Arc burns on high strength, high pressure pipelines 1

[Olefazz]

Why are arc burns removed from pipelines during construction or maintenance welding? As a pipeline welder for forty years, I have yet to get an answer from inspectors or engeneers that any two people are in agreement. It seems that about 90% of the Gas Transmission Companys have no arc burn repair procedure. The other 10% have procedures and allow repair.
API Specification for Line Pipe, API 5L, 43rd. Edition allows repair of arc burns, ( see API 5L, 7.8.11 and 9.9 ).
What exactly does an arc burn cause in X42 through X70 line pipe when put in service? Thanks, Olefazz

 

[metengr]

Arc burns (aka, arc strikes) are actually small metallurgical notches that can result in crack initiation sites in service. The metallurgical notch is created because of local melting that results in very rapid heating to the surrounding base material and subsequent cooling that can harden this region.

Either grind them out and verify no cracks by surface NDT or if they are deep enough the excavations may require local weld repair.

 

[SectionIX]

Not withstanding the metallurgical aspect, arc burns are the result of poor welding practices that pipeline companies want to avoid.

Their remediation includes grinding, an acid etch (nital or ammonium persulfate),NDE (generally MPI) and case-by-case evaluation...obviously pipe integrity needs to be maintained, and this becomes a costly concern.

Additionally, a deep arc strike on an in-service line could require a sleeve or a cut-out if deep enough. Wall thickness restoration via weld deposit is not a practice embraced by the industry.

 

[metengr]

Wall thickness restoration via weld deposit is not a practice embraced by the industry.

Why? For most materials this is an acceptable repair practice provided you have suitable controls for such activity, and follow sound engineering practices regarding preheat and PWHT.

 

[stanweld]

Hydrogen induced cracks have initiated in the martensitic zone created by arc strikes resulting in failures - some catastrophic. Arc strikes from copper welding grounds or magnetic particle test prods (copper induced into the grain structure) have caused failures in pipe. Most are best removed by abrasive or machining (rotary file) methods and PT or MT examined after removal to assure freedom from cracks.