Welding question Root passes vs. time
Welding question Root passes vs. time
(OP)
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.





RE: Welding question Root passes vs. time
RE: Welding question Root passes vs. time
RE: Welding question Root passes vs. time
RE: Welding question Root passes vs. time
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?
RE: Welding question Root passes vs. time
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.
RE: Welding question Root passes vs. time
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.
RE: Welding question Root passes vs. time
RE: Welding question Root passes vs. time
RE: Welding question Root passes vs. time
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.