The influence of graphite morphology on the hardenability and austemperability of cast irons

Coral (CG), Compacted/Vermicular (C/VG) and Nodular Graphite (NG) shapes were analyzed as to degree of compactness and it was found that the cumulative factor (L) is more representative as it considers not only area and perimeter but also maximum size of graphite particles. Simultaneous presence of NG and C/VG in iron structure strongly influences the graphite/matrix interface of entirely graphite phase, which reaches the highest level for CG. Two suitable hardenability criteria were used (Jominy end quench test), based on careful microstructural study and hardness/microhardness measurements along the length of end quenched bars-a first-ferrite criterion and a half-hardness criterion. Graphite morphology was observed to have significant effect not only on the as-cast structure (ferrite/pearlite ratio) but also on the hardenability, which decreases in the following order: NG : C/VG : CG = 1.0: (0.6...0.9) : (0.4...0. 6), as hardness evaluation. First-ferrite criterion is more efficient in hardenability prediction of irons, especially at lower graphite compactness. The use of hardenability-austemperability relationship established for ADI [2] could be also extended for other graphite morphologies, mainly based on the results concerning the total and reacted retained austenite evaluation, pro-eutectoid ferrite incidence at highest graphite-matrix interface, martensite occurrence due to un-stable austenite, etc. Alloying grade and/or quench severity in austempering process must be increased for C/VG Iron and especially for CG Iron, which is difficult to austemper (limited to thin-wall castings).