Experimental Investigation and Elastic-Plastic Analysis on Connection Strength of Zirconium Tube-Tubesheet Joints

Zirconium tube is widely used in heat exchanger equipments in petrochemical process for significant corrosion resistance. The connection joint of tube-tubesheet is the weakest parts in a heat exchanger. The experiment and numerical analysis of different materials (zirconium tubes, titanium tubes and 16MnR tubesheets, 316L tubesheet) joints were performed in this paper. The expansion joints specimens were prepared at the pressure ranging from 28MPa to 38MPa. And pulling out test was performed from 20°C to 300°C. The finite element model of tube-tubesheet joint was established. The effect of expansion pressure, temperature and groove on the pulling out strength of joints was analyzed. Both the experiments and the finite element analysis show that the pull-out strength increases with the increasing expansion pressures. Working temperature also has a great effect on the connection strength of tube-to-tubesheet joints, especially for the zirconium and 316L joints, which have the most significant difference of thermal expansion coefficient between tube and tubesheet. The residual contacting pressure on the contact surface between tubes and the tubesheet is not uniformly distributed and two tightness bands are found near the surfaces of the tubesheet or at the two brinks of the groove on the tubesheet hole. Compared with the ungrooved joint, the residual contacting pressure on the tightness bands for the grooved joint is much higher, indicating a grooved joint has better tightness.

Crystallographic Texture Change of Pilgered Zirconium Alloy Tubes with Heat Treatment

Crystallographic texture of pilgered zirconium alloy tubes was analyzed by neutron diffraction, electron backscatter diffraction (EBSD) and X-ray techniques to study bulk and local texture change with pilgering and heat treatment above re-crystallization temperature. Pilgering resulted in slightly inclining (001) planes to sample normal direction, and aligning effectively (100) planes to the normal of radial direction, respectively. (001) planes of the zirconium tube uniformly exist in radial and tangential directions, however, the (001) planes moved from tangential to radial directions after pilgering followed by heat treatment for 20 hours at 540°C. Some of (001) and (100) planes of pilgered tube were paralleled to longitudinal direction by re-crystallization.

Low Cycle Fatigue Behavior of Zirconium Tube under the Cyclic Pressurization

Study was attempted to produce stress-life curve of the zirconium tube under the cyclic pressurization and to analyze its behavior. The internal pressurization machine was devised and the low cycle fatigue test at the zirconium cladding tube was carried out. The result showed that the O’Donnell and Langer relationship can be fitted to the fatigue behavior of the zirconium cladding under the cyclic pressurization, where the fatigue limit of the Zircaloy-4 at 350oC was shown at 341.6MPa. From the analysis of the diametral changes and the fractographic observation, a combined creep-fatigue interaction rather than a pure fatigue had an influence on the failure of the zirconium cladding under the cyclic pressurization at 350oC.