Hot Cracking Susceptibility of 800H and 825 Nickel-Base Superalloys during Welding via Spot Varestraint Test

Hot cracking susceptibility of fillers 52 and 82 in 800H and 825 nickel-base superalloys was discussed using the Spot Varestraint test. The fillers of 52 and 82 were added into nickel-base superalloys via a gas tungsten arc welding (GTAW).Experimental results showed that the hot cracking sensitivity of the nickel-base superalloys with filler at high temperature was lower than that without filler. The hot cracking sensitivity had a slight effect when the filler 82 was added. The total length of crack was increased, the liquid-solid (L-S) two-phase range is higher so that the hot cracking susceptibility will be raised. The morphologies of cracks included the intergranular crack in the molten pool, molten pool of solidification cracking, heat-affected zone of intergranular cracks, and transgranular crack in the heat-affected zone.

Study of Pore Modality of Irradiated UO2 Fuel Assembly

UO2 fuel pellets may be swelling and recrystallization during irradiation. Density, dimension and distribution of pores are main factors to induce irradiation swelling, especially the size distribution of pellet pores plays an important role. 4×4-4 fuel assembly was a high performance fuel assembly which was self-designed and manufactured, the average burn-up of the fuel assembly was 42GWd/tU.For studying the effect of irradiation on pore modality, the specimens of irradiated UO2 fuel pellets were taken from 4×4-4 fuel assembly after dismantling, microscopic structure and distribution of pores for UO2 fuel assembly by scanning electron microscopy were studied in this paper. The results showed that there were many cracks in fuel pellets, most micro-cracks were transgranular crack. The release rate of fission gas with burn up were augmentation, which was consistent with the porosity of diversity burp up fuel rod. Pores were distributed non-uniformly in irradiated fuel pellets, gathered at local area and more obvious connectivity of pores. The size of pores after the irradiation was between 0.2~1.2 μm, and mostly distributed at 0.3μm ~0.6 μm; The pores at grain boundary of two adjacent grains was less, the pores at grain boundary were distributed by the way of triangle or quadrilateral. The size of pores was increased than pre-irradiation, but ratio of pores and density of pores were decreased obviously, the phenomenon of irradiation densification was occurred in fuel pellets after irradiated. Recrystallization and Rim structure effect were not found.