Evaluation of factors contributing to the failure of cast iron water mains Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9c67wr205

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  • An evaluation was made of factors contributing to the service failure of a 12 inch diameter cast iron water pipe in Pendleton, Oregon. The quality of the pipe was found to be acceptable when compared to the acceptance limits established by the American National Standards Institute, Inc. However it exhibited numerous casting defects such as blowholes, gas porosity, lapping zone discontinuities and poor casting surface indicating a poorly cast product. Metallurgical study also revealed a discontinuous solidification pattern which produced a thin separated metal layer at the pipe's outer wall. The cause of the failure, however, was not attributed to any of these defects. Between 71 and 88 inches on the fracture surface, an unusual black surface appearance was observed. Microscopic examination and electron microprobe analysis of the corrosion deposit in this area revealed a very thick adherent layer of corrosion products, found nowhere else on the fracture surface, and a penetrating type corrosive attack. This corrosive attack was determined to be characteristic of graphitic corrosion that occurred over a long period of time. Based on the results of this evaluation, it was concluded that the failure was not caused by any metallurgical defect, by corrosion alone, or by service stresses. It appeared to be caused by the presence of a longitudinal hairline crack on the inner pipe wall that was formed as a result of impact received before or during installation. Over the four and one-half years of service that the pipe was in service, corrosion and service stresses increased the depth and length of the crack until the pipe was so weakened that a seven feet four inch longitudinal crack opened all the way through the pipe wall over a short period of time.
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