• Theoretical modelling of ultrasound columnar grain boundary scattering and validation of models with experiment,

• Metallurgical and acoustic microscopy investigations of columnar grain structure,

• Inspection qualification during defect assessment by ultrasound on welded austenitic test pieces.

Four groups of materials comprising the full scale of industrially relevant stainless steels have been selected to be investigated: Austenitic stainless Cr-Ni steels, fully austenitic stainless steels with increased Ni-content, Nickel-base-alloys, and Duplex steels (Ferritic-austenitic steels).

Based on the plane wave ansatz, for the first time three-dimensional ray tracing techniques have been developed, to trace the path of the ultrasound in the transverse isotropic medium. Experimental sound ray tracing serves to validate the ray tracing calculations.

However, because of convergence problems during ray tracing ("mirage effect") 'grain sizes' had to be assumed unrealistically small. Therefore modelling of the ultrasonic amplitude, which is attenuated due to grain boundary scattering, by the present ray tracing technique is excluded.

Therefore theoretical concepts, better suited to the problem, have been followed to model the ultrasonic amplitude, which is attenuated due to grain boundary scattering:

• A statistical approach (extension of the Stanke \& Kino unified theory),

• The 'Transfer Matrix' approach specified by A. N. Nayfeh.

Additional causes of ultrasound scattering may include different degrees of segregations within a bead and grain boundary precipitation. However, these are still under investigation. Validation of the models is by measuring attenuation of the main beam in austenitic weld metal. The results of amplitude modelling will be introduced later to the ray-tracing algorithm. Test specimens, both flaw free welded specimens and welded specimens with reference reflectors and realistic defects, have been prepared and currently are under inspection using various techniques.

A comment on ultrasonic examination of welds in austenitic steels has been already submitted to the European standardisation bodies CEN.