94 SCIENCE PROGRESS 



In a typical mild steel the mass of the metal is composed of 

 grains of ferrite between which lie patches of a material which 

 appears homogeneous under a low magnification but is really 

 an intimate mixture of ferrite and cementite. Fig. 5 represents 

 a section of a mild steel plate cut in the direction of rolling 

 and etched to show the structure. The arrangement of the 

 grains of ferrite is seen to follow the direction of rolling, whilst 

 the intervening patches of conglomerate are not sufficiently 

 magnified to reveal their internal structure. The manner in 

 which the pearlite and cementite are intermixed in this con- 

 glomerate varies with the heat-treatment to which the steel 

 is subjected. In steels quenched from a high temperature, the 

 carbide is in a state of ultramicroscopic subdivision termed 

 "emulsified carbide" by Arnold. It then becomes black on 

 etching and is commonly called troostite. If the cooling be less 

 rapid, the carbide becomes coarser and a granular conglomerate, 

 termed sorbite, is obtained. This condition is favourable to 

 toughness and is preferred in steel rails. Thoroughly annealed 

 steels contain the iron and carbide in a very finely laminated 

 form, like the surface of some diatoms or of mother-of-pearl 

 and hence termed pearlite. This, although generally regarded 

 as the typical condition of the conglomerate, is not physically 

 stable and if the annealing process be prolonged, the laminae 

 break up, the cementite becomes gathered into relatively coarse 

 granules, segregation continuing until the original finely divided 

 mixture has disappeared entirely and the steel no longer con- 

 tains any constituent but ferrite and isolated masses of cementite. 

 As each of these structures corresponds with a distinct set of 

 physical and mechanical properties, the importance of the 

 microscopical examination of steel used as a structural material 

 is obvious. 



A further example of the breaking-up of a homogeneous 

 solid during cooling may be taken from the alloys of copper 

 with zinc containing about 40 per cent, of the latter metal, to 

 which Muntz-metal and manganese bronze l belong. Like the 



1 The necessity of a more systematic nomenclature of alloys is clearly seen 

 in this instance. Bronze is historically and in general usage an alloy of copper 

 and tin. Manganese bronze, however, is an alloy of copper and zinc to which 

 a minute quantity of manganese has been added to remove oxygen. Manganese 

 may be absent from the finished metal. Such absurdities are frequent in the 

 current technical nomenclature of alloys. 



