Abstract
This portfolio of work presents an investigation of some of the factors influencing the service life and performance of cast iron potable water pipes (trunk mains) in order to inform the management of such important assets. A literature review was carried out in order to clarify the direction of the investigation. This covered the metallurgy of cast irons, mechanical properties and corrosion behaviour, as these are all important parameters to consider in the context of the performance of cast iron pipes in service. From the review, it was concluded that a better understanding of microstructure property relationships was required, in particular with regard to fracture and fatigue behaviour and the performance in a corrosive environment. Hence lines of research were developed to investigate these phenomena, using cast iron material sourced from nine, ex-service, trunk mains all of which had experienced some degree of in-service corrosion. The nine pipes were considered to represent reasonable samples of the types and ages of trunk main that are still in service in the Thames Water region. In the course of the microstructural work, the composition and metallurgy of the nine cast irons (proportions of ferrite, pearlite, graphite and eutectic present) were examined using a range of techniques and the graphite morphologies were characterised in accordance with ASTM standards. The sample set showed a wide range of microstructures, in particular the pearlite content ranged from 13% to 80%. A variety of flake and rosette graphite morphologies was revealed. With regard to the mechanical properties, a detailed investigation of the fracture and fatigue behaviour has been carried out using a Single-Edge-Notch Bend geometry. Fracture toughness values in the range of 15 to 25 MN/m3/2 were measured, with the higher values corresponding to higher pearlite content. Fatigue crack growth was investigated using crack propagation gauges and the Paris relation parameters were determined for a representative set of materials. Calculations of fatigue life based on the integration of the Paris law were then carried out and compared with previously published data for samples from cast iron distribution mains. Based on the results, it is suggested that fatigue crack growth in isolation may not be a major contributory factor to the failure of pipes in service. The corrosion behaviour of a number of cast irons in environments (aerated water, static water and a salt solution) representative of a range of service conditions was also investigated. The corrosion rates and associated mechanisms were influenced by environment and cast iron microstructure. The measured corrosion rates were in the range 0.07 to 0.7 mm/year. It is suggested that pipes with pearlitic microstructure and rosette graphite morphology are more susceptible to corrosion. Over decades in service these corrosion rates will lead to significant loss of section and have implications for mechanical performance and, as a consequence, asset management.