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The relevance of creep crack growth data obtained from uniaxial laboratory experiments to the assessment of defects in plants is being investigated in a pressure vessel research program involving full size half-percent chromium-molybdenum-vanadium components containing machined defects. The first vessel in this program contained external circumferential notches in a large pipe and was tested at 565°C and 62.5 MN/m2 steam pressure, during which time crack growth and deformation in the vessel were monitored. Final failure occurred in an explosive manner after 1583 h. This paper concentrates on the fractographic and metallographic aspects of the failure analysis.
Examination of the vessel revealed three fracture modes across the failed ligament. Intergranular creep fracture occurred immediately ahead of the machined notch, followed by 45-deg ductile shear involving extensive deformation and finally low ductility shear fracture causing fast failure. These observations are interpreted primarily in terms of the mechanisms involved, with some consideration given to the mechanics of the fracture. The intergranular creep fracture mode is considered typical of that associated with plant failures and the significance of this in defect assessment is mentioned. The transfer to ductile shear is discussed in terms of the formation of void sheets, and net section yielding is related to uniaxial tensile data. The change in mode to fast shear fracture, attributed to gross overloading, produced an explosive failure.
The main implication of the failure analysis is that defects in this material can be assessed using mechanics describing intergranular creep fracture. Even so, it is demonstrated that in a nominally ductile situation final failure can be rapid and catastrophic.
fractography, metallography, fractures (materials), pressure vessels, steels, elevated temperature, creep properties, voids, crack propagation, plastic deformation, shear properties
Research officer, Marchwood Engineering Laboratories, Marchwood, Southampton, Hampshire