2 Transactions of the Society. 



aid of the Microscope that the causes of these variations may be 

 explained. Metallography is intended to augment, rather than sup- 

 plant chemical analysis. The Microsco'pe enables us to ascertain 

 much about the mechanical and thermal treatment the metal has re- 

 ceived, which in commerce is often of the utmost importance. 



It is well known that specimens of both steel and iron, produced 

 under apparently the same conditions, often display totally different 

 properties. This is especially the case with steel, which, on account 

 of its more complex character, is easily affected by small alterations 

 in the conditions of its manufacture. 



The causes of the variation in properties of similarly produced 

 samples of metal may often be explained by the aid of the Micro- 

 scope, when all other methods of investigation have failed. 



To be able to determine the quality of the quenching of a steel 

 is of vital importance, especially in the case of large masses of metal, 

 e.g. the ingots used in the manufacture of ordnance. If a gun-tube 

 is quenched below its critical point, it will be soft, and consequently 

 very unsafe for firing purposes, on account of its low elastic limit. 

 The Microscope would, however, be invaluable in such circumstances, 

 as it would enable one to say definitely whether the metal had been 

 properly quenched and tempered or not. 



For the engineer, the Microscope is especially useful in deter- 

 mining the influence exerted by thermal treatment on varieties of 

 steel of different composition. It is also useful for detecting slag 

 patches, defective welds, " cold rolling " effects, minute blow-holes, 

 cracks, flaws, and allotropic changes in the metal. 



It will, perhaps, be as well at this period to give a short account 

 of the constitution of iron and steel, and also to show the effect of 

 altering their thermal treatment during manufacture. 



Steel is composed of iron, which may contain from • 05 to 2 

 per cent, of carbon, together with other impurities, such as man- 

 ganese, sulphur, phosphorus, silicon, and arsenic. As the presence 

 of these impurities complicates the subject considerably, it will be 

 better in this demonstration to neglect their influence altogether. 

 Carbon-frae iron is most difficult to obtain, and can only be prepared 

 by depositing it electrically, or by reducing ferric oxide by alu- 

 minium. 



Cast iron contains from 2 to 4 • 5 per cent, of carbon, and may be 

 either white, mottled, or grey, according to the state in which the 

 carbon is present ; this state is modified by the thermal treatment 

 the iron has received during its manufacture. Grey iron under 

 certain conditions may be made to absorb as much as G per cent, 

 of carbon. 



In steel the whole of the carbon is combined with a portion of the 

 iron, forming the iron carbide Fe a C. This Fe 3 C contains by weight 

 about 7 per cent, of carbon. The carbide in a slowly cooled steel is 

 distributed throughout the balance of the iron. 



