Photomicrography of Iron and Steel. By W. H. Merrett. \\ 



Steel may therefore be considered as a mixture or an alloy of iron 

 and iron carbide (Fe 3 C), and it has been shown in the Reports of 

 the Alloys Research Committee to the Institution of Mechanical 

 Engineers* that if studied from this standpoint, the formation of its 

 structures will be found to follow the laws which govern the forma- 

 tion of frozen saline solutions. 



In order to understand the analogy between frozen saline solu- 

 tions and steel, it will be necessary to refer to the work of Guthrie in 

 1876:— 



It is well known that by dissolving common salt (sodium 

 chloride) in water, the freezing-point of the water is lowered. By 

 increasing the amount of salt the freezing-point of the resulting 

 mixture is, at first, correspondingly lowered, until it contains a cer- 

 tain percentage of salt. The lowest possible freezing-point of a 

 solution of sodium chloride in water is then reached, and further 

 addition of salt will gradually raise the freezing-point of the brine. 

 Dr. Guthrie found that the mixture which has the lowest freezing 

 point contains about 23*50 per cent, of salt; and as the hydrate 

 containing 10 molecules of water would require 24 ■ 50 per cent, of 

 salt, Dr. Guthrie inferred that the solution of lowest freezing-point 

 was a hydrate of the formula NaCl +10 H 2 0. He proposed for 

 it, and for all similar mixtures, i.e. for all saline solutions of lowest 

 freezing-points, the name of cryohydrate or eutectic, by which he 

 meant to imply that they can only exist in the solid state at a low 

 temperature. AVhat actually takes place in the freezing of solutions 

 of salt and water may, perhaps, be better explained by the aid of a 

 diagram.! Fig. 1 shows how a thermometer, plunged in the solu- 

 tion, falls as the solution cools down. The dots along the lines are 

 points of retardation in the fall of the mercurial column ; it is seen 

 that in most cases there are two points for each stage of concentra- 

 tion. 



The diagram consists of two branches, the one marked " ice," and 

 the other " salt." It will be best explained by taking two solutions 

 containing two definite amounts of salt on either side of the point B 

 where the branches meet. If, for instance, a thermometer be placed 

 in a solution of 10 per cent, of salt in water which is being slowly 

 cooled down by means of an external freezing mixture, the mercury 

 will stop in its fall at about — 8° C. or 18° F. ; this is due to the 

 separation of pure ice. This gives the point d on the branch A B. 

 The mercury then continues to fall until the temperature of — 22° C. 

 or — 8° F. is reached, and the cryohydrate or eutectic of ice and salt 

 solidifies. This eutectic consists of alternate laminae of ice and salt 

 in juxtaposition, and is merely a mechanical mixture, not a chemical 



* See 4th and 5th Reports of the Alloys Research Committee to the Institution 

 of Mechanical Engineers. 



t Reproduced from Sir W. C. Roberts- Austen's Reports to the Alloys Research 

 Committee, by permission of the Council of the Institution of Mechanical Engineers. 



B 2 



