Properties of as-cast and heat-treated nodular graphite cast irons, melts treated with CaSi-CaF_2 alloy

摘要

An attempt has been made to evaluate the microstructure and mechanical properties of as-cast and heat-treated grades of nodular graphite cast irons produced through nodularising treatment with CaSi-CaF_2 alloy. The results showed that up to 92 percent nodularity in graphite could be obtained by using a pre-alloy inoculant with 8.10 wt percent CaSi-CaF_2 (ratio 5:2) optimum concentration. An average hardness of 327 Hv, Charpy-V-Impact energy of 5.60 J, tensile strength of 611 MPa and a corresponding elongation of 3.05 percent have been obtained for the as-cast iron, whereas the microstructure appeared completely ferritic. Although annealing and normalizing treatments produced significant growth and enhancement in nodularity of graphite, there was little improvement in mechanical properties. The iron's response to the applied marquenching transformation schedules was equally poor, with a maximum strength of 830 MPa and a corresponding elongation of 2.10 percent obtainable after 45 mm of transformation. The iron's poor response to heat treatment and mechanical properties resulting thereof, were apparently due to the effect of the rather high silicon content (4.05 percent) on movement of the reaction fronts and hardenability in the case of marquenching and ductility in other heat treatments. Excessive grain boundary cleavage, accompanied by large smooth areas separated by wide cleavage steps and other characteristic features of brittle fracture, appear to be the main mode of fracture, for both as-cast and heat treated grades of the iron.