On the Microstructnre of Hypo-eiUectoid Steel. 269 



form of MnS, which, although not a desirable constituent, is 

 nevertheless the safest form the sulphur can assume, since MnS 

 is about the first to solidify when the molten steel is crystallizing 

 ■out to form the ingot. The freezing-point of pure MnS is generally 

 accepted as being 1620° C, whilst that of pure iron is 1500° C. 



When sulphur combines with the iron to form FeS it is 

 highly detrimental if in any quantity. Now, whereas MnS sepa- 

 rates out and appears as " dove-grey " islands in the ferrite, FeS 

 tends to form membranous sheaths surrounding the pearlite 

 •crystals in such a manner as to seriously interfere with the vitally 

 necessary adhesion of one pearlite crystal to another, or the pearlite 

 to the ferrite, and consequently it is a source of weakness, and 

 renders the steel unreliable. The melting-point of i'eS is about 

 ■950° C, so that during the rolling process, which generally com- 

 mences at a higher temperature than this, it will be seen that 

 whilst the ingot as a whole has " set " (though still plastic) the 

 membranous FeS is still in a liquid state, and so destroys the 

 cohesion which is absolutely essential whilst the ingot is under- 

 going the required reduction by rolling. In extreme cases the red- 

 hot ingot will break into several pieces. That such effects are 

 produced by sulphur has long been known to the steel-maker, 

 although I believe I am correct in saying that the cause was some- 

 thing in the nature of a mystery iintil the microscope defined the 

 form of MnS and FeS. Where the latter is present with its low 

 melting-point it is now known that it is rather like attempting to 

 bind two solids together with a liquid. Eather a difficult propo- 

 sition 1 



As in this case we had clear indications of the doubtful areas as 

 found by sulphur-printing. Three pieces were cut for examination ; 

 the first containing the marked segregation, the second containing 

 what was evidently a branch of it in the web, and the third was 

 from the foot. Visual examination previous to " etching " revealed 

 the presence of the MnS enclosures, which can be^ seen in the first 

 photomicrograph (fig. 5). It must be understood that this illus- 

 tration was not representative of the whole. It was selected from 

 a part particularly rich in these enclosures in order that the various 

 formations could be observed. In the centre, where evidently the 

 rolling-stresses have been least exerted, it will be seen that there is 

 quite a tendency for the MnS to partly retain the " globular " form, 

 whilst further away the stresses have apparently tended to break 

 the larger globules up into smaller units, with a tendency to form 

 into lines. A glance at the first sulphur-print (fig. 2) will 

 show that there are fine though well-defined sulphur " lines " in 

 the web, and in order that the elongated shape of the MnS could 

 be seen another photograph was taken in the web. 



It will be observed in another photograph from the same speci- 

 men (fig. 6) that, with one exception, the MnS exists in small 



