Mechanical behavior of alloy 230 at temperatures relevant to NGNP program

Identification and selection of suitable structural materials for heat exchanger application within the purview of the next generation nuclear plant (NGNP) program constitute a major challenge. This challenge stems from the lack of many desired metallurgical and mechanical properties of conventional metallic materials and alloys for applications at temperatures approaching 950 oC. Nickel (Ni)-base Alloy 230 has been highly recommended as a suitable structural material for such application due to its excellent resistance to high-temperature plastic deformation and superior corrosion resistance in many hostile environments. Systematic studies on tensile, fracture toughness, creep, stress-rupture and creep-fatigue behavior of this alloy have been performed in this investigation. A gradual reduction in yield and ultimate tensile strength has been observed with increasing temperature, as expected. The room-temperature fracture toughness of this alloy was relatively lower compared to that of other Ni-base alloys. The results of creep testing indicate that Alloy 230 may be resistant to plastic deformation at 750, 850 and 950 οC at applied stresses not exceeding 10% of its yield strength (YS) at these temperatures. At 0.25YS, this alloy exhibited an enhanced creep deformation at 850 and 950 οC. The results of stress-rupture testing, performed at 750, 800 and 850 οC under applied stress levels of 20, 25 and 30 ksi, respectively, have also been presented using different parametric extrapolation techniques. The Larson-Miller (LM) parameter was found to be very useful in predicting the rupture time. However, another approach based on the Minimum Commitment Method (MCM) was also applied that proved to be quite efficient in predicting the creep-rupture behavior of this alloy. Further, the effect of combined creep-fatigue loading on its cracking susceptibility has been studied by imposing different hold times on a triangular waveform associated with cyclic loading under a constant stress-intensity-factor range. These results indicate that the crack-growth-rate of Alloy 230 may be significantly enhanced at higher temperatures even after holding for very short durations. As to the fracture morphology, its mode of failure was changed from transgranular to predominantly intergranular due to the introduction of longer hold times and/or increasing temperature. (Refer to PDF file for exact formulas.

GIS Based Approach to Determine the Changes of Water Hyacinth (Eichhornia crassipes) Cover and Relation with Lesser Whistling Teal (Dendrocygna javanica) Assemblage at Santragachi Wetland, West Bengal

The present investigation is conducted to study the year wise (2011 to 2018) changes of water hyacinth (Eichhornia crassipes) cover at Santragachi Lake a Wetland under National Wetland Conservation Programme of India. Further the relationship between water hyacinth cover and the most abundant migratory waterbirds of Satragachi, Lesser Whistling Teal (LWT; Dendrocygna javanica) is assessed because this bird species is fully depended on water hyacinth mat for their roosting. The study comprises of eight satellite images procured from Google earth (2011 to 2018) to explore this relationship. A marked decline in the number of LWT at Santragachi wetland is observed in the year of 2017 and 2018. It is very interesting fact that from 2017-2018, the water hyacinth mat of this wetland is almost cleared before winter and the result of cluster analysis supports this fact. Significant positive correlation is also observed within LWT number and water hyacinth cover area (r = 0.7481 at p< 0.05) along with the total perimeter (r = 0.8648 at p< 0.05) of the water hyacinth islands at Santragachi wetland. However, open water area is also needed for diving, swimming, food searching for the LWT and other waterbirds. Therefore, more study is needed to optimize the clearing operations, focused on optimizing the shape and size of water hyacinth islands for proper management of the waterbirds habitat

Mechanism of Creep Deformation of Alloy 230 Based on Microstructural Analyses

Creep testing of Alloy 230 was performed at 750, 850 and 950 °C under applied stresses equivalent to its 10% and 25% yield strength (YS) values. The results indicate that this alloy satisfied the maximum allowable creep strain of 1% when loaded to 0.10YS level at these temperatures. Microscopic evaluations revealed the formation of second phase particles including superstructures and carbide precipitates as a function of temperature. The enhanced plastic strain at 0.25YS stress level at 950 °C was attributed to dislocation glide, subgrain formation, and recrystallization

Community Structure of Migratory Waterbirds at Two Important Wintering Sites in a Sub-Himalayan Forest Tract in West Bengal, India

The waterbird community structures of two sub-Himalayan wetlands (Nararthali and Rasomati) situated in forested areas were compared during the wintering period. These wetlands had similar geophysical features but were subject to different conservation efforts. Sixty species of waterbirds, including four globally threatened species, were recorded during the study. Nararthali was found to be more densely inhabited (116.05±22.69 ind./ha) by birds than Rasomati (76.55±26.47 ind./ha). Density increased by 44.6% at Nararthali and by 59% at Rasomati over the years of the study, from 2008 to 2015. Winter visitors increased considerably at Nararthali (66.2%), while a 71.1% decrease at Rasomati clearly indicated degradation of habitat quality at that site during the later years. Luxuriant growth of Eichhornia crassipes, siltation, poor maintenance and unregulated tourist activities were the key factors leading to the rapid degradation of Rasomati. Nararthali, on the other hand, a well-managed wetland habitat, showed an increasing trend in bird densities. Therefore, poor habitat management and rapid habitat alterations were observed to be the main reasons for depletion of bird density in the wetlands of eastern sub-Himalayan forest regions