( 535 ) 



is in internal oqiiilibrium, there will also be internal equilibrium in 

 the other coexisting -phase. 



If we apply this consideration to the system iron-carbon, we may 

 observe that as the liquid phases on the line AB coexist with the 

 mixed ciystal line Aa, these latter solid [)hases will also be in inter- 

 nal equilibrium. But this holds then for the whole mixed crystal 

 region, and then martensite mixed crystals are nothing but solid 

 solutions of graphite and cementite in iron, and these three substances 

 w\\\ be able to form an internal equilibrium also in the solid phase. 



It follows from this that internal equilibrium can prevail also in 

 the mixed crystals on the line Sa and in those on S'a', and this 

 conclusion now furnishes a clue to the explanation of Royston's 

 observations. 



Let us, namely, suppose that malleable cast iron has been heated 

 to 1030° for a long time, so that the stable equilibrium between 

 martensite mixed crystals Srj and graphite C^ has set in. If we 

 now do not cool slowly enough, it is possible that the deposition of 

 graphite fails to appear entirely or partly, in consequence of which 

 the mixed crystal phase does not follow the line as, but another 

 course, e. g. S,j Sc and now in consequence of the greater carbon 

 concentration in the solid phase the internal equilibrium may lie so 

 much inore to the carbide side than in case of stable equilibrium 

 with graphite at the same temperature, that the solid phase becomes 

 supersaturate also with respect to cementite, and the latter deposits 

 in Sc 



Now it is of course possible that also the deposition of cementite 

 can be retarded, but as this does not give rise to the formation of 

 new phases, this is of no importance here. 



If the cooling is not too rapid, the metastable mixed crystal eutectic 

 point will be reached at about 690°, where the conversion : 



martensite -^ ferrite + cementite 

 takes place, and the original concentration of the martensite mixed 

 crystal phase Sg lying on the right of that of the eutectic point S\ 

 the structure elements perlite -j- cementite must be found, and by 

 the side of them of coui-se graphite, so exactly what was observed. 



It is clear that this is peculiar about the just discussed phenomenon 

 that though the martensite crystals are in contact with graphite, no 

 graphite, or not a sufficient quantity, deposits with not very slow 

 cooling. 



It has, however, been stated already several times that a super- 

 saturate liquid in contact with the substance with respect to which 

 the liquid is super-saturate can sometimes remain so for a comparatively 



36* 



