Welding question Root passes vs. time

[wagnermj]

Does anyone see a drawback or issue with welding a root pass on small diameter pipe (<3") carbon steel pipe with base metal thicknesses < .250" and leaving overnight before returning to complete the weld ? Welding process is GTAW using ER70S-2 filler metal. Ambient temperature >80F. Fabrication is in controlled shop environment with minimal pre-load on the piping at time of fit up.

 

[kingnero]

What do you call carbon steel? If you don't preheat (indicating low %C), it shouldn't be a problem.

 

[racookpe1978]

Best practice is root pass + cover pass before you stop. What filler material? How many total passes?

 

[metengr]

For this size pipe and assuming a low CE (carbon equivalent) carbon steel pipe material, leaving the root pass is acceptable. Before resumption of welding, I would perform a surface NDT (wet fluorescent MT) of the weld root; if acceptable proceed with welding.

 

[kingnero]

@ metengr:
When (from what size pipe) would you say that it is not acceptable, and why (load, self-weight, ...) ?

In this case (3" and assuming low CE), and little or no load during assembling, I cannot see the reason for a NDT. Not if you continue directly after the root pass, but also not if there is a night inbetween the root and following passes. That additional 8-12 hours sure won't change anything?

 

[weldstan]

In a pipe fab shop, there is no problem and it is often done.

In a field setting, it can be a very different story depending on pipe loads (use of a 5T come-along, etc.) ambient temperature and wind conditions. See metengr.

 

[metengr]

kingnero;
Interrupted welding for carbon steels is common, and most codes and standards do not address it. Interupted welding for low alloy steel on the other hand is a different matter. With that said, determination of when to interrupt welding for low CE carbon steel base materials is on a case-by-case basis. In the above example, I cited NDT before welding because this is more conservative and with wet MT it can be done quickly. I have seen miss cracked weld roots on visual examination. Cracking of weld roots can be caused by poor alignment, fit-up, hydrogen, high tensile and bending stresses from system applied loads, etc.

 

[weldstan]

I would not recommend delaying the addition of at least a hot pass for welding heavy wall, low carbon steel pipe > 3/4", nor for welding large diameter, greater than standard wall, microalloyed X65, X70, X80 pipe. After completing the two weld layers, interuption of welding can readily be done.

 

[tr1ntx]

A possible reason I can imagine for an immediate/soon second pass is that you limit the atmospheric exposure time of the new surface, which will end up being buried within the weld pool. If it is exposed for some length of time, I suppose in some best practices criteria, the surface should be wired brushed or ground prior to welding another pass.

 

[weldstan]

tr1ntx,
The root pass weld chemistry is most affected (diluted) by the base metal chemistry. Carbon equivalency of the deposited weld metal may increase to a point where the microstructure is martensitic (subject to hydrogen embrittlement/cold cracking/delayed cracking). The heavier the wall thickness, the more rapid the cooling rate will be, which further effects martensitic transformation. The thin root is subject to high residual tensile stress which increases with increasing base metal thickness. The purpose of the second pass is to temper the root pass and provide greater meat (weld thickness) to resist the welding residual stress and thus mitigate the potential for cracking. While the GTAW process can be considered a low hydrogen process, hydrogen can be dissociated through the arc when moisture is present on the weld joint or the dew point of the gas is high, possibly due to leaks in the shielding gas tubing, using commercial grade argon, or contaminated by other means.