Me2, thanks for your explanatory comment. 
Prof. Verhoven was aware of the problem with the volume fraction, because on the same page where he defines volume fracture he writes a footnote comment "Strictly speaking this formula gives weight fraction ferrite." (In my copy page 22). For me it means, that strictly speaking all estimates based on the Fe-C diagrams are expressed in weight % C.
It is not difficult to calculate that pure cementite contains some 6.7 weight % C*, but it is difficult to calculate volume % C. I am not quite sure how to do it.
Jan
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* Cementite has a formula Fe3C, atomic weight of Fe is 56 while atomic weight of C is 12.
Hence the Weight % C = 12 / (3*56 + 12) = 0.067 i.e. 6.7%.
Prof. Verhoven was aware of the problem with the volume fraction, because on the same page where he defines volume fracture he writes a footnote comment "Strictly speaking this formula gives weight fraction ferrite." (In my copy page 22). For me it means, that strictly speaking all estimates based on the Fe-C diagrams are expressed in weight % C.
It is not difficult to calculate that pure cementite contains some 6.7 weight % C*, but it is difficult to calculate volume % C. I am not quite sure how to do it.
Jan
_______________________________________________________________________________________
* Cementite has a formula Fe3C, atomic weight of Fe is 56 while atomic weight of C is 12.
Hence the Weight % C = 12 / (3*56 + 12) = 0.067 i.e. 6.7%.