TRANSACTIONS OP SECTION G. GG7 



eection, sbowiug the true configuratiou to be tliat of geometrically oriented steps, 

 were shown. The application of the method to the study of fractures was next 

 dealt with, and photo-micrographs of transverse sections of typical tensile, shock, 

 and bending fractures of iron and mild steel shown under magnifications ranging 

 from 100 to 1,800 diameters. By means of these sections, the exact path taken by 

 the fracture among the various micro-constituents can be traced in the most 

 minute detail, thus throwing fresh light on the mechanical behaviour of the 

 various constituents. Thus, in the case of mild steel, it was shown that in all 

 forms of fracture which are preceded by severe plastic deformation, the fracture 

 passes through ferrite and pearlite areas almost indiflerently, while in the case of 

 fractures produced by shock, in such a way that fracture is not preceded by serious 

 deformation, the path of the fracture lies almost entirely through the ferrite areas. 

 The author suggested the explanation that in the case of considerable plastic 

 deformation, the lower plasticity of the pearlite results in the formation of internal 

 fissures just before the adjoining ferrite breaks, so that the ultimate fracture tends 

 to follow these fissures through the pearlite ; microscopic evidence in favour of 

 this view was shown. In conclusion, the author pointed out the possibilities 

 opened up by this method in studying the mechanical behaviour of metals having 

 a complex structure, and also in the study of ' mysterious ' fractures occurring in 

 practice or in testing. 



i. Segregation in Steel Ingots, and its Effect in Modifying the Mechanical 

 Properties of SteeU By 3. E. Stead. 



5. Structural Changes in Nickel Wire at High Temperatures.'^ 

 By H. C. H. Carpenter. 



The research has been carried out in order to ascertain, if possible, the reason 

 for a fundamental change in the mechanical properties of nickel wire used as the 

 heating coil of an electrically heated porcelain tube-furnace. 



The wire contained 98'60 per cent, nickel, 1'22 per cent, iron, 0-16 per cent, 

 manganese, and a trace of cobalt. Some dissolved gas or gases were also present. 

 The diameter of the wire was yigth of an inch. The ultimate tensile stress was 

 35-2 tons per square inch, with a percentage elongation of 34-4 on 3f inches and 

 a percentage reduction of area of about 70. The resistivity at 0° 0. was 9-2 

 microhms-cm. In building the furnace the wire is wound round an imglazed 

 porcelain tube (1| inch external diameter), which is inclosed in a wider one, the 

 intervening space being filled with crushed quartz. The ends of the furnace con- 

 sist of porcelain slabs which fit into the wider tube, and are bored so as just 

 to allow the passage of the narrower tube. In actual use the wire carries 

 20 amperes at 50 volts pressure, and a temperature of 1200°-1300° 0. can be 

 obtained in the tube. With care the life of such a furnace is usually three or 

 even more months. But sooner or later it breaks down. The wire is then usually 

 found to be so brittle that it can be snapped between the fingers. Occasionally it 

 is still tough, but has become perfectly fibrous. These changes of mechanical pro- 

 perties are accompanied by structural changes which have been studied with the 

 microscope. They are the result of the combined influence of heat and electricity, 

 and are not produced by either of these agencies singly. 



It appears that the changes are due mainly to two effects, viz., recrijstallisation 

 and the penetration of gases, which are themselves the result of heat and elec- 

 tricity on the metal. The frequent association of brittleness with gross crystallisa- 

 tion has long been known. But the evolution of dissolved or combined gas or 

 gases from nickel and their mode of penetration through and eventual exit from it 

 by means of cracks between the gross crystals are, it is thought, described here 



' Published in the Proceedings of the Cleveland Institution of Engineers, 

 * Published in Engineering, August 17, 1906. 



