Gray Cast-Iron Water Pipe Metallurgy

Abstract

The results of a metallographic study of gray cast-iron water pipes are reported. These pipes were installed between 1885 and 1973 in eight different water systems. Each pipe had been extracted during scheduled maintenance or failure repairs to provide data for a larger study to produce a methodology for determining the residual life of gray cast-iron pipes. This metallographic study was conducted to determine the causes of variations in the mechanical properties of these pipes. Pit-cast and spun-cast pipes were found to have distinctly different types of graphite flakes, flake sizes, and metallic matrices. These differences were directly responsible for the variations in the mechanical properties between the two types of pipes, with the larger flake sizes of the pit-cast pipes, in particular, producing weaker material. Examples of anomalous pipes that did not have the standard appearance of either type of manufacture were also found and the reasons for their appearance identified. The results of the study show that the metallurgy of the pipes may be a major contributing factor, along with external forces such as corrosion or poor installation practices. Metallographic analysis therefore can assist water utilities managers in making decisions on repairs, rehabilitation, and replacement.