560 TRANSACTIONS OF SECTION B. 



The carbides separated in the manner previously described contained : — 



(1) (2) 



Per cent. Per cent. 



Carbon 616 3-00 



Sulphur 0-09 008 



Silicon 0-97 793 



Percentage of carbides insoluble in acid . 27 "5 50-00 



The repeated acid treatment in this, as in all previous cases, no doubt dissolved 

 a portion of the carbides, and what was actually weighed represented only a part 

 of those actually present in the alloys. 



In No. 2 alloy, after polishing and ' heat-tinting,' the microscope proved the 

 presence of a few fir-tree crystallites embedded in a ground mass of cementite and 

 a eutectic containing the two kinds of cementite, the No. 1 specimen containing a 

 much smaller proportion of the cementite rich in silicon than No. 2. 



As the metals had been somewhat rapidly cooled the alloy No. 2 was remelted, 

 and was then allowed to cool in the crucible, so as to obtain a more coarsely 

 crystallised eutectic. When cold, on polishing and ' heat-tinting,' the eutectic 

 was clearly seen. There were the remains of large primary silico-austenite 

 crystallites, plates of the red-coloured cementite, and a well-developed eutectic 

 consisting of the (red) coloured and (white) cementites. 



The cooling having been slow, this compound constituent had suffered partial 

 decomposition in isolated patches into graphite and silico-ferrite, whilst the 

 cementite coloured red remained intact. 



There can be little doubt that the residue left insoluble in acid consisted of the 

 two cementites, but in what proportion it is impossible to tell, as a method for 

 isolating them has yet to be found. 



Had the alloy contained a greater proportion of carbon the amount of cementite 

 rich in silicon would have been in much greater proportion. 



The trials, incomplete and necessarily imperfect as they are, go far to prove, 

 just as Gontermann premised, that during the solidification of high silicon pig- 

 irons two cementites fall out of solution together as a eutectic mixture. 



They also have proved that the carbo-silicides are exceedingly unstable, 

 breaking up into silico-austenite almost as soon as formed. It is the instability 

 of these silico-carbides which is mainly responsible for the graphitic character 

 of grey irons rich in silicon and low in sulphur. 



Summary and Conclusions. 



1. The experimental results advanced show proof that carbide of iron in 

 presence of iron sulphide crystallises with a minute quantity of sulphur not 

 exceeding about one-thousandth part of the weight of the carbide, but the 

 nature of the iron-carbon-sulphur compound has not yet been determined. 



2. It seems almost, if not absolutely, certain that it is the sulphur crystallised 

 with the carbide which makes the latter stable. 



3. The evidence appears to support the view, long held by some and more 

 recently accepted by others, that during the freezing of iron-carbon-hypo-eutectic 

 alloys after the crystallisation of the primary austenite, and in the eutectic 

 and hypereutectic alloys, it is the carbide and not graphite which primarily 

 forms and that the carbide afterwards decomposes into graphite and austenite. 



4. It has been proved by chemical methods that when the hypo-eutectic alloys, 

 low in silicon, freeze, nearly all the silicon crystallises out with the primary 

 austenite ; and it follows that on gradually increasing the carbon so as to reduce 

 the quantity of primary austenite, the silicon remaining constant, the austenite 

 which does form must be as gradually enriched in silicon up to saturation-point ; 

 and, when that point is reached, the excess silicon crystallises out with a portion 

 of the carbide of iron to form carbo-silicide of iron. Other elements remaining 

 constant, the same result must follow on gradually increasing the silicon. 



5. In the alloys of eutectic proportion and in the hypereutectic alloys, as no 

 primary austenite can form, the silicon crystallises primarily with the carbide. 



6. In Cleveland pig-iron containing about 1'5 per cent, phosphorus, a ternary 

 eutectic of iron-carbon-phosphorus takes the place of the iron-iron-carbide eutectic. 



