Septemher S, 1 910] 



NATURE 



jirovcd by Turner, and, moreover, as will be shown 

 prtsently, it is combined with iron in solid solution before 

 the carbide is decomposed. 



Oontermann ' found that on adding pure silicon to molten 

 iron, the iron and silicon combined with considerable rise 

 in temperature, and I have noticed the same thing even 

 ■when adding it to carburised iron. 



The same authority, who has made a most careful study 

 of the ternary alloys of the iron-carbon-silicon series, has 

 shown that the eutectic freezing-point rises with the silicon 

 from 1130° when silicon is absent, to about 1150° when it 

 reaches 10 per cent., and to 1175° when it is about 17 per 

 cent., and that the carbon in the eutectic of the alloys con- 

 taining between o per cent, and 10 per cent, silicon, falls 

 as the silicon rises by about o'j per cent, for each unit of 

 silicon. 



The same author proved that the pearlite reversion point 

 in these alloys rises with the silicon on an average of 

 about 30° C. for each unit of silicon in the alloys con- 

 taining between 0° and 6 per cent, silicon. He concluded, 

 but did not actually pro%-e, that in the region of the curve 

 of unvarying equilibrium two cementites crystallise ; one a 

 solid solution of the carbide and silicide of iron ; and a 

 second, a mixture of this with another ternary iron-silicon- 

 carbon solid solution. 



If the composition of the alloy lies between the curve of 

 saturated silico-austenite and the curve of non-varying 

 equilibrium, saturated silico-austenite primarily forms ; and 

 following this a secondary crystallisation of a binary 

 eutectic consisting of this saturated austenite and silico- 

 cementite. 



In the year 1901 I described certain unique idiomorphic 

 ■ciystals which had been found in the hearth of a disused 

 blast furnace at Blaina. The crystals were more or less 

 o.\idised on their exterior surfaces. 



The analysis was as follows :— 



.After deducting 

 the (l.vygen, &c. 



Manganese 

 Iron 

 Carbon 

 Silicon 



ioo"oo 



.A. micro-examination proved the crystals to be quite 

 homogeneous mixtures, or solid solutions. It was difficult 

 to assign to them any definite chemical constitution. They 

 may be considered as silico-carbides of manganese and 

 iron, and, as will be shown presently, bear a close relation 

 10 similar crystals which primarily form during the 

 freezing of iron-carbon-silicon alloys. 



Having briefly referred to the work of a number of 

 authorities, I now propose to describe my attempts to 

 supplement our knowledge in this direction by a purely 

 micro-chemical research. 



In order to understand the remarks which follow, it is 

 necessary briefly to describe the changes which occur when 

 pure iron-iron carbide allovs pass from the liquid to the 

 solid state as are indicated by the researcehs of Osmond, 

 Roberts-.Austen, Stansfield, and of Carpenter and Keeling. 



In the iron alloys containing less than the eutectic pro- 

 portion of 4'3 per cent, carbon, described as hypo-eutectic 

 nlloys, austenite octohedral crystallites of the fir-tree type 

 first fall out of solution, and these continue to grow until 

 the liquid is so impoverished of iron and enriched in carbon 

 that when the eutectic proportion of 4^3 per cent, carbon is 

 reached, the liquid solidifies and breaks up into carbide of 

 iron and austenite. 



The hypereutcctic alloys, containing more than the 

 eutectic proportion of carbon, on cooling, first yield carbide 

 of iron crystals, and these continue to grow until, by 

 removal of the excess carbon, the eutectic proportions of 

 iron and carbon are reached. The eutectic in its turn then 

 freezes. 



For the purpose of my research it was necessary to 

 select pig metals, grey and high in silicon and white with 

 high sulphur. These were kindly supplied by Messrs. 

 Wilson, Pease and Co. 'and Messrs. Cochrane and Co., 

 Middlesbrough. They were made from Cleveland ironstone 

 and contained : — 



1 " Anorganische Chem>e, Bd. 59, 1908. 



NO. 2132, VOL. 84] 



It may be accepted that the sulphur in the white iron 

 undoubtedly is the cause of the whiteness of the iron, 

 whilst the excessively high silicon and low sulphur are 

 equally responsible for the graphitic condition of the carbon 

 in the grey irons. 



The micro-structure of the high silicon metal was charac- 

 teristic of all phosphoretic, high-silicon, carbon alloys. 

 Curved plates of graphite cut the mass in many directions, 

 whilst the binary eutectic of phosphorus and iron remained 

 in irregular patches, generally midway between the graphite 

 plates. The ground mass occupying the space between the 

 eutectic and graphite plates consisted of silico-ferrite. 



The interesting feature about the structure of the white 

 iron is that there was no iron-iron-carbide eutectic. This 

 had been replaced by the ternary eutectic of iron-phosphorus 

 and carbon, which, according to Dr. Wiist, contains 

 about : — 



Iron 



Phosphorus 



Carbon 



91 



There was evidence that' the primary crystals of austenite 

 of the octohedral skeleton type had been the first to fall 



out of solution, that the second crystal to form consisted of 

 short plates of carbide of iron (cementite) ; whilst the 

 ternary eutectic of phosphorus, carbon, and iron was the 

 last to freeze and occupied spaces between the cementite 

 plates and the primary crystals. 



Dr. Carpenter and his assistant, Mr. Edwards, of 

 Victoria University, Manchester, kindly obtained, for the 

 purpose of this address, the cooling curves of these two 

 typical metals. These were as follows : — 



Grey Iron. 

 The long arrest at 1118° indicates a change of state, but 

 is also coincident with important chemical changes. The 

 second long arrest at 945° is due to freezing of the iron 

 phosphorus carbon eutectic. The arrest at 850° indicates 

 the formation of pearlite, and corresponds closely with the 

 arrest in a similar alloy examined by Gontermann. The 

 arrest at 690° is probably due to the formation of pearlite in 

 the eutectic of iron and phosphorus, and is of great interest, 

 for it points to the conclusion that silicon is not a 

 constituent of the austenite of the ternary eutectic. 



