecules to best attain tenacity in the life and wear of iron subjected to heat. While it is true that we find in present practice that hardness is generally obtained by the higher sulphur, as can be seen from many of the analyses shown herein, and others recorded, still wherever manganese can be applied in preference to sulphur, to affect the carbon, in giving hardness to chill rolls, etc., better results in preventing surface cracks, etc., may be expected. A chill which is chiefly promoted by manganese will prove more yielding to strains and not so liable to chill-crack from heat as a chill which has been chiefly promoted by sulphur.
Then again, manganese causes a -more gradual decline from the white to the grey in chilled castingsly the same depth.... Shop scrap     ......	15            x ro           x	1.50 i. So	22.50 iS.oo