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Title: The high temperature performance of ½Cr ½Mo ¼V/ 2¼Cr1Mo steel weldments
Author: Parsons, A. W. J.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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The majority of high temperature, high pressure steam piping systems, in British plant, are currently operating at times approaching the nominal design life of 100,000 hours. A major proportion of in-service failures have been found to be associated with service times of between 60,000 and 80,000 hours. These failures have been shown to be associated with cracking in the intercritical region of the weldment heat affected zone, termed type IV failure. Research to data has failed to consistently reproduce failures typical of those encountered in service, even in uniaxial cross-weld creep tests of up to 10,000 hours duration. The current programme has reproduced type IV failures in cross-weld tests of between 70 and 8000 hours. From such tests it has been possible to deduce the material characteristics, test conditions, and specimen geometries that increase susceptibility to this form of in-service damage. It was found that as-welded and post weld heat treated weldments showed no susceptibility to damage accumulation in the intercritical region of the heat affected zone, despite extensive type IV damage in ex-service weldments under the same conditions. A programme of investigation into the tempering performance of 1/2Cr1/2Mo1/4V / 21/4CrlMo weldments revealed that tempering performance was specific to each microstructural region. For tempting times, simulating up to 100,000 hours of service, the weldment microstructures tempered in such a way that hardness variations recorded across the heat affected zone decreased with time at elevated temperature. For simulated periods beyond 100,000 hours, the weldment was shown to produce a region of decarburization within the heat affected zone. The location of this zone was such that, in service butt welds, a circumferential region of soft ferrite would be formed. The potential of this microstructural change for producing a mode of long term in-service failure is assessed. The condition of the weldments tested is also presented in terms of a remenant life assessment of the ex-service components supplied for the project by BP Chemicals Ltd.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available