3201.     SIROVICH, G.  [Polymorphic Transformation of Iron at 370° and the Possibility of Solution of Cementite in a1-Iron.]  Gazz. chim. ital., vol. 53, 1923, pp. 674-688; Chem. Abs., vol. 18, 1924, p. 960.

        In a previous paper, Metall. italiana, vol. 14, 1922, pp. 3-17, Sirovich and Ariano demonstrated that a-Fe exists in 2 forms.  One of these, called a₁, is stable and magnetically resistant and in it C is soluble in the solid state; the other, the a₂-form, is formed at 370°, is relatively soft, is magnetic, but does not dissolve C in the solid state.  The above and other conclusions are confirmed in this paper by further results (1) on the existence of an A₀ point at 370° indicating polymorphic transformation of Fe and (2) on the possibility of causing solution of cementite in a₁-Fe without having recourse to tempering.  Earlier work on (1) is reviewed.  It has been considered generally that there is a complete continuity n the dilatometric behavior of Fe from ordinary temperatures up to 700°.  The results given here were obtained with the photographically registering dilatometer of Montemartini and Bosana.  A parallelpiped (4.7 cc. in vol.) of electrolytic Fe was placed in fused Sn for the determinations.  Other details are given.  The volume as registered showed a distinct break at 370°.  The same results were obtained with a fused Fe (C 0.04, Mn 0.048, S 0.052, P 0.034%), showing that the effect is not due to any H₂ present in the other specimen of Fe used.  The Sn was then replaced with melted paraffin in order to eliminate any possible effect of the Sn, but without changing results.  The same determination when carried out with a specimen of Mn-containing Fe (C 0.07 Mn 0.49, Si trace, S 0.035, P 0.04%) showed only a slight change in direction at 370°-380° but no discontinuity.  Sirovich suggests that previous failures by other workers to obtain a transformation at 370° are due to the effects of impurities as in the case of Mn here reported.  In the experiments on the solution of cementite in a₁-Fe steels containing cementite and ferrite in extreme dispersion, a hypoeutectoid steel with C 0.53, Mn 0.51, Si 0.11, S 0.039, P 0.06 was used.  It was heated at 850° for 0.5 hr.  The external decarbonized part was ground off and Brinell hardness test was made (182).  The piece then was introduced into a fused Sn bath kept at 300°±5° (heated in an electric coil).  The Brinell test and the micrographic structure were determined after 288, 528, and 3,501 hr., at which times the hardness was 216, 241, and 241, respectively.  The micrographs showed a progressively increasing homogeneity, but completion of the process is prevented by retarded diffusion.  The possibility of obtaining some solution of cementite in a₁-Fe in the field of stability of the latter with pearlitic steels was established.