268 / . THE AMERICAN MONTHLY [December, 

 Microscopic Structure of Steel. 



By p. H. DUDLEY, 



NEW YORK CITY. 



[From Journal of the New York Microscopical Society, October, 1891.] 



The subject is so vast and important that I shall confine my remarks^ 

 principally to rail steel. Besides the iron forming the basis of ordinary 

 steel rails, they have a large range in chemical composition, as shown 

 by the following table : 



Parts of I per cent. 



Carbon 25 to .50 



Manganese / 30 to 1.50 



Silicon 0410 .30 



Phosphorus 03 to .15 



Sulphur 04 to .10 



Copper ID to .80 



Traces of other minerals are present. 



This alloy, being formed by fusion and cast, is, of course, crystalline 

 in structure. The texture will be fine or coarse, according to composi- 

 tion, size of ingot, rapidity of solidification, amount of mechanical work 

 given to the inetal in reducing to the section of the rail, shape, and size 

 of the latter, and the temperature at which the rail is finished. In a 

 section of rail of which the ingot or bloom was maintained at too high 

 temperature, the crystallization becomes coarse and often sharply de- 

 fined, the matrix enclosing or joining the crystals weak and breaks upon 

 the surface, instead of pulling out the portions which penetrated the 

 large pseudo-crystals. The delicate matrix enclosing the pseudo-crystals 

 is shown in specimen No. i with low powers, but few traces of lines 

 from the matrix are seen to penetrate to the interior of the polygons. 

 This specimen is from the tire of a steel wheel, is very hard, and con- 

 tains 6 per cent, of manganese. Wheels of such steel have given a very 

 large mileage. Specimen No. 2 is from a rejected heat of rails, and 

 contains .90 of carbon. The ingot was maintained at too high a heat 

 in the reheating furnace, coarse crystals resulting in the head of the 

 rail, and, as will be seen, they are quite well defined. In the fracture 

 the crystals have separated from each other through the matrix without 

 breaking the individual crystals. This shows first the overheating of 

 the steel, which, in a large head of a rail, does not receive sufficient 

 work to break up this coarse crystallization. The distance across the 

 axis of the crystals measures iWd to iVn'^o of an' inch, which, for a good 

 wearing rail, I consider coarse, though the majority of rails in the track 

 are much coarser. Specimen No. 3 is from the same rail, taken near 

 the top of the head, and shows that the mechanical treatment of rolling 

 has broken up the coarse crystallization, rendering it practically amor- 

 phous, very fine in texture ; in fact, much finer than can be expected in 

 rails imtil higher carbons are reached than has been until recently con- 

 sidered advisable to' put in the tracks. As we inciease the carbon in 

 rails we increase the hardness, raise the elastic limit and tensile strength, 

 but, on the other hand, decrease the elongation, and, without great care, 

 render the steel brittle under shock instead of retaining suflicient tough- 

 ness in the rails to render them safe in the track during winter in this 

 latitude. The element phosphorus tends to render rails brittle or cold- 

 short, and as the carbon is increased the phosphorus must be decreased. 



