Halo effect on pipe welds 1

[exCLP]

It has always been a concern to me about the halo effect of pipe welds, or any welds for that matter. The heat effect of the weld in the adjacent region of the pipe weakens the pipe strength, as with any heat weld.

On a practical basis, there does not seem to be a problem, especially because of the safety factors used in establishing pipeline maximum operating pressures (MOP's) and because in mills using pipe strong enough to meet the pipe strength classification slightly stronger steel than the rating is used (even examples of say a pipe failing an X-65 test being down-rated and sold as X-60, as problematic as that is).

On the day-to-day, good API 1104 tensile breaks seem to go in the halo area, as opposed to near the clamps where there is some surface damage (stress concentration points) due to the clamps.

Question: Anyone know of this issue being officially addressed in standards? It is not critical...only a long-time thought.

 

[BigInch]

I believe its only addressed under the manner of qualifying the welding procedure.

http://virtualpipeline.spaces.msn.com

 

[metengr]

It has always been a concern to me about the halo effect of pipe welds, or any welds for that matter. The heat effect of the weld in the adjacent region of the pipe weakens the pipe strength, as with any heat weld.

This is not really a totally correct statement related to welding. The heat input from fusion welding will alter the original base metal mechanical properties and microstructure (this is the structure of the metal as viewed using an optical microscope). However, the altered base metal structure is typically highly localized or confined to a narrow region surrounding the fusion zone of the weld.

Since the region of the locally altered base metal (aka the heat affected zone - HAZ) is a relatively small band in comparison to the surrounding base material (unaffected from heat), the HAZ should behave in similar fashion as the surrounding base material. An example of this is when local softening of the base metal occurs within a narrow band of the HAZ. Despite the softened region, the surrounding base metal and weld metal, which are higher strength in comparison will provide reinforcement to this lower strength region.

Local hardening of the HAZ is more difficult to deal with from welding but can be controlled by preheat requirements and post weld heat treatment (to soften this hardened region). The bottom line is that depending on the chemical composition of the base metal, the ultimate proof of the weld process is demonstrated by weld procedure qualification testing using tensile and bend testing, as stated above.