176 
thereon and gave the first comprehensive melting point diagram, 
without, however, a complete unanimity of opinion being attained. 
There has been a difference of opinion particularly as to the question 
whether iron carbide Fe,C is a stable or more likely a metastable 
compound. Baknurs RoozeBoom took it to be stable below 1000° and 
in agreement therewith E. D. CAMPBELL} found by means of thermic 
determinations a positive heat of formation. CHarpy, BENnepicks and 
others have, however, argued that cementite must be metastable, 
which conclusion is confirmed by the experiments of Royston ®). 
Moreover the subsequent measurements of Rurr and Grrsten*) 
have taught us that Fe,C is endo- 
thermic (—~15.1 cal.). Hence 
we may assume with a fair 
‚153 
degree of certainty that the 
solid phase Fe,C is metastable 
in regard to iron and graphite 
and probably also in regard 
to iron and amorphous carbon. 
In Fig. 1 we thus have the 
diagram of condition. In many 
changes in condition, however, 
such as in a not exceedingly 
slow cooling, the carbide (cemen- 
tite) is often formed instead of 
graphite + iron which, once it 
is formed, passes but exceedingly slowly into the stable phases‘). 
In considering the ternary system we will, therefore, have to take 
into account the possibility of formation of Fe,C. 
Fe —>C 
Fig. 1 
The isotherms for temperatures below 700°. 
The equilibria in the ternary system at constant temperature may 
— as it has been done with the systems metal-sulphur-oxygen *) — 
be represented in an equilateral triangle of which Fe, C and O are 
the apexes. [ 
Below 700° martensite, the solid solution of carbon in iron, is not 
yet stable. Carbon and iron are thus in equilibrium with each 
1) Journ. Iron and Steel Institute 59, 217 (1901). 
*) Journ. Iron and Steel Institute 1, 166 (1897). 
3) Ber. d. D. chem. Ges. 45, 63 (1912). 
4) Also see A. Smits Z. f. Elektrochemie 18, 51 (1912). 
) Reinpers and Reinpers and GOUDRIAAN, |. c. 
