Effect of Elemental Segregation on the Microstructure and Mechanical Properties of Heavy Section Compacted Graphite Iron

The effect of elemental segregation on the microstructure and properties of heavy section compacted graphite iron (CGI) was studied to develop a type of CGI cylinder liner for marine low-speed diesel engine. It is found that the nodularity in heavy section of the cylinder liner showed obvious difference due to the different cooling rates and segregation of cerium (Ce) and magnesium (Mg). The exterior and interior samples of the heavy section with higher cooling rate and lower Ce and Mg segregation had the lower nodularity of 8% and 13%, respectively. While the nodularity of central samples was up to 20% and the nodule size was also larger than those of exterior and interior samples. Ce and Mg segregated into the post solidified molten iron, resulting in the formation of bulk or nodular graphite. The CGI matrix was composed of pearlite and a small amount of eutectic carbide. Segregation of carbide forming elements promoted the formation of carbides at eutectic boundaries. The carbide content of central sample was 3%, higher than those of exterior sample (2.1%), and interior sample (2.5%). The tensile strength, elongation, and hardness of central samples were 383 MPa, 0.42%, and 248 HBW, respectively, which were lower than exterior sample of 400 MPa, 0.52%, and 252 HBW, and interior sample of 388 MPa, 0.43%, and 250 HBW. The fracture morphologies of tensile samples were characterized by cleavage facets and a few torn edges, showing the typical brittle fracture. The carbides and shrinkage porosities promoted the initiation and propagation of cracks.