Phased Array of Corrosion Resistant Alloy (CRA) Welds

NDE Associates, Inc. Phased Array UT of CRA Welds

Corrosion resistant alloy (CRA) pipes are used for transport of high sour gas and oil that has high percentage of hydrogen sulfide.  CRA piping can be weld overlay or metallurgically bonded .  CRA pipes  are carbon steel with carbon steel or CRA welds. Carbon steel provides the strength and CRA clad is only for corrosion resistance. The CRA materials are A625 or A825. Unlike carbon steel pipes inspected with a single technique and  sensitivity, the CRA pipes require inspection not only for  weld fill passes but also integrity of the CRA root. Failure of CRA portion of the weld root, that is only 3-5 mm, will expose the sour fluids to carbon steel and can have disastrous effects. Because of this reason, inspection of these pipes should pay special attention to root integrity.

CRA welds are anisotropic and not favorable to conventional shear wave transmission.  Inspection of these welds is done by refracted longitudinal waves. Refracted longitudinal waves have a limitation. They cannot be used be in full vee mode. Inspection is therefore limited to 1/2 vee mode and requires removal of weld cap for 100% coverage.


Figure. Cross-section of a weld with metallurgically bonded A625 clad and A625 weld

PHASED ARRAY TECHNIQUES FOR DIFFERENT CRA WELD CONFIGURATION
Carbon Steel Pipe + CRA weld + WOL – 1/2 vee Refracted L wave
Carbon Steel Pipe + carbon steel weld + WOL – 1/2 vee Refracted S wave
Solid CRA (duplex SS) – 1/2 vee refracted L-wave
Note – All techniques require removal of weld cap

CALIBRATION OF L-Waves. Side drilled holes vs Notches
Refracted longitudinal waves are typically used for inspection of corrosion resistant alloy welds in piping using phased arrays. Sensitivity calibration is done either on side drilled holes or notches. Figure 7  from from the paper (Ref 1) shows refracted longitudinal waves mode convert when incident on notches. When incident at 45 degrees on ID notches, refracted longitudinal waves mode convert and loose 16 dB. This drop is significant and results in a large difference in reflection between side drilled holes and ID notches. Figure 7 (d)  of the paper shows barely any detection of the 1 mm notch with refracted L-waves when sensitivity is established on the side drilled holes

ID CRACKS CAN BE MISSED IN CRA WELDS
Refracted longitudinal waves when calibrated on side drilled holes as per some engineering codes (Ref 2)  have low ID sensitivity and can miss ID cracks. Detection of LOF or ID cracks in root are very critical.  Sensitivity should be established on ID notches when inspecting ID connected discontinuities. Paper (1) recommends update of engineering codes where sensitivity calibration is based only on side drilled holes.

L-WAVE UT OF CRA WELDS WITH CONVENTIONAL PROBE
Refracted dual element longitudinal wave angle beam probes are recommended by engineering codes. The main issue with such probes is that they result in ID roll from weld overlay clad. The ID roll can mask ID indications. This can be overcome by using phased array probes with focusing on the ID surface.

1. Anmol Birring, “Selection of Calibration Reflectors for Corrosion Resistant Alloy (CRA) PipingMaterials Evaluation, March 2016, p 361-366

2. DNV-OS-F101, Appendix D, Section B411