THE STRUCTURE OF METALS 93 



recrystallisation of the deformed material and a return, in great 

 measure, to the original structure of the casting. The forma- 

 tion of new crystals in the worked material, like the original 

 process of solidification, sets in from distinct centres or nuclei 

 and spreads outwards until the crystalline growths from neigh- 

 bouring centres meet and interfere, giving rise to crystal grains 

 as in the original process of solidification but the complex 

 interlocking of boundaries, which is so conspicuous a feature 

 of many cast metals, is less usual after annealing and an 

 approach to simple rectilinear polygonal forms is noticeable in 

 most worked and annealed metals, especially when they have 

 been subjected to a high temperature. If the metal be worked 

 mechanically before annealing, the crystals that are produced 

 are not simple but frequently twinned, the repeated twinning 

 being similar in effect to that observed in felspars in rock 

 sections. Fig. 4 represents a rolled and annealed specimen of 

 German silver, a homogeneous mixture of copper, nickel and 

 zinc ; both the rectilinear boundaries of the crystals and the 

 repeated twinning planes are apparent. 



Another class of alloys, although crystallising from the 

 molten state in the form of a homogeneous solid, as in the 

 metals just described, undergoes such further changes in the 

 solid state that an entirely new structure is produced. To this 

 class belong most of the varieties of steel. All steels solidify 

 in the first instance in the form of crystal grains of uniform 

 composition, if certain minor impurities be, for the moment, 

 neglected. It is, however, rare that such a structure persists 

 during the process of cooling down to the ordinary tempera- 

 ture. Manganese steel, containing 13 per cent, of manganese 

 and 1 per cent, of carbon, which finds such important applica- 

 tions, on account of its resistance to abrasion, in crushing- 

 machinery, tramway crossings, etc., is an example of a steel 

 which retains its polygonal structure permanently ; but this is 

 quite an exceptional case. Ordinary carbon steels, from the 

 softest structural material to the hardest varieties of tool steel, 

 have undergone transformation to a greater or less extent, so 

 that the original polygonal grains have been more or less 

 resolved into a complex structure the principal constituents 

 of which are ferrite (iron alone or uniformly associated with 

 small quantities of silicon, manganese, phosphorus and other 

 elements but not carbon) and iron carbide or cementite, Fe 3 C. 



