Ultrasonic Examination of a Weld Repair in Ni-based Alloy

The anisotropic, heterogeneous and coarse granular structures of austenitic stainless steel welds lead to important disturbances of the ultrasonic propagation: beam skewing and splitting, attenuation, backscattering and spurious echoes. These disturbances have been studied by EDF R&D for a few years in order to improve the ultrasonic NDT in nuclear applications. In particular, a methodology for modelling ultrasonic inspection of such welds was developed and applied with success to Shielded Metal Arc Welding (SMAW process). This paper presents a laboratory study on the ultrasonic examination of a Ni-based alloy weld repair (Inconel 82). The weld was realized using an automatic Gas Tungsten Arc Welding (GTAW) process, in a horizontal-vertical position. Such a process is considered for local repairs of nuclear components. This work combines both experimental and numerical approaches. Indeed after experimental acquisitions using several UT probes performed on a representative weld repair mock-up, their numerical mirrors were computed using a FEM code (ATHENA, developed by EDF) which simulates the propagation of the ultrasonic beam in the weld and calculates the defect/beam interaction. The input description of the weld was obtained from a metallographic study. In particular, data provided by Electron BackScatter Diffraction (EBSD) allow determining numerous material characteristics such as grain orientation, grain size, elastic properties or even attenuation coefficients. The use of several ultrasonic probes allows the evaluation of the influence of wave characteristics on the detection of side drilled holes for both the experimental and the numerical studies. Finally a good agreement with the experimental results is showed, which gives the opportunity to initiate a parametric study of the weld structure effects on the ultrasonic performances. Limits of 2D numerical approach to deal with the GTAW ultrasonic inspection are also discussed. CONTEXT AND OBJECTIVES EDF has launched about ten years ago a consistent research program on ultrasonic inspection of austenitic stainless steel welds. In particular, a methodology was developed to improve the understanding of the main disturbances of the ultrasonic propagation (beam deviation, distortion and attenuation) in components welded with a SMAW process (Shielded Metal Arc Welding) [1]. This methodology, based on UT modelling, was applied to various components such as: a butt weld of the surge line [2], the branch pipe weld connecting the Chemical and Volume Control System (CVCS) and the primary coolant piping of PWR circuits [3], or even nickel-based alloy welds of the steam generator divider plate [4] and of the reactor vessel clevis [5]. This paper presents a laboratory study on the ultrasonic examination of a weld repair in alloy 82 and realized with a GTAW process (Gas Tungsten Arc Welding). The objectives are to study the UT performances in this weld, to improve the understanding of the ultrasonic disturbing phenomena and to check if the methodology developed for the SMAW process can be applied to GTAW process. This work is based on both experimental analyses on welded mock-ups and the use of a finite element modelling tool (ATHENA 2D code) which allows realistic simulation of the propagation phenomena in complex mediums. WELD CHARACTERISTICS AND CHARACTERIZATION The characteristics of the mock-up are presented in Figure 1. In particular, dimensions of the repair cavity after machining are indicated. The weld repair was performed using an automatic GTAW process, in a