THE ROLE OF MANGANESE AND COPPER IN THE EUTECTOID TRANSFORMATION OF SPHEROIDAL GRAPHITE CAST-IRON

The decomposition of austenite to ferrite plus graphite or to pearlite in spheroidal graphite (SG) cast iron is known to depend on a number of factors among which are the nodule count, the cooling rate, and the alloying additions (Si, Mn, Cu, etc.). This study was undertaken in o rder to deepen the understanding of the effect of alloying with Mn and /or Cu on the eutectoid reaction. For this purpose, differential therm al analyses (DTAs) were carried out in which samples were subjected to a short homogenization treatment designed to smooth out the microsegr egations originating from the solidification step. The effect of vario us additions of copper and manganese and of the cooling rate on the te mperature of the onset of the stable and metastable eutectoid reaction s was investigated. A description of the conditions for the growth of ferrite and of pearlite is given and shows that these reactions can de velop only when the temperature of the alloy is below the lower bounda ry of the ferrite/austenite/graphite or ferrite/austenite/cementite re lated three-phase field. The experimental results can be explained if the appropriate reference temperature is used. The cooling rate affect s the temperature of the onset of the ferrite plus graphite growth in the same way as for the eutectic reaction, with a measured undercoolin g that can be extrapolated to a zero value when the cooling rate is ze ro. The growth undercooling of pearlite had values that were in agreem ent with similar data obtained on silicon steels. The detrimental effe ct of Mn on the growth kinetics of ferrite during the decomposition of austenite in the stable system is explained in terms of the driving f orce for diffusion of carbon through the ferrite ring around the graph ite nodules. Finally, it is found that copper can have a pearlite prom oter role only when combined with a low addition of manganese.