390 SUMMARY OF CURRENT RESEARCHES RELATING TO 



previously placing particles of salts of known melting points on the 

 rear surface of the specimen. An examination of the particles when 

 cold, revealed whether they had melted or not. 



"Growth " of Cast Irons after repeated Heatings.*— H. F. Rugan 

 and H. C. H. Carpenter have made extensive experiments on different 

 commercial cast irons and on numerous alloys containing varying 

 amounts of carbon and silicon, to ascertain the cause of the familiar 

 phenomenon of growth. Long continued heating is not accompanied 

 by continuous growth ; successive heatings and coolings are necessary 

 for growth to take place. White irons of low silicon content altered 

 in volume very little in repeated heating and cooling, while cast irons 

 containing notable amounts of silicon, and accordingly grey, in- 

 creased in volume in some cases more than GO p.c. The exclusion of 

 gases, by carrying out the repeated heatings in a vacuum, prevented 

 the growth of some specimens, but others still gave some increase of 

 volume. In these cases the growth is ascribed to the effects of the 

 gases originally present in the metal. A theory of the mechanism of 

 growth is outlined ; the following are the chief points : Commercial 

 grey iron consists essentially of a solid solution of iron silicide in iron, 

 intermingled with which is a quantity of graphite. Heating in air or 

 in flame gases, leads to a penetration of gases, which, probably on cooling, 

 are absorbed by the solid solution and oxidise the silicide of iron. This 

 oxidation is accompanied by an increase of volume, causing incipient 

 disintegration and probably driving the graphite into holes originally 

 present in the metal. A second heating brings about further gas absorp- 

 tion, and the reaction proceeds through numerous heatings until the 

 whole of the silicon and some of the iron are oxidised. The metal has 

 now lost the mechanical strength of cast iron, and can be sawn like 

 chalk ; its microstructure has been revolutionised. 



Iron-carbon Alloys. | — N. Gutowsky has sought to determine accu- 

 rately the solidus curve of this system — the temperatures at which solidi- 

 fication is complete on cooling, or at which melting begins on heating. 

 While the temperatures of commencing solidification and the eutectic 

 temperatures were clearly indicated in cooling curves taken by the author, 

 the points of final solidification could not be discerned in alloys not con- 

 taining eutectic. For this reason, and also because of the difficulty of 

 ensuring complete equilibrium in a solidifying alloy, the cooling-curve 

 method was abandoned in favour of the method of examining micro- 

 scopically alloys after quenching at known temperatures, the samples 

 being previously annealed to bring them into a condition of equilibrium. 

 The solidus curve thus obtained for the range, to 2 p.c. carbon, is con- 

 siderably more concave than that given by Heyn and others, the tem- 

 peratures at which melting begins, in intermediate compositions, being 

 lower than the temperatures formerly assumed. A series of cementation 

 experiments, and a study of the phenomena of melting and solidification 

 of white and grey iron, were also carried out. 



* Journ. Iron and Steel Inst., lxxx. (1909) pp. 29-143 (15 figs.). 

 t Metallurgie, vi. (1909) pp. 731-6, 737-43 (35 figs.). 



