In this paper, a sensing device specifically for measuring deformations of high temperature pipes is designed, and its applicability is verified both experimentally and theoretically. First, the design procedure and the working principle of the sensing device are described in detail. Then, experiments are carried out to prove the accuracy and the long-term stability of the sensing device. To verify the accuracy of the device, numerical simulation of the deformation of a pipe model is carried out using finite element method. Results from the experimental measurements are in good agreement with results from the numerical simulation. The long-term stability of the device is validated by monitoring the deformation. Conclusions are drawn that the designed sensing device has high accuracy and excellent stability and can be used for measuring deformations of high temperature pipes in power plants.
Direct diffusion bonding of 316L stainless steel was performed at 850-1100°C for 1-3 h
under a pressure of 10MPa in this study. The effect of bonding temperature and holding time on
mechanical performances of the joints was investigated. Tensile tests were conducted to evaluate
strength and elongation of the joints at room temperature and elevated temperature of 550°C. The
microstructure and fracture surfaces of the joints were examined by optical microscope (OM) and
scanning electronic microscope (SEM). The results indicated that the elongation of the joints
increased with the increase of bonding temperature and holding time. However, overlong holding
time had a side effect on the strength of the joint. Moreover, the change of the mechanical properties
was closely related to the variation of the microstructure of the joints. The X-ray diffraction (XRD)
analysis revealed that FeCr and Fe0.64Ni0.36 were formed at the DB6 joint during bonding process. It is
suggested that FeCr should be detrimental to the improvement of high temperature strength of the
joint.
Two types of joining specimens with and without Ni foil interlayer between 316L–SS bar have been prepared by diffusion bonding in a temperature range of 850- 1050°C, under a uniaxial pressure of 10MPa for 1hours. The relationship between the bonding parameters and the tensile strength of the joints at elevated temperature was studied. Optimized processing parameters were suggested based on the testing results. It was found that the introduction of the interlayer may reduce the room temperature strength but increase the high temperature strength. This was attributed to the transformation of Fe0.64Ni0.36 formed in bonding process into FeNi3 at high temperature.
The dynamic measurement model for the interface cavities of the diffusion welded joint, which based on the effective resistivity, is established by using the damage factor and the Derby Model of the Bridgman Law. Then the quantitative relationship, which between the direct current potential and the effective area of the joint at high temperature and high pressure, can be obtained. In addition, on the basis of the dynamic four-probe DC potential method and the fieldbus technology, the computer measuring system is established, so that the creep propagation behaviour for the interface cavities of the 316L stainless steel can be monitored in real-time.
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