?,oS 



NATURE 



[SEPTEMUtR 8, I9IO 



It was the micro-structure of the while portion, how- 

 ever, which was of unique interest. On " heat-tinting " 

 two kinds of hard crystals appeared, one more readily 

 coloured by heating than the other. The more resistant 

 crystals were idiomorphic, and were furnished with their 

 terminal angles, but as they were embedded in the 

 surrounding metal it was impossible to form any eNact 

 idea of the crystalline system to which they belonged. 

 - The second order of crystals had evidently solidified at 

 a later period, as their forms were interfered with by 

 those of the idiomorphic type ; they were much like 

 ordinary plales of carbide of iron. The ground mass con- 

 tained indications of octahedral on fir-tree crystallites and 

 a well-developed phosphorus iron eutectic of the honey- 

 comb type. This eutectic was the last to freeze, as it 

 filled the spaces between the plates of the hard crystals. 

 There was no pearlite excepting in the <;utectic of phos- 

 phorus and iron. We can only tentatively conclude that, 

 of the two cementites. the idiomorphic crystals contained 

 the greater part of the silicon, because of their greater 

 resistance to o.xidation, and probably consisted of carbo- 

 silicide of iron, Avith sufficient sulphur in them to make 

 them stable; also that the second crystals were carbideof 

 iron, possibly containing a lesser quantity or no silicide 

 in solid solution. 



A further series of experiments was made on a portion 

 of the same metal. In this case the molten iron was mixed 

 and agitated with free sulphur instead of sulphide of iron, 

 and the metal was at once poured into a sand mould in 

 a thin layer. When cold it was quite white in fracture, 

 and had large, brilliant cleavage faces. 

 It had the following composition : — 



Pf r r^nt. 



Combined carbon • 2-60 



Manganese ... ... -•■ ••• ••■ Trace 



Silicon <'-6s 



Sulphur 0-93 



Phosphorus ... ... ..- ••- ••• 2-08 



The sulphur Iiad evidently effected concentration of the 

 silicon, phosphorus, and carbon by removing some of the 

 iron, as sulphide of iron was actually formed and floated 

 on the surface of the iron. It was fractionally dissolved 

 as described in previous cases, and the residue (72 per 

 cent, of the weight of the original metal) was tested and 

 found to contain : — 



P r cent. 



Carbon 292 



Manganese Trace 



Silicon <>70 



Sulphur 0-062 



Phosphorus 1-410 



This insoluble fraction evidently consisted of both classes 

 of crystals, together with some phosphide of iron. Efforts 

 were made to separate the crystals by chemical means, 

 but without success. 



On the long and continued action of strong hydrochloric 

 acid a residue was obtained containing a little less carbon 

 and more silicon than were present in the mixture, an 

 indication that the less soluble portion is different from 

 that more soluble." 



The micro-structure was similar to that of the metals 

 of the previous trial, but as the carbon and silicon were 

 higher the carbo-silicide was in greater quantity. It 

 crvstallised in long flat plates, and not in relatively short 

 idiomorphic crystals. 



It is probable exception may be taken, with some justifi- 

 cation, that the sulphur does not simply arrest the decom- 

 position of the cementites, which I have premised primarily 

 form, but may act in some other unknown way. An 

 attempt was therefore made to find out whether they 

 could be obtained by some other method without the aid 

 of sulphur. As it is known that the ternary eutectic of 

 iron, phosphorus, and carbon melts at about 945° C, it 

 appeared probable that if silicon in small quantity were 

 to be melted with an iron-carbon-phosphorus alloy very 

 rich in phosphorus, the two kinds of cementites would fall 

 out of solution at a lower temperature, and would prob- 

 ably not decompose into graphite and silico-austenite in 

 cooling down after their formation. To ascertain whether 

 or not this would be the case, a fusible iron-phosphorus- 



NO. 2132, VOL. 84] 



carbon alloy 

 of carbon w 



ontaining more than the eutectic proportion 

 made, ll h:id the following composition: — 



Iron qi-89 



Phosphorus... ... ... ... ... ... 5-37 



Carbon ... ... ... ... ... ... 2-62 



Silicon, &c. ... ... ... ... ... o-io 



Sulphur ^ 0-02 



100-00 

 Four hundred grams wc>re melted with sufiicient silicon 

 alloy to yield in the mixture : — 



Carbon ... ... ... ... ... ... 2-4 



Phosphorus ... ... ... ... ... ... 5-0 



Silicon ... ... ... ... ... ... 2-90 



.Sulphur ... ... ... ... ... ... 002 



When melted, a portion of it was cast in a sand mould ; 

 the remainder was allowed to cool in the crucible- 



When cold, that cooled in the crucible was quite grey, 

 whilst the portion cooled in sand was white at the lower 

 part and grey on the top part of the casting, results which 

 proved that the alloy was very unstable, and that decom- 

 position of the lower part of the casting was arrested by 

 the slight chilling effect of the cold sand. 



On microscopic examination of the white portion, the 

 ground mass was found to consist of the binary phosphorus 

 iron eutectic, whilst two different cementites were 

 embedded in it; one much more rapidly coloured on " heat- 

 tinting " than the other. The colours of the constituents 

 of the properly heated and polished melal were as 

 follows : — ■ 



Cementite (a) ... While 



{/>) Ked 



Phosphide of iion ... ... ... .. Purple 



Iron pearlite crystallites ... ... ... Grey 



The part which broke with a grey fracture consisted 

 of octahedral crystallites of silico-pearlite, the binary phos- 

 phorus iron eutectic, and undecomposed (red) cementite 

 crj'stals, but there was a complete absence of the (white) 

 cementite crystals. Graphite was also present in exceed- 

 ingly fine plates, resembling what is known as temper 

 graphite. 



The evidence here is conclusive that even in the absence 

 of sulphur : — 



1st. Two cementites had formed. 



2nd. That one cementite is much more unstable than 

 the other variety, and decomposes in advance into silico- 

 austenite and graphite. * 



Having proved that two different kinds of cementite do 

 actually form and crystallise in the phosphorus eutectic, it 

 remained to ascertain in what way these crystallise in the 

 absence of the phosphorus eutectic. 



For this purpose two hypo-eutectic alloys were prepared 

 without any phosphorus, but with sufficient sulphide of 

 iron to check the decomposition of the carbides. 



They contained : — 



Pel 



Carbon 



Silicon _ . 



Sulphur I-2I 0-82 



Phosphorus O'oz 002 



These when cold, after casting in sand, broke with 

 white fractures. 



The carbides separated in the manner previously 

 described contained : — 



Carbon 



Sulphur 



Silicon 



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 



