Inspection of Weld Butter
An example of Phased Array Automated Ultrasonic Testing (PA-AUT) of butter welds would be the inspection of sub-sea components which have buttering applied directly to the part prior to making the final weld.
There are basically two types of weld butter, conventional and the butter used as a means of protection to the part in the form of a Corrosion Resistant Alloy (CRA).
Conventional butter is used to improve the mechanical properties of a material, such as a forging. The butter is applied directly to the end of the part and will eventually become the weld bevel. A carbon steel alloy is applied to the forging or pipe, is then inspected and if the weld butter is acceptable, the butter weld receives Post Weld Heat Treatment (PWHT) to improve the mechanical properties of the final part. Following PWHT, the butter is re-inspected to ensure any acceptable flaws which may have been present did not grow or propagate during the PWHT cycle and to ensure no cracking initiated during the PWHT. The actual weld butter is inspected in a single scan.
The part is then machined to reveal the buttered bevel which is now ready to join the two components. Once the final weld is complete, the remaining portion of the butter is re-inspected along with the new final weld.
CRA buttering is used to enable the welding of Dissimilar Metal (DM) welds are parts that have a CRA clad overlay for protection in sour gas applications. A CRA alloy, such as Inconel 625® is applied to the inside portion of a carbon steel pipe or valve body which would then be welded using Inconel 625® to join the parts while maintaining the required corrosion resistance.
The effects that the DM interface has on the PA-AUT can be clearly seen at both the clad and weld metal interfaces. In this image, a 3mm flaw (angled flat bottom hole) at the far side of the weld interface can be seen.
