THE STRUCTURE OF METALS 195 



subjected. The effect of hot-rolling on steel has been referred 

 to in the former article (photograph 5), where it was shown 

 that the grains of iron and the areas of pearlite are elongated in 

 the direction of rolling. Such flow-structures are of frequent 

 occurrence in rolled metals, so that the direction which a 

 specimen originally occupied in the rod or plate from which it 

 was cut is readily determined by microscopical examination. 

 Naturally, enclosures of slag or sulphide and similar impurities 

 are also elongated in the direction of rolling when the tempera- 

 ture is so high that they are in a liquid or plastic state at the time. 



A metal which has only been worked while hot has 

 properties which differ but little from those which the metal 

 possesses in a fully annealed state ; it differs from a cast metal 

 in being more compact and generally more uniformly crystal- 

 lised but the elastic properties are not greatly modified, except 

 in so far as they depend on the crystallisation. 



The effect of mechanical work on the properties of a metal 

 becomes more pronounced as the temperature falls. So long as 

 the temperature of working is above a certain limit, different in 

 the case of each metal and alloy, internal strain is removed as 

 fast as it is produced by a process of recrystallisation whereby 

 the equilibrium is re-established. At lower temperatures this is 

 not the case : the properties of the metal undergo more or less 

 permanent alteration, until at the ordinary temperature nearly 

 all metals are very appreciably " hardened " by the process of 

 hammering, pressing, rolling or drawing into wire. The term 

 " hardening" here denotes a change in many properties which 

 are closely associated with one another. The actual minera- 

 logical hardness — that is, the resistance to scratching — as a rule 

 is little affected but the elasticity is increased and the ductility 

 diminished, whilst the electrical conductivity is also lessened 

 and important changes are produced in the electro-chemical and 

 thermo-electric properties. Such a metal is said to have been 

 " cold-worked," although the temperature of working may be 

 considerably above the atmospheric temperature provided that 

 it is below that at which recrystallisation occurs freely. The 

 crystals of such a metal as copper or 70 : 30 brass are crushed 

 and deformed, the extent of the deformation naturally varying 

 with the degree of cold-working, whilst the structure of alloys 

 containing two or more micrographic constituents becomes 

 extremely confused and small areas of an eutectic may be 



