206 SCIENCE PROGRESS 



a normal condition by heating until the austenitic region is 

 entered and then cooling. A fine grain is obtained in this way 

 and dangerously brittle steel, if not burnt, may thus be made 

 equal in quality to steel which has not been rendered coarse at 

 any time. The minimum temperature for the purpose varies 

 from 950 for very mild steels to 8oo° for hard steels. 



When the proportion of carbon is somewhat greater, so that 

 the pearlite forms a considerable fraction of the entire mass, a 

 second factor enters, namely the condition of distribution of the 

 carbide. Rail steel containing about 0*45 per cent, of carbon 

 may be taken as an example. A slowly cooled or annealed rail 

 contains its carbide in the form of laminated pearlite. Prolonged 

 annealing not only causes an increase in the size of the grains 

 but if it be conducted at a temperature below that at which the 

 carbide is absorbed, it has the further effect of causing segre- 

 gation of the carbide, a final state of equilibrium being reached 

 only when the whole of the carbide has been gathered into 

 isolated masses which lie between the grains of ferrite. Such a 

 condition is eminently favourable to brittleness, on account of 

 the facility with which the cleavages opened in one grain can 

 be propagated. It has been found that the maximum toughness 

 is obtained when the steel is cooled so rapidly that the carbide, 

 instead of forming parallel laminae, remains in a minutely 

 granular state in the condition known as sorbite. Such a 

 condition may be obtained by a process of semi-chilling, in 

 which the cooling is not sufficiently rapid to harden the steel 

 but is too rapid to allow the carbide to segregate. 



Similar considerations apply to other metals and alloys. 

 Heating to a high temperature increases the size of the grains, 

 whilst hot-working destroys the large crystals, so that a fine- 

 grained structure may be obtained by selecting a suitable 

 finishing temperature. Fig. 3 represents a transverse section 

 cut from a rod of Muntz metal, which has been rolled hot and 

 has a fine grain, the a and /3 constituents being arranged very 

 uniformly, with little or no tendency to rectilinear groupings. 

 Fig. 4 represents a rod of the same alloy heated to S50 and slowly 

 cooled without applying work. The magnification is the same 

 in both cases. It is obvious that the size of grain has increased 

 enormously, whilst the a-crystals also show a strong tendency 

 to assume rectilinear forms and to become arranged parallel with 

 the cleavages of the ^-crystals from which they have separated. 



