THE STRUCTURE OE METALS 205 



accurate indication of the presence or absence of brittleness in 

 a specimen of steel than the ordinary tensile test. 



Brittleness in steel may be due to the presence of impurities, 

 principally phosphorus, which has a remarkable effect in 

 coarsening the structure and developing the cleavages. The 

 influence of these dangerous elements is thoroughly well 

 understood and the control of metals by chemical analysis is 

 largely designed to guard against danger from this source. 

 Other less well understood factors remain, among them the 

 influence of nitrogen, to which some authorities have attributed 

 the brittleness which occasionally develops in mild steel plates 

 with age. This is a point which has not yet been fully investi- 

 gated, although certain remarkable changes of structure, 

 including the production of large and conspicuous cleavages, 

 have been recognised as associated with the presence of 

 nitrogen, which is apparently retained by the iron in the form 

 of a homogeneously distributed nitride. Apart from these 

 chemical conditions, the principal factor which determines the 

 toughness or brittleness of a given steel is the size of grain 

 and this is in turn dependent on the thermal treatment. Con- 

 sidering first a steel containing only a small proportion of 

 carbon, heating to a high temperature within the austenite 

 range, say to 1200 or 1300 , produces a coarse structure, the 

 size of the grain being approximately proportional to the 

 temperature and this coarseness is retained after cooling to 

 the ordinary temperature. In fact, the size of grain is a func- 

 tion of the maximum temperature to which the steel has been 

 exposed, provided that no mechanical work has been applied. 

 If, on the other hand, the metal be rolled or forged while hot, the 

 mechanical treatment breaks up the crystal grains while it 

 continues and the final size of grain is a function of the 

 "finishing" temperature and not of the maximum temperature. 

 Coarse crystallisation due to overheating is thus obliterated by 

 work, provided always that the metal has not been " burnt," in 

 which case the grains, separated by films of oxide, do not reunite 

 during cooling. Steels very low in carbon also become coarse 

 and brittle if annealed for a long time at a low temperature, the 

 growth of the grains being extremely rapid somewhat above 

 700 , as was shown in the previous article. 



Steel which has been overheated or which has been annealed 

 for too long a period at a low temperature may be restored to 



