TWI Industrial Member Report Summary 914/2008
By C Nageswaran and C Bird
Background
Dissimilar metal welds are an important class of joints used in several key industries. This includes joints between nuclear reactor pressure vessel nozzles and primary circuit pipes, and welds in corrosion resistant clad pipes used in the petrochemical industry. There are many engineering issues related to welds between ferritic (primarily the low alloy 2.25Cr-1Mo steel) and austenitic (eg Alloy 800 or 316 stainless) components; considerable research has been performed to ensure that the metallurgy is adequate for the required mechanical properties across the weld. The non-destructive testing (NDT) of such welds to assure weld quality/integrity at the point of fabrication and during service is complicated by their inherent inhomogeneity and anisotropy. Ultrasonic inspection is currently the key NDT technique in use for the inspection of thick section austenitic welds and this project investigated the detection and characterisation of defects in an example of such a weld using phased array based techniques.
Austenitic weld material is composed of large (several millimetres) columnar grains, which grow along thermal gradients during solidification of the weld. The large grain boundaries have an appreciable effect on the propagation of sound. The increased scattering of sound energy is translated into noise on received ultrasonic signals. Furthermore, the sound velocity varies across the boundary between two grains due to the anisotropy imparted by the crystallographic orientation.
Past experimental studies have shown that the longitudinal wave is generally best suited to inspection of austenitic welds; transverse waves are more sensitive to the scattering and are severely attenuated over small distances and, hence, find limited use in industrial inspection of austenitic material. To mitigate the effects of scattering, low frequency ultrasonic transducers are used such that the longitudinal wavelength is suitable for inspection of the coarse-grained material; the conventional single crystal probes recommended for austenitic inspection do not exceed 2.5MHz. At grain boundaries the longitudinal waves experience reflection, refraction and mode conversions (into transverse waves), and the trajectory of the sound beam can be altered, leading to an effect known as beam skewing. Beam skewing can lead to errors in the positioning of echoes, and interference between echoes from various interfaces (including that from the back wall) can give rise to false calls within the weld volume. Hence, it is important to understand how the sound field is distorted by the weld, as this has implications for the evaluation of defect sizes from echo amplitudes and their positioning within the weld.
In order to improve the state of affairs with respect to the inspection of austenitic welds (and inspection through austenitic cladding), researchers have proposed the use of focused beams, twin crystal probes and signal processing methods as potential strategies to overcome the issues. Whilst it is generally recognised that no single solution exists, it should be possible to gain more confidence in inspection results through the application of new technologies. The core challenge lies with the coarse, textured grains that constitute the austenitic weld; current welding technology cannot deposit the weld with pre-designed textures for the grains. Hence, it is not possible to accurately predict the sound field distortion due to scatter and anisotropy such that flaws can be accurately sized and located.
The signal-to-noise (S/N) performance is an important indicator of the quality of a technique for the inspection of austenitic welds, as it quantifies the reliability of the detection and characterisation of small flaws (such as stress corrosion cracks (SCC) that have been identified as critical). In this project two ultrasonic techniques using phased array technology (two dimensional matrix arrays and focused time-of-flight diffraction (TOFD)) were used to investigate the benefits of advanced inspection technology to S/N and flaw sizing performance.
Objective
Establish whether phased array transmit-receive longitudinal (TRL) and focused TOFD techniques provide improvements in detection and sizing of defects in a dissimilar metal weld in comparison with current best practice techniques.