3D modeling of the crystalline structure of thick multi-pass austenitic welds made in position by GTAW process. Application to ultrasonic Non-Destructive Testing. – MINA3D

The demonstration of safety and the extension of the lifetime of complex industrial devices (nuclear...) are based on the periodic non-destructive testing (NDT) of welded parts. When there are thick welds (30 to 70 mm), in austenitic stainless steel, the ultrasonic method for defect detection is the only one possible. It is however complex because the heterogeneous and anisotropic nature of these thick multi-pass welds induces strong perturbations in the propagation of the acoustic beam which distort the diagnostic.

The best (non-destructive) solution to overcome this difficulty is obtained by modelling the ultrasonic propagation, but this requires the detailed description of the real crystalline structure of the weld. The current 2D weld models, except LMA’s work, provide either a simplified description of the crystallographic growth, based on a symmetry assumption, or a more realistic description, but at the cost of high instrumentation and computation time. Moreover, no model exists for a weld made in position, when the solidification is also governed by gravity.
The objective of the project is to produce a realistic 3D model for welds made with GTAW process in all positions, from minimalist input data (those given by the DMOS) and with a calculation speed compatible with industrial needs.
Gravity induces inclinations of the texture not only in the direction perpendicular to the weld, but also in the welding direction. The transition from 2D to 3D is therefore not a simple evolution or adaptation of MINA 2D, because the gap is very important. The study will be progressive: some mock-ups will be manufactured for a narrow chamfer (U-type) which allow a stacking of a single pass per layer, and open chamfer (V-type) geometry, in vertical-up and horizontal groove welding position. Specific instrumentations (embedded camera, optical microscopy, EBSD) will help us to understand the solidification kinetics and the grain growth, and then to create the model, the challenge being linked to the various length scales present (weld, grain, dendrite).
The objective is to determine a link between the pool shape (gravity, welding energy, ...), the thermal gradient (part temperature, chamfer, preheating, ...), and the crystal growth (crystal competition, ...).
The orientation of the grains will be ultimately calculated from information voluntarily restricted to the welding notebook which describes the welding procedure (geometry of the chamfer, sequence of passes, etc.), to be in adequacy with the industrial practice, which cannot afford to instrument each welding carried out in a complex way.
The micrographs simulated by the model will be compared to the real micrographs and will thus allow to validate it. A second validation will also be sought by comparing the ultrasonic propagation predictions obtained by associating the MINA 3D model with a 3D ultrasound propagation model, with experimental data. The prediction of the deviations and divisions of the ultrasonic beam will then be mastered, bringing a significant improvement of the ultrasonic testing.
The MINA 3D project perfectly fits with the research axis B.4. One innovation concerns the increase in knowledge of the material, but the main innovation is in the application, and therefore in the consequent improvement of the potential of NDT by ultrasound. The 6 partners of the project are the best French specialists in the field and used to work together.