PRESIDENTIAL ADDRESS. 557 



It has long been known that on dissolving grey ferro-silicon containing even 

 six 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 decom- 

 posed the silicide part of it would become diluted with the iron of the decom- 

 posed 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 varvo at 960° C, chemically combine, a fact I have fully confirmed, although 

 it is impossible to get silicon to combine 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 3'5 per cent, of carbon, was mixed with 10 per cent, by weight of a 

 silicon alloy containing 20 per cent, of silicon ( = Fe,Si) 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 : — 



In white metal alone .... 

 „ ,, and silicide . 



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 contained 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 which 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 premised, 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. 1 grey glazed metal was melted, 

 and when fluid a little sulphide of iron was mixed with it. The mixture was then 

 cast in sand. 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 analyses were as follows : — 



White part. Grey part. 



Per cent. Per cent. . 



Combined carbon 206 0-60 <" " 



Graphite Trace 1-46 



Manganese 0-03 0-03 



Silicon 5-41 5-40 



Sulphur 0-88 0-91 



Phosphorus 1-50 1-50 



1910, p O 



