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Title: The suitability of ferritic oxide dispersion strengthened tubular components for heat exchanger applications
Author: Rees, M.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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The investigation focused upon determining the circumferential creep rupture strength of two ferritic Oxide Dispersion Strengthened tubular materials, namely MA956 and ODM 751, and classifying the degree of anisotropy displayed by MA956 at 1100oC. The most critical part of such an assessment was that tubular components be tested under internal pressures, as the hoop strength of the tube is design limiting. Testing of this nature necessitated that the tubular specimens be made pressure retaining, hence the initial experimental work focused upon successfully diffusionally bonding tubular MA956 to both Nimonic 105 and Alloy 800H. The joint design was incorporated into a tubular component design which enabled a series of internally pressurised creep tests to be performed. The tensile creep properties were determined in a purpose built direct loading rig where test sections cut from the tube wall could be tested. The circumferential creep strength of ODM 751 was approximately three times greater than MA956. Both alloys failed in a low ductility intergranular fashion due to creep cavity development and coalescence on longitudinal grain boundaries, which was surmised to be strain controlled. The longitudinal creep rupture strength of MA956 was an order of magnitude greater than the tube's circumferential strength. MA956 displayed circumferential Norton stress exponents in the range of 3.8-6.7, compared to a longitudinal stress exponent of 36.2. The contrasting stress sensitivities resulted from the differing grain morphologies and the degree of constraint afforded to cavity growth by the adjacent matrices. The longitudinal threshold stress theory was calculated to be 35.7 MPa. However, the threshold stress concept was not applied to the circumferential data of MA956, as the threshold stress seemed to be dependent upon grain geometry and the stress required to cause cavitational development, rather than dislocation/dispersoid interactions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available