. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . erestill in good condition. The rolling tends to break down the grain and give a finer structure, but im-mediately after the work stops the grain commences to grow again, consequentlythe lower the finishing temperature the smaller the grain size. If the steel isworked below the critical point, strains are developed which injure the metaland may rupture it. Work at too low a temperature distorts the grain orflattens and elongates the crystals in the direction of t
. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . erestill in good condition. The rolling tends to break down the grain and give a finer structure, but im-mediately after the work stops the grain commences to grow again, consequentlythe lower the finishing temperature the smaller the grain size. If the steel isworked below the critical point, strains are developed which injure the metaland may rupture it. Work at too low a temperature distorts the grain orflattens and elongates the crystals in the direction of the rolling. * N. Ljamin, Chtiin. Zoit, 1899, Baiimatorialion, 1S99, imda the tenacity in differeut steels variesdirectly as the size of the pcarlite grains, at the same finishing temperature. INFLUENCE OF DETAIL OF MANUFACTURE 425 To thoroughly understand the effect of the roUing, it is necessary to studythe structural changes that take place in the cooling metal.* Let us first consider the cooUng curve of a copper bar, shown in Fig. find here no evidence of any sudden change in the nature of the cooling copper. 200 150. 20 10 100 -10 0Fig. 303. 20 40 60 Time - Cooling Curve of Solid Copper.(J. W. Mellor.) -20 \c V Freezing \ \ \ ^ 0 20 40 60 Ti:me Fig. 304. — Cooling Curve of Water.(J. W. :MeUor.) If, however, a curve is drawn for water cooling down from 20° to —20° C,we get a terrace in the cooling curve, as shown in Fig. 304. This tells us thatsome change has taken place in the nature of the substance at 0°. We see at 1000 800 600 400 Ar 20 40 60 80 too Time Fig. 305. — Recalescence. (J. W. Mellor.) once that this change corresponds with the passage of water from the liquidto the solid state. When a steel bar is coohng, an evolution of heat occurs at about 690° amount of heat evolved is so great that the metal visibly brightens in phenomenon is called recalescence. The coohng curve is shown inFig. 305. * See The Crystallization of Iron and St
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