Micro-Metallography. By J. E. Stead. 277 



piece of machinery, for instance, may have perfect mechanical pro- 

 perties, a perfect microstructure, be of correct mechanical design, 

 and yet fail when in use, because of some slight depression or 

 flaw in the surface of the metal. When these are present, it is of 

 very little use to make further investigation, because these irregu- 

 larities are almost always sufficient to account for the failure. If 

 they are absent, it is important that the metal near to the initial 

 point of fracture should be selected for microscopic examination, 

 for it is quite possible that this particular part may have been 

 weakened by some kind of incorrect thermal or mechanical 

 treatment, when possibly no other part of it may have been so 

 affected. 



It often happens that engineers and others, who wish to have a 

 micro-examination of the metal, cut off a piece perhaps several 

 inches away from where the fracture initiated, and expect the 

 metallographer to diagnose from the structure the cause of fracture. 



It is important also in metals which have been rolled or forged 

 that longitudinal as well as cross sections should be examined, for 

 it not infrequently happens that what cannot be seen in the cross 

 section becomes quite evident in the specimen cut longitudinally. 



In selecting specimens from worn surfaces, such as rails, tyres, 

 and other similar metals, two such surfaces should be placed face 

 to face, so that the worn parts of the metal constitute a line 

 between the two sections. After placing face to face the two pieces, 

 1 cm. by £ cm. by •£ cm., they are held in position in a vice, and a 

 little solder placed on the back of them in sufficient quantity to 

 keep then in position. Care should be taken to place the lower 

 part, opposite to the soldered end, against a metal plate, so that 

 the surface to be examined does not become heated. The specimen 

 is then ground and polished in the usual way, and a section of the 

 worn or crushed surface can be examined. If this precaution of 

 placing the pieces together is not taken, and an attempt is made to 

 polish the metal in the usual way, the worn surface will become 

 rounded on the polishing blocks, and it will be difficult to study it 

 properly. 



In the preparation of samples of wire, first of all it is necessary 

 to obtain pieces of steel about the same hardness as the wire, 1 cm. 

 square and h cm. in thickness. In the centre of this a hole is 

 drilled of exactly the same thickness as the wire. The latter is 

 inserted in the hole, the metal placed on a steel block, and the 

 back of the specimen secured in position by solder. The section is 

 then polished in the usual way. Longitudinal sections are prepared 

 by soldering several pieces of wire on a small block 2 cm. by ^ cm. 

 by 1 cm., but only at the extreme ends, so as to avoid heating the 

 central parts, which alone are examined. All the pieces are then 

 ground down to half their diameter, and the surfaces are polished 

 and etched in the ustial way. 



