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Failure Analaysis
Investigation
Close inspection of the insulator caps indicated that several of them had suffered multiple arc strikes, with associated melting, cracking and metallurgical changes in the melted and heat affected zones. These aspects are illustrated below. Figures 4 and 5 shows small secondary cracks associated with the melted zone of an arc strike, while Figure 6 shows an etched metallographic section through the edge of an insulator cap, showing similar small cracks. The cracks can be seen to largely be associated with the HAZ of the arc strike region in which the microstructure has been changed.
Metallography These caps were manufactured from a malleable ferritic cast iron, containing irregular graphite nodules. Figure 7 shows an unetched section of the microstructure near a fracture surface and the graphite nodules can clearly be seen. Etching such a section (Figure 8) indicates clearly that the HAZ associated with an arc strike changes the microstructure from a ferrite matrix (RHS of the picture) with small regions of pearlite to one that is largely pearlitic (LHS). In the melted zone (Figure 9) refinement of the graphite nodules has also occurred.
Mechanical Properties Malleable ferritic cast iron is made from white cast iron via an annealing heat treatment at about 910°C for between 40-60 hours to decompose the original cementite. The casting is then cooled to 760°C for 2 hours and then to 700°C over another 20 hours. This process gives rise to a structure of ferrite and irregular graphite shapes (called temper carbon) which has poor toughness, a tensile strength typically between 278-355 MPa and elongation to failure of < 10%. Mechanical property information for malleable cast irons is summarised on the Casting Source website, which contains a number of informative articles on cast irons. The fracture toughness of malleable iron, whilst low, is adequate to resist fracture under the calculated values of arc energy, at least in an undamaged condition. The critical issue in causing these failures, the occurrence in service of multiple arc strikes, is considered in Activity 1. Note: For metallographic information on white cast irons, see the Manchester Internet Microscope project, and the Metallography.com website, while information on cast irons generally can be found at the Key to Steel site. Proceed to third part of case study.
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