Effect of Ultrasonic Scattering on Inspection of Austenitic Welds

Record Control Number : 31119

Quality Validation Date : 16-Sep-1997

Update Date : 04-Nov-1997

Title : Effect of Ultrasonic Scattering on Inspection of Austenitic Welds

Subject Index Code : Measurement Methods; Reference Materials; Standards

General Information : Specification of Industrial Problem: There is a strong incentive to perform reliable ultrasonic inspection of austenitic stainless steel welds to detect and classify defects which could cause weld failure. This needs codes and regulations specifying the ultrasonic inspection in the same way as it is usual for normal ferritic welded components and plant.
Specification of codes and regulations for such welds are mainly hampered because of severe attenuation of the ultrasound. The sound is scattered and mode-converted at the boundaries of the columnar grains of the austenitic weld metal This has several - basically less understood - effects for evaluation of ultrasonic signals:
- The amplitude of the ultrasonic echo is influenced by ultrasound scattering, phase-changes, and mode-conversion in a characteristic way.
- The spectrum of the ultrasonic pulse will be distorted depending on the (unknown) transfer characteristics of the columnar grained weld metal.
- Ultrasonic grain boundary back-scatter is measured as noise decreasing the signal-to-noise ratio during ultrasonic testing.
The rules recommended for evaluating the ultrasonic indications in examination of welded austenitic components up to now are - strange to say - based on the physical laws of sound propagation in isotropic materials neglecting the anisotropic properties of the weld metal. Defect acceptance/rejection criteria of the approving authorities are therefore conservative and it happens that welded austenitic components and plant are repaired or even rejected although the true size of the defect might have been acceptable in terms of fitness-for-purpose.
Objectives
1. A general examination procedure for austenitic stainless steel welds shall be developed. This requires understanding of ultrasound scattering in columnar grained austenitic stainless steel weld metal, which shall be achieved in the frame of this project by
1.1 theoretical modelling of ultrasound columnar grain boundary scattering and validation of models with experiment,
1.2 metallurgical investigations of columnar grain structure,
1.3 performance demonstration during defect assessment by ultrasound on welded austenitic testpieces.
2. A general examination procedure for austenitic stainless steel welds on the basis of the results shall be written and introduced in the currently intended revision of the International Institute of Welding(IIW)}- 'Handbook on the Ultrasonic Examination of Austenitic Welds'

Start Date : 01-Mar-1996

End Date : 28-Feb-1999

Duration : 36 months

Project Status : Execution

Programme Type : 4th FWP (Fourth Framework Programme)