Graphite Nodularity Evaluation in High-Si Ductile Cast Irons

Ferritic high-Si ductile cast irons replace an unstable mixed ferrite-pearlite matrix with a unique combination of high elongation, strength and hardness (ideal for automotive drive train components) and resistance to oxidation and corrosion at high temperatures (automotive exhaust and turbocharger systems). The present paper analyses the graphite parameters of 4.5%Si, un-inoculated ductile cast iron (4.7%CE, 0.035%Mg_res) as an effect of the casting section size. The structure is characterized by 10.5-11.2% graphite and 464-975 nodules/mm², at more than 70% ferrite and no carbides, including at 3 mm wall thickness. The lower the wall thickness is, the higher the nodule count is and, consequently, the higher the ferrite amount is. The Roundness Graphite Shape Factor (RSF = 0.65-0.68) illustrates the presence of Slightly Irregular Spheroidal Graphite (Form V ISO 945). There is a big difference between the graphite nodularity evaluated according to ISO 16112:2017 [CGI] (NG₁ = 79-86%) and according to ISO 945-4-2019 (DI) (NG₂ = 65.2-74.6%), both of them based on RSF. Graphite Nodularity (NG₃), calculated with the ISO 945-4-2019 [DI] formula, but replacing RSF with SSF, Sphericity Graphite Shape Factor, has an intermediary position. The higher the imposed minimum RSF or SSF is, the lower the Graphite Nodularity (NG₄, NG₅): 80-90% for min. 0.50 (minimum Form IV or Intermediate Graphite), 60-80% for min. 0.60-0.65 (minimum Form V graphite) and 35-70% for min. 0.80 (minimum Form VI graphite). The SSF is more representative than the RSF for Si-alloyed ductile cast iron, so it is recommended to use a graphite nodularity calculus considering SSF instead of the RSF formula (stipulated by ISO 945-4-2019), with SSF replacing RSF.