Investigation of a failure problem in cold-bent boiler riser and supply pipes

Abstract

A small number of failures have been reported at the extrados of cold-bent carbon/carbon manganese steel riser and supply pipes operating at elevated temperatures in power plant boilers worldwide. These failures occurred at a very low frequency and between service durations of 1000-10 000 h and more. They are generally associated with minor surface defects such as laminations or hammer marks. Detailed study of some of the failed bends had revealed that the failure was by some slow crack growth mechanism(s). The present work was undertaken to identify and isolate the failure mechanism(s) responsible and to suggest possible palliative measures. Time-dependent mechanisms were studied using standard laboratory specimens; creep crack growth has been identified as the most likely failure mechanism. Other mechanical properties such as fracture toughness, impact properties, etc., were also studied to establish the baseline material data. A number of casts of two carbon manganese steels, namely BS3602—HFS35 and BS3602—490Nb, used by the United Kingdom power generating industry were studied. Differences in their characteristics have been explained in terms of their chemical composition, hardness level and pipe fabrication/heat-treatment conditions. The conclusion from this work is that the failure mechanism is creep crack growth. By comparison with the information available on failed bends, it is suggested that it is possible to eliminate this type of failure by controlling the pipe chemical composition and hardness level together with adequate control of pipe bending, installation and in-service inspection practices.