Shi TtMliKK' S, 1910] 



NATURE 



507 



varieties of metal in acid differed in character — that from 

 the white portion was dense, whilst that from the grey 

 metal was much more voluminous. The white metal con- 

 tained the eutectic proportion of carbon, and therefore it 

 could not contain any austenite crystallites ; indeed, with 

 the silicon obo per cent, also present, it irust be regarded 

 as a hypereutectic alloy, and on that account we are 

 forced to conclude that the silicon must have crystallised 

 with the carbide. 



It has long been known that on dissolving grey ferro- 

 silicon containing even 6 per cent, silicon the silica 

 gelatinises, whereas when the silicon approaches 10 per 

 cent, much of the silica remains in a dense form. It is 

 almost certain that during the solidification of the grey 

 part of Mr. Hogg's pig iron a rich silicon cementite must 

 have primarily formed, for the high carbon would not 

 allow the formation of any primary silico-austenite ; when 

 this cementite decomposed the silicide part of it would 

 become diluted with the iron of the decomposed carbide. 

 It was, no doubt, this diluted solid solution in the cold 

 grey metal which yielded the gelatinous silica. 



That silicon does diffuse into iron, even at relatively 

 low temperature, was proved by Lebeau. He found that 

 free silicon and iron, when heated together in vacuo at 

 960° C, chemically combine, a fact I have fullv con- 

 iirmrd, although it is impossible' to get silicon to com- 



FiG. 7. — An Iron^Carbon-Silicon Alloy, free from 



Phosphorus, made more stable by Sulphur. 



Groken-up £,tructure in the centre=the euteclic of two 



cementites, silico-cariiide and carbide. 



Half-tone=lhe carbide cementile. 



Dark area=decomposed eutectic. 



Light portion at right lower corner= crystallite of 



silico-pearlite. 



bine with iron on heating them together in a cementation 

 furnace where oxidising gases have access to the silicon. 



To determine whether silicide of iron would diffuse into 

 and precipitate the graphite in white iron, a sample of 

 crushed white iron free from impurities, containing 35 

 per cent, of carbon, was mi.xed with 10 per cent, by 

 weight of a silicon alloy containing 20 per cent, of silicon 

 ( = Fe,Sij, also in powder. The mixture, after compression 

 in a short piece of iron tube, was heated for two hours 

 at 1000° C. in an atmosphere of hydrogen gas, and was 

 then removed and cooled in air. 



For comparison, a portion of the crushed white iron 

 was treated in the same way. 



The combined carbon in the metals before and after 

 heating were as follows : — 



NO. 2132, VOL. 84] 



Not only does this trial prove that silicide does diffuse 

 into carbide of iron and precipitate graphite, it has also 

 an important bearing on the question as to why silicon 

 in pig iron, even in small quantities, causes the carbide 

 to be decomposed. In the experiments with the chilled 

 part of a casting containing only 0-7 per cent, silicon and 

 3-75 per cent, carbon, it was shown that the carbide con- 

 tained only 0-028 per cent, silicon, and that 98 per cent, 

 of the total silicon was concentrated in the pearlite ; yet 

 this white iron, on heating to 1000° C, became quite grey. 

 Are -we not justified in concluding that it was the diffusion 

 of silicide of iron from the silico-austenite into the carbides 

 wliich caused the separation of graphite? 



As I had proved, first that sulphur crystallises with 

 and makes the carbide of iron more stable, and second 

 that in the presence of a fusible mother liquor rich in 

 phosphorus, after the austenite crystallisation is complete, 

 the carbide crystallises out in plates and not as iron carbide 

 eutectic, it appeared probable that if, as Gontermann pre- 

 mised, two kinds of cementite actually form during the 

 solidification of iron-carbide-silicon alloys, it might be 

 possible to obtain them in a separate state by melting the 

 rich silicon alloys with a little sulphur. 



In order to test this, a portion of the No. i grey glazed 

 metal was melted, and when fluid a little sulphide of iron 

 was mi.xed with it. Thf niixturt- \\,i~ ili- n 1 r-; i'l ~ind. 



Fig. 8.— Pure Iron-Iron Carbide Eutectic, the co .ling 

 of which was arrested before the complete de- 

 composition of the Carbides into Austenite and 



White=carbide of iron. Black lines = graphite. 



Half-tone = pearlite. 



Owing to the rapidity of the melting, some of the graphite 

 escaped and floated on the surface of the metal. 



When cold it was found that the lower part of the 

 small casting gave a white fractured surface, whilst the 

 upper part was close grey. 



The analvses were as follows : — 



Combined carbon 



Graphite 



Manganese 



Silicon 



Sulphur 



Phosphorus 



The grey part, although slowly attacked by cold acid, 

 did dissoWe, yielding much voluminous silica. The white 

 part w-as almost inert, and only dissolved in strong hydro- 

 chloric acid with difficulty, and when the iron was dis- 

 solved out the remaining silica was of the dense variety, 

 from which it would appear that the effect of the sulphide 

 Is akin to that of sudden quenching. 



