. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . Let us now consider what takes place when steel containing per centcarbon cools from 900° C. The cooling curve shows nothing very remarkableuntil a temperature of about 720° C. is attained. Here the critical pointsArs and Ar2 of pure iron coalesce into one. At this point pure iron, orferrite, separates from the solid solution. The separation of ferrite goes onalong the curve AP (Fig. 308) until the temperature reaches about 690° C,when another recalescence po
. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . Let us now consider what takes place when steel containing per centcarbon cools from 900° C. The cooling curve shows nothing very remarkableuntil a temperature of about 720° C. is attained. Here the critical pointsArs and Ar2 of pure iron coalesce into one. At this point pure iron, orferrite, separates from the solid solution. The separation of ferrite goes onalong the curve AP (Fig. 308) until the temperature reaches about 690° C,when another recalescence point occurs (Ari). No other essential change, asfar as we are concerned, occurs as the system cools down to the normal tem-perature of the atmosphere. Fig. 308 is derived from Roozebooms diagram,* the carbon-iron diagi-am, * H. W. B. Roozeboom, Zoit. Phys. Cliom., , 1900; improved in Zoit. Eloktrocliom.,, 1904; MotalloRraphist, , 1900; H. Ic Chatolicr, ibid., , 1900; , 1901; F. Osmond,, 1901; H. Jiiptner von Jonstorff, ibid., , 1902. INFLUENCE OF DETAIL OF MANUFACTURE 427 1000 800 600 400. 1-5% Carbon given by Howe,* based upon later researches, shows the temperatures some-what higher than those of the figure. When the temperature is above the line APB the iron is in a form knownas austenite. Whatever carbon is present is dissolved in this austenite,which is what is called a solid solution as distinguished from a mechanicalmixture or conglomerate, just as salt and water, when brought in contact, mergein each other and pass from the condition 1:200°of a mixture or conglomerate to that of asingle substance. As the iron with per cent carboncools and the hne AP is reached theaustenite begins expelling from itself partof its iron in the form of ferrite. As theferrite thus expelled is nearly or quite freefrom carbon, the remaining austenite be-comes relatively richer in carbon, until,when the temperature reaches Ari, itcontains per cent
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