PHYSICS — howe;. 235 



boundary is at about 725° C, and the upper varies from 725° to 1130°, ac- 

 cording to the carbon content, this conglomerate mass is completely trans- 

 formed, its several components merging to form the solid solution called 

 austenite, vv'-hich in slow cooling again splits up into a conglomerate like that 

 of its initial state; but when it is cooled rapidly from above this transforma- 

 tion range, differentiation into the old conglomerate condition is obstructed, 

 so that the metal is arrested in some condition intermediate between that of 

 that conglomerate state and austenite. 



It has long been known that the free ferrite or cementite often forms well- 

 marked cell-walls, inclosing kernels of pearlite often containing irregular 

 grains or islands of ferrite or cementite. At other times this cellular struc- 

 ture is either fragmentary or wholly lacking, so that the mass consists solely 

 of irregular grains of its several constituents. It has also long been known 

 that the properties of the steel are affected profoundly by the size and 

 arrangement of the constituent particles. But our fragmentary knowledge 

 has been much confused by failure to distinguish between these composite 

 cells and the individual grains. 



The present investigation aims to trace the life history of these cells and 

 grains. I have enunciated or re-enunciated the previously supposed or 

 known laws, verified a number of those previously insufficiently proved, and 

 discovered several additional laws. These laws follow : 



(i) The cellular structure arises normally in one of the two differentia- 

 tion processes, (a) solidification and (&) transformation from austenite into 

 pearlite, etc. During this transformation each grain of mother austenite 

 rejects to its outside the ferrite or cementite to which it gives birth, and thus 

 sets up the cellular structure. 



(2) This structure is unstable and thus represents an early stage in struc- 

 tural development. It breaks down through the spheroidizing and coales- 

 cence of the ferrite or cementite in its walls and that scattered through the 

 kernels. 



(3) The cell-size increases with the length and height of the heating 

 above the transformation range. The rate of growth evidently increases as 

 some relatively high power of the temperature is reached. 



(4) The rate of cooling affects the cell-size only in so far as it affects the 

 total opportunity for growth above the transformation range. 



(5) Both long and high heating increase the stability of the cellular 

 structure, 



(6) The cellular structure is slower to develop and is more stable in 

 hyper-eutectoid than in hypo-eutectoid steel. 



The presence of manganese — 



(7) Increases the extent of the areas free from coagulated and hence 

 recognizable ferrite ; 



(8) Lessens the size of the individual cells, and of the individual ferrite 

 grains ; and 



