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Influencing Parameters for Ultrasonic Inspection of Austenitic Welds

TWI Core Research Programme Project 1153/2021

TWI undertook a Core Research Programme project to generate informative data for the selection of suitable inspection techniques for the testing of austenitic stainless steel welds. This case study presents the scope and activities of the project as well as subsequent follow-on work.

Overview

Austenitic stainless steel (SS), duplex stainless steel (DSS) and nickel alloy (NA) welds and cladding are increasingly being used in the oil and gas, power and petrochemical sectors, and in desalination equipment and wastewater treatment facilities.

Ultrasonic techniques are commonly used for non-destructive testing (NDT) of these materials, as they provide volumetric coverage and are able to interrogate large component thicknesses and complex geometries which preclude radiographic techniques. However, the welds typically display an austenitic structure, which consists of textured large grains, and this results in high ultrasonic scattering associated with mode conversion effects, beam distortion and a variation of ultrasound velocity with direction and position in the weld. Therefore, extensive parametric analysis is required to identify best practice inspection methods for such materials and welds.

It is known that welding procedures, geometry and position are known to have a strong influence on the inspection capabilities of ultrasonic methods. Quantifying the influence of these parameters on the performance of the ultrasound gives an advantage to inspection engineers and asset managers, in terms of selecting suitable inspection techniques and anticipating inspection performance more efficiently. It also reduces the requirement for representative demonstration mock-ups.

 

Objectives

  • Ascertain the relationship between materials and welding parameters, and ultrasonic inspection capabilities and performance
  • Establish optimum and best-practice ultrasonic inspection techniques using specimens
Figure 1. Ultrasonic beam fingerprint experimental setup on a welded component
Figure 1. Ultrasonic beam fingerprint experimental setup on a welded component

Solution

Eighteen representative calibration and welded specimens were fabricated with a variety of differences in material (316L, 304L), welding process (MMA, NGT) and position, namely vertical upwards (PA), downhand (PF) and vertical/horizontal (PC). Defects were introduced within the weld, representing foreseeable defects that could occur during fabrication.

A methodology was established to allow consistent signal-to-noise (SNR) analysis and ultrasonic beam analysis. Each weld specimen went through an extensive set of experiments, including a through transmission hydrophone setup, which allowed for the mapping of the ultrasonic beam after transmission through the weld. Figure 1 shows the setup used to undertake the fingerprint of the welded specimens. Figure 2 shows an example of the ultrasonic beams recorded after transmission through the weld at different sound frequencies.

The defect specimens were then inspected using phased array ultrasonic testing (PAUT) and total focusing method (TFM) techniques using commercially available equipment.  The scans made use of dual matrix array (DMA) PAUT probes.  The DMA probes have no roof angle, so beam steering was achieved via delay laws in the passive axis.  This provided flexibility for the crossing, i.e. focal, point of the transmit and receive beams.  In additional, plane wave imaging (PWI) scans were performed using the 5L64 probe with L-wave transmission.

 

Conclusion

A methodology was established to characterise the austenitic welds with regards to noise analysis, through transmission beamplot generation and defect inspection in representative specimens. The beamplot analysis showed that ultrasonic beam at the recommended low frequency still presents beam splitting where the major part of the ultrasonic energy propagates downwards towards the weld root. This effect is more pronounced for weld fabricated in vertical upwards (PA) and downhand positions (PF). The noise analysis of the welds showed that the welding position described as vertical/horizontal (PC) presented a significant difference due to the large amount of cross sectional asymmetry in the grain structure. Taking into account the difference in terms of efficient SNR and sizing accuracy between frequencies ranking from 2MHz to 5MHz, it is recommended that different inspection techniques are used for comprehensive detection and sizing capabilities.

 

This project was funded by TWI’s Core Research Programme.

Figure 2. Results of the ultrasonic beam fingerprint
Figure 2. Results of the ultrasonic beam fingerprint
Avatar Capucine Carpentier Consultant, Non-destructive Inspection and Site Services

Capucine joined TWI in 2006 on completion of an undergraduate degree in Mechanical Material Engineering, carried out in the Non-destructive Testing (NDT) Technology Group of the Common Research Centre, EADS in France. She is a qualified ISO: 9712, Level 3 Inspector in the field of ultrasonic testing of welds. Since May 2020, Capucine has led a team of engineers who analyse inspection challenges and develop solutions using the latest knowledge and technologies. She is also a Technical Consultant in TWI’s Non-destructive Examination (NDE) Group, providing direct engineering services in the application and development of non-destructive inspection techniques and procedures for welded structures, in carbon steel and stainless steel. Primary areas of expertise include complex geometry, phased array ultrasonic, austenitic and dissimilar weld inspection, modelling, and NDT code and standards.

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