If we full weld a stiffener ring on the outside of a round machined pipe (pipe roundness after welding is important), which welding process, SMAW or GTAW, will give less distortion?
Assuming the fillet weld size on both sides of the stiffener to be the same for either process.
Regards
Boon
The one with the highest travel speed :)
Hello Boon, to some degree weld progression may have a large effect on distortion as well. If you can employ 2 welding operators onto this joint configuration and position them 180 degrees apart from one another and welding in the same direction you may notice a "reduced" distortional effect. If you could use 4, one starting at each 90 degree point, it might even be better. In either of these instances the welding occuring directly across from each other helps to negate some of the distortional effect that occurs. The scenario that I have described would relate to the "roundness", as to the other types of distortion, that caused by fillet welding, the other replies that you have gotten would address those issues.
Best of luck on your project and regards, Allan
I would expect GTAW to produce the greater distortion due to the low travel speed and highest heat input that results.
The typical travel speed for GTAW is considerably lower that that of SMAW because the energy transfer of SMAW is higher than that of GTAW, i.e., more energy goes into melting the electrode, so the deposition efficiency is greater for SMAW than GTAW.
GTAW, with its slower travel speed will heat up a larger area of base metal, thus more metal is heat hot enough to exceed the yield strength of the base metal, hence contributing to greater distortion. I played with the numbers and estimate the base metal only needs to be heated by a delta T of 220 degrees F to reach the yield point of the base metal. Any base metal that experiences a delta T of 220 degrees or more contributes to permanent deformation.
Since the GTAW is likely to heat up more base metal surrounding the weld, more distortion results. A good example of this contrast is comparing the heat input and resulting distortion of GTAW and an electron beam weld ( a study in contrast!).
The secret is delta T; if one preheats the base metal to an elevated temperature, the delta T is reduced from one location to another and the distortion is reduced.
E= stress/strain = 29 million (for all steel)
By substitution:
E= yield strength/(coefficient of expansion x delta T)
29 000 000 = YS/(6.5 X10^-6 x delta T)
Delta T = YS/(29 x 6.5)
Delta T = 40 ksi/(29X6.5) = 212, round up = 220 degrees F
It is an approximation, but since expansion can be considered to be linear for small changes in temperature provided no phase change is involved, the equation is valid enough for our purposes.
SMAW, with its higher travel speed heats up a smaller area of base metal thus less distortion results.
This is all predicated on the resulting weld being the same size using a single pass and no preheat. As mentioned, preheat reduces the delta T and therefore reduces the distortion.
Best regards - Al
Topic Welding Industry / Technical Discussions / Comparing distortion between SMAW vs GTAW processes
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