Improving Phased Array Inspection Reliability of Alloy 625 Welds

Carbon steel pipes are used for transporting hydrocarbon fluids. Hydrogen sulfide (H 2 S) in these fluids can have adverse effect on carbon steel pipes. The carbon steel pipes can become susceptible to corrosive attack when the sour content in the hydrocarbons crosses a certain concentration limit. To protect the carbon steel, corrosion resistant alloy (CRA) clad is applied on these pipes. Cladding protects carbon steel from corrosive attack. The most common CRA material for cladding is UNS N06625, commonly known as Alloy 625 which is used both for cladding and welds. Codes and standards provide some guidance on austenitic weld ultrasonic inspection but do not fully address this weld/clad configuration. Phased array ultrasonic test is one of the methods that is very effective for this weld/clad configuration. Application of phased array method requires proper design of the calibration block and selection of the phased array probe. The inspection of such welds is done in the refracted longitudinal wave mode. Phased arrays provide complete weld image so discontinuities are easy to interpret compared to conventional ultrasonic instruments with A-scan displays. Calibration is applied so that proper sensitivity is established not only in the weld but also the weld root in the clad.