

THE EUTECTOID TRANSITION POINT OF CARBON STEELS. 191 



temperature tends always to be less than the amount which would be present if the 

 solution had the uniform concentration corresponding with complete equilibrium. 



The calculated fraction of the whole mass which deposits as eutectoid being c/c, 

 it follows that when c is small the amount of solid solution remaining when the 

 eutectoid temperature is approached is also small. 



Consequently, the effects arising out of the comparative slowness of diffusion are 

 not likely to be considerable. But when c is larger, these effects may become 

 important. We shall then have comparatively thick layers of solid solution just 

 above the eutectoid point, and there may be an appreciable difference of concentration 

 between their centres and their surfaces. 



As the temperature falls, however, the surface concentrations rise and the layers 

 become thinner. Each of these effects tends to increase the concentration gradients 

 from the surface inwards. Hence the diffusion tends to accelerate. This acceleration 

 will be maintained because the iron crystals will grow and maintain the surface 

 concentration of the carbide in solution as the layers thin down. Hence the trans- 

 formation of the last of the pre-eutectoid iron will be rapid and difficult to distinguish 

 thermomagnetically from that of the eutectoid. 



Such considerations enable us to see that the amount of magnetisation acquired 

 after Q in the figures is passed may easily be greater than corresponds with the 

 amount of iron contained in the eutectoid. 



10. Experimental Evidence of the Effects of Diffusion. 



In order to confirm the existence of incomplete equilibrium of the kind pictured 

 above, we performed the experiments indicated by the thinner-lined curves of 

 figs. 6 and 7. 



Instead of allowing the temperature of the material to fall continuously as before, 

 the cooling was now interrupted repeatedly. After each interruption the temperature 

 was slowly raised some 10 or 20 degrees, and then allowed to fall slowly to a point 

 below that at which the interruption took place. This process was repeated several 

 times in each steel as shown in the curves. 



It will be obvious at once that the general result of every interruption and re-heat 

 is to add to the amount of magnetisation shown by the steel when the temperature 

 of interruption is regained. 



Each cycle of temperature change tends to reduce the concentration differences 

 within the solid solution and to make the amount of iron set free correspond more 

 nearly with that required for complete equilibrium. It thus becomes possible to 

 distinguish more clearly between the real contribution of the excess iron and that of 

 the eutectoid iron to the magnetism of the material as a whole. 



From the positions of the lines A and B with respect to the " interrupted " curves, 

 it will be seen that there is no reason to doubt (l) that in these steels, as in that of 



