Influencing Parameters for the Ultrasonic Inspection of Austenitic Welds
TWI Industrial Member Report 1153-2021 [pdf / 6384KB]
By J Lambert and C Carpentier
Industrial Need
Improving the ultrasonic inspection of austenitic stainless steel (and nickel alloy) welds requires a good understanding of the influence of welding parameters on the metallic structure of the weld. By understanding the impact of these parameters on inspection capability, both welding and inspection procedures can be designed with due consideration for the required level of quality in the joint.
Key findings
- In order to improve the likelihood of detection across the full volume of the weld, it is recommended that several techniques should be utilised to cover the same region of weld. Low frequencies are more suitable for detection of discontinuities, whereas higher frequencies are better suited for any sizing requirements.
- Generally for weld thickness in the range considered in this project, and likely for lower and higher thicknesses, the recommended sound frequencies is between 1 and 5MHz. Frequencies outside this range can be utilised but with likely loss in resolution and increased sensitivity to attenuation.
- The metallurgical structure of the weld appears to ‘guide’ the sound towards the root, which was observed on all welding positions regardless of the level of asymmetry in the solidification structure. Along with other factors related to focusing and convergence point of transmit-receive probe types, this appears to increase the likelihood of detecting root flaws.
- Even though only stainless steel was included in this project, similar results could be expected for other austenitic material that develop textured grain growth guided by cooling gradients. This statement is supported by experience in TWI, but it should be noted that the use of nickel alloy in welds and cladding are generally lower in thickness than the 60mm joints in this project, so it is possible that effects could be more pronounced in comparison to 300-series stainless steel in similar thicknesses.
- It is possible to measure through-wall height of planar flaws and using tip diffraction signals. However, it should be noted that in cases where the weld can only be inspected from a single-side, there is a risk that flaws on the opposite side may not be detected and, if detected, may not be sizeable. Care should also be taken with root flaws, which may be under sized, in particular with lower frequency probes. This again reinforces the earlier recommendation that several techniques and frequencies should be used to inspect austenitic welds.
- Imaging techniques such as FMC/TFM and PWI did not demonstrate a clear benefit in comparison to the different phased array techniques, but further work should be done to seek any such benefit, because there are many operational modes that remain to be investigated and under development.
- The evidence in this report supports the need for stringent qualification of selected techniques when the welds to be inspected are of a sufficient critical nature, to establish the necessary performance for detection, and sizing if necessary. Several formal methods for qualification are available to industry.
Scanning a test block in the immersion tank to establish the influencing parameters for inspection of austenitic welds
