Flawed Welds Research Articles

Testing defects in automated ultrasonic testing and radiographic testing

For high-value pipeline projects, the NDT system used for weld inspection is frequently qualified by testing the system with flawed welds made deliberately for the purpose. Such deliberately fabricated flaws cannot be made to be completely realistic. Past work for pipeline inspection has shown a degree of tolerance to this when testing automated ultrasonic testing (AUT) systems, but this is not always the case. Comparing the shapes of the flaws from metallurgical sections with their ultrasonic detection suggests that shape is not the only determining factor and shows that ultrasonic testing (UT) diversity is key to a robust performance. Indeed, it is shown that a well-designed AUT system can be tolerant to a wide range of differences in materials, welding and geometry.

Crack driving force prediction in heterogeneous welds using Vickers hardness maps and hardness transfer functions

Flawed welds often require an Engineering Critical Assessment (ECA) to judge on the necessity for weld repair. ECA is a fracture mechanics based prediction of the integrity of structural components with defects under operating conditions. Adding to the complexity of a weld ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification allows for a more accurate assessment compared to common (standardized) practice, which assumes welds to be homogeneous. Hardness measurements allow to quantify weld strength heterogeneity given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of relations (‘transfer functions’) between hardness and strength, and recognize substantial scatter in hardness based predictions of strength. Within this context, this paper investigates the suitability of Vickers hardness mapping to perform an accurate weld ECA for high strength low alloy steel. To overcome the scatter associated with standardized transfer functions, this paper suggests an experimental calibration procedure based on all weld metal tensile tests. Finite Element (FE) analysis has been conducted on welds originating from steels to simulate their crack driving force response in Single-Edge notched Tension (SE(T)) specimens. Vickers hardness maps and hardness transfer functions are combined to assign element-specific constitutive properties to the model. The transfer function calibrated by all weld metal tensile tests yields a better agreement with experimental load-CTOD curves than transfer functions mentioned in standards and codes. Finally, a step-by-step procedure facilitates a practical adoption of the methodology.

Effect of root flaws on the fatigue property of friction stir welds in 2024-T3 aluminum alloys

The fatigue experiment of friction stir (FS) welds in 2024-T3 aluminum alloys were performed to investigate the influence of root flaws on the fatigue strength and life of FS welds. The test results of welds with flaws (flawed welds) were compared with the results suggested by the International Institute of Welding (IIW) recommendations and the welds without root flaws (flaw-free) in the published research reports. It was found that there was always existed flaws at the roots of FS welds because of unsuitable welding parameters and the vertical length of the flaws is about 0.31–0.33 mm for the FS butt-welded joint of 4 mm in thickness. The fatigue life of flawed welds is 33–80 times shorter than that of flaw-free welds, and the fatigue characteristic values have decreased from 120.6 MPa for flaw-free welds to 54.7 MPa for flawed welds at 2 × 10 6 cycles.

Weld overlay repairs from conception to long-term qualification

The weld overlay was initially developed in 1982 as an interim repair for intergranular stress corrosion cracking (IGSCC) flawed welds in boiling water reactor stainless steel piping. Since that time extensive work has been performed to qualify the weld overlay repair technique for longer-term service. Analytical studies of the weld overlay process, experimental programs on laboratory and field specimens, tests to demonstrate the beneficial effects of the weld overlay over its service life, and programs to develop nondestructive examination (NDE) techniques for use in inspecting weld overlays have all yielded positive results. These programs have been sponsored by the utility industry, either individually, through EPRI sponsorship or as part of the BWR Owners' Group (BWROG) effort, and by the US Nuclear Regulatory Commission (USNRC). Weld overlay repairs serve several design functions. These include: structurally reinforcing the flawed location to restore code margins to failure, providing an IGSCC-resistant barrier to crack growth, and imposing a favorable residual stress distribution in the inner portion of the flawed component to inhibit further flaw initiation and growth. The qualification programs confirm that these design functions are maintained with long-term service. A regulatory barrier to long-term operation with weld overlays was the need periodically to inspect overlays and thereby demonstrate continued integrity. Although IGSCC flaws have generally been detected by ultrasonic methods, the repair of these flaws by weld overlay made continued monitoring of the repaired flaw by conventional ultrasonic techniques difficult. Recent developments in ultrasonic examination equipment and techniques, including surface preparation criteria, have enhanced the inspectability of weld overlay repairs and the underlying base material. The original acceptance of weld overlay repairs by the USNRC was for limited service (one or two fuel cycles of operation). However, more recent regulatory guidance, including the recently issued NUREG-0313, Revision 2, recognizes the above technical advances, and provides criteria for acceptance of weld overlays as long-term repairs. Several US utilities have been successful to date in obtaining USNRC approval for extended operation with weld overlays. Some overlays at these plants are currently entering their fourth successful fuel cycle of operation.