The Structural Integrity Assessment Of A Dissimilar Metal Welded Overlay Safe End And The Failure Analysis Of The Bourdon Spring Of Electrohydraulic Servo Valve

This paper mainly analysis two typical mechanical components, the integrity of the dissimilar metal weld overlay between pipe-nozzle of nuclear pressure vessel and safe end, and the failure reason of the bourdon spring of electrohydraulic servo valve in aerospace. Primary water stress corrosion cracking (PWSCC) has often been produced in the dissimilar metal welded joints between pipe-nozzles of nuclear pressure vessels and safe ends. The weld overlay of Alloy52M with higher corrosion resistant has usually been made to repair or mitigate this PWSCC. Based on the detail three-dimensional finite element analyses, the failure assessment curves (FACs) of the dissimilar metal welded overlay structure were constructed, and the effects of the weld overlay thickness, crack depth and crack location on the FACs have been analyzed; the LBB curves and ligament instability lines of the dissimilar metal welded overlay structure were constructed, and the effect of the weld overlay thickness on the LBB safe margin has been analyzed. In aerospace, the electrohydraulic servo valve is a very important part of electro-hydraulic servo systems, the bourdon spring is most probably to failure in the system. The second part of the paper mainly analysis three typical failure bourdon spring, using the SEM and optical microscope, observing the whole deformation, damage, crack appearance, surface conditions, fracture characteristics, and the microstructure of bourdon spring, study the failure mechanisms and factors, and analysis the failure and fatigue reasons by finite element software ABAQUS. The main results are as follows:(1) With increasing the weld overlay thickness, decreasing the crack depth and moving the crack location to the pipe-nozzle, the failure assessment curves shift upward, which increases the safety of the structure. To accurately assess the integrity of the DMWJ structure with weld overlay, the FACs related to the weld overlay size, the crack size, the crack location and the material properties should be constructed.(2) The LBB curves and ligament instability lines with the weld overlay are located above those without the weld overlay. With increasing the weld overlay thickness, the LBB curves and ligament instability lines both shift upward, and the LBB safe margin of the dissimilar metal welded joint is increased(3) The failure mechanism of the bourdon spring is brittle fatigue crack growth. The inclusion composition of the material caused the fatigue crack nucleation and growth; the unevenness of the grain, the amount of the excessive grain whose geometry size exceed30μ m, and the accumulate grain boundary may also result in the fatigue failure.(4) In normal circumstance conditions, the maximum tensile stress of the whole bourdon spring structure are less than the fatigue strength, which would not cause the fatigue nucleation. In failure circumstance conditions, the fatigue nucleation probably happen because of the increasing of the stress. In fluid experiment mode, as the internal pressure increases, which cause the increasing of the hoop stress, may result in the transverse crack nucleation and growth.