THE STRUCTURE OF METALS 99 



phosphor-bronze, the minute quantity which is actually present 

 being completely masked but ordinary grey cast-iron contains 

 an appreciable quantity of phosphorus in the form of iron 

 phosphide, Fe 3 P, which is distinctly visible as a brilliantly white 

 constituent disposed in characteristic reticulated patterns which 

 represent the eutectic alloy that is the last portion of the cast- 

 iron to solidify on cooling from the molten state. Other non- 

 metallic elements occur principally as impurities and are there- 

 fore considered below. 



The types briefly enumerated above comprise nearly all the 

 principal metals and alloys encountered in engineering practice, 

 with the exception of white pig-iron which has an eutectic 

 structure peculiar to itself — and of hardened steels — the complica- 

 tions of which are too intricate for discussion within the limits 

 of a short article. The variety in this instance is due to the 

 fact that hardened steels are not in a condition of chemical and 

 physical equilibrium and that many stages may be recognised in 

 the return to the stable condition. It is possible by examining 

 a polished and etched surface of such a steel to form an accurate 

 judgment of the heat-treatment to which the specimen has been 

 subjected. The newer " high-speed " tool steels, containing 

 chromium and tungsten or molybdenum as essential constitu- 

 ents, have structures differing considerably from those of carbon 

 steels and present difficulties of interpretation that have not yet 

 been overcome. 



A metal or alloy which has been subjected to heat treat- 

 ment bears in its internal structure a record of its immediate 

 history and the interpretation of the record is one of the most 

 important applications of metallography to technical practice. 

 As an example, the influence of annealing on the microscopic 

 structure of mild steel may be considered. The temperature at 

 which annealing has taken place may be inferred, other things 

 being equal, from the average size of the crystal grains. It has 

 been found l that the rate of growth of the ferrite grains is a 

 maximum at slightly above 700 , growth being less rapid either 

 above or below that temperature. Prolonged annealing at 700 

 produces an extremely coarse grain. When the proportion of 

 carbon is higher, as in the rail steel, containing 040 per cent, of 

 carbon, the ferrite forms " cells," filled with sorbite or pearlite. 



1 J. E. Stead, /. Iron and Steel Inst. 1898, i. 145 ; A. Joisten, Metallurgie 

 1910, 7, 456. 



