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• The present study compares the thermal creep behaviour of different generations of the Zr-2.5Nb pressure tube used in Indian Pressurized Heavy Water Reactors. • Pressure tubes manufactured from new route were found to be more creep resistant than the tubes manufactured from old route. • Both grain size and aspect ratio of the new route tubes were higher than that of old route tubes. • Higher extrusion ratios incorporated in fabrication of new route tubes produced stronger texture, which resulted in better creep properties. • Stress exponent and activation energy values revealed that the creep deformation mechanism was dislocation creep mechanism (dislocation climb – rate controlling mechanism, dislocation glide – strain producing mechanism). Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. The deformation of PT in PHWR occurs due to irradiation creep, irradiation growth and thermal creep. In the present study, the different generations of PTs used in IPHWR are tested for its thermal creep behaviour. The accelerated creep rupture tests were carried out using flat tensile specimens with its axis parallel to the longitudinal direction of the PT in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The minimum creep rates were evaluated along with the stress exponents and activation energies. The values of stress exponent and activation energy revealed the underlying creep mechanisms responsible for the creep deformation. Microstructural examination was carried out to understand the need and efficacy of the new fabrication route. The observed thermal creep behaviour of all the generations of pressure tubes were rationalised in terms of their respective texture and microstructures. Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. In the present study, the thermal creep behaviour of different generations of PTs used in IPHWR is examined by carrying out accelerated creep rupture tests in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The study revealed that the rupture life of the PTs following the new routes were higher than that of old route at 350 and 400°C. At 450°C all PTs had similar rupture life. A similar correspondence was made with minimum creep rate. The tubes manufactured by the new routes were more creep resistant than the old route tubes. [Display omitted] [ABSTRACT FROM AUTHOR]

Creep rupture properties of austenitic stainless steel foil, 347SS, used in compact recuperators have been evaluated at 700 °C in the stress range of 54-221 MPa to establish the baseline behavior for its extended use. Creep curves of the foil show that the primary creep stage is brief and creep life is dominated by tertiary creep deformation with rupture lives in the range of 10-2000 h. Results are compared with properties of bulk specimens tested at 98 and 162 MPa. Thin foil 347SS specimens were found to have higher creep rates and higher rupture ductility than their bulk specimen counterparts. Power law relationship was obtained between the minimum creep rate and the applied stress with stress exponent value, n = 5.7. The value of the stress exponent is indicative of the rate-controlling deformation mechanism associated with dislocation creep. Nucleation of voids mainly occurred at second-phase particles (chromium-rich MC carbides) that are present in the metal matrix by decohesion of the particle-matrix interface. The improvement in strength is attributed to the precipitation of fine niobium carbides in the matrix that act as obstacles to the movement of dislocations. [ABSTRACT FROM AUTHOR]

Creep-rupture properties of a columnar-grained nickel-base superalloy have been evaluated over a wide temperature range (1033 to 1311 K) and stress levels (80 to 850 MPa). Creep data analyses based on the conventional approach as well as on a new graphical method—the Β-envelope method (Ref 1)—have been carried out for creep strain and life estimation purposes. The relation between minimum creep rate of the alloy with the applied stress obeys simple power law, whereas the rupture data of the alloy fits well to the Larson-Miller parameter. Also, the Monkman-Grant relation between the minimum creep rate and the rupture life produces a trend with some degree of scatter in the data. The latter relation in its generalized form by the Β-envelope method exhibited the best correlation with significantly reduced scatter in the data. [ABSTRACT FROM AUTHOR]