Simulations of the Influence of the Grains Orientations on Ultrasounds

In some austenitic stainless steel welds grains orientations cause deviation and splitting of the ultrasonic beam. It is especially true in the case of multipass welds when the remelting process after each pass causes complex solidification process. With the new model MINA (Modelling anisotropy from Notebook of Arc welding) we show that we are able to predict thoroughly grains orientations. This model has been validated with microstructural analysis and ultrasonic measurements. With these orientations we define a precise description of the heterogeneous and anisotropic material. Grain orientation give the Cartesian coordinates system in which we expressed the elastic constants. Incorporating this description in an ultrasonic propagation code allows simulating ultrasonic inspection. Comparisons are made with previous models given by several authors. The reference grain structure is obtained by macrograph analysis. Different propagations of the elastic waves induced by a modification of the evolution of the grains orientations are shown. The best simulation results are obtained with MINA model. We thus provide a very interesting model dedicated to multipass welding to improve understanding of ultrasounds propagation in a very heterogeneous medium. This work ensures a better reliability of ultrasonic testing. Introduction: The ultrasonic assessment of structural integrity in nuclear power station progresses with advances in ultrasonic wave propagation codes. It makes ultrasonic testing more accurate and reliable. Several modelling codes have been proposed to forecast the propagation of ultrasounds. Authors use ray tracing codes, semi-analytical codes or finite element codes. Developments concerned three steps: a calculation of ultrasonic fields radiated by transducers, a calculation of the ultrasonic wave propagation and a simulation of the various echo-formation mechanisms. Descriptions and comparisons of these modelling techniques are found in literature [1-3]. An efficient simulation depends on an accurate description of the material properties. It is well known that inspection of components composed of austenitic steel can cause difficulties. Several large demonstrating programmes highlighted this phenomenon: the projects PISC (Programme for the Inspection of Steel Components) [4-5] or the Defect Detection Trial [4]. Theses major international efforts were made to improve the assessment of capability and reliability of procedures for non-destructive testing. Best results were obtained in the case of ferritic steels with easy access. Performances decreased when access difficulties increased. Lowest results were obtained with cast austenitic steel welds [6-8]. It was concluded that there is a growing need for precise description of the weld material to improve the capability of simulating real weld testing. The source of the difficulties lies in the grains structure of austenitic steels. The ultrasonic wave equation could be written in the form: