Evaluation of Sub-Surface Residual Stress by Ultrasonic Method and Finite-Element Analysis of Welding Process in a Monel Pressure Vessel

Abstract Welding of nickel-based alloys is increasingly used in the industry to manufacture various important structures in the marine industries, chemical processing, etc. This study investigates evaluation of sub-surface residual stresses, which are produced by the welding process in a pressure vessel made from Monel 400 alloy. The residual stresses are experimentally measured by ultrasonic method in which longitudinal critically refracted (LCR) waves are propagated inside the specimen to evaluate the effect of stress on the wave velocity. Any difference in the wave velocity could be transformed to the material stress by using acoustoelasticity relations. A nondestructive hydro-test process is used to measure the acoustoelastic constant, which is an important material property needed to be embedded in the acoustoelasticity relations. By using a different frequency range than the ultrasonic transducers, the LCR wave penetrates in different depths of the specimen to measure the sub-surface stresses. The welding processes are also numerically analyzed by a 3D thermo-mechanical finite-element (FE) model, which is validated by hole-drilling stress-measurement method. The residual stresses calculated by FE simulation are then compared with those obtained from the ultrasonic stress measurement and an acceptable agreement is achieved. It is demonstrated that the sub-surface residual stresses of the Monel pressure vessel could be accurately evaluated by combination of the FE simulation and stress measurement implemented by the LCR waves.