Preticipation of Spheroidal Graphite during Cooling from Liquid State in Hyper Eutectoid Steel

The process of formation of graphite during cooling mainly in hyper-eutectoid Fe-C-Si alloys was studied. Some experiments were carried out to examine changes in the nodule number and the shape of graphite taking place under the effect of Mg addition and changing cooling rate below the solidus temperature.Experimental results obtained were as follows:(1) The nodule number of graphite formed in specimens with hyper-eutectoid composition decreased with decreasing cooling rate below the solidus.(2) In case of specimens containing Mg, the shape of graphites was almost spherical in the initial stage of the formation process of graphites in the range of chemical compositions in the present work. This initial form of graphites, however changed into irregular shapes during cooling in the later stage only under the condition of slow cooling (8.3×10−3 K/s). This phenomenon more markedly appeared in the hyper-eutectoid composition than in the hypo-eutectic.(3) In case of specimens containing no Mg, the shape of graphite changed from that agglomerated and like grain boundary to that like films and then to a flaky form with increasing total amount of C and Si.(4) The results obtained suggest that the form of graphites mainly depends on the shape of precipitation sites of graphites which are defects such as grain boundary, voids and gas cavities.