6010 by 7018 vs 6010/6010

[engineeringguy]

I am being asked to compare the lifespan of a 6010 x 7018 weld with a 6010 x 6010 weld in a pipe that will thermally cycle once every other week.

Where is a good starting place for evaluating the two weld types?



"If experience was always the only factor, how would we get to the moon?"

http://www.piping-designer.com

 

[metengr]

engineeringguy;
This is one of these questions that has so many variables I am not sure what type of of evaluation one could make with reasonable accuracy.

The first issue is quality of weld metal. Welds are cast structures that contain inherent flaws.

Second, is the level of residual stress which is a function of preheat, weld deposition sequence, etc.

Here is how I would approach this problem - assuming two welds with similar inherent flaws and similar residual stress levels (same base material, same dilution effects, same preheat, same deposition sequence;

the 7018 deposited weld will exhibit higher tensile and yield stregnth in comparison to 6010, this is based on 70 Ksi minimum UTS for 70xx versus 60 Ksi minimum UTS for 60xx weld rod. Since for wrought steel, the high cycle fatigue strength is approximately equal to 50% of the ultimate tensile strength, one could assume that the weld deposited with 7018 as the bulk (6010 root + 7018 fill), will generally have higher fatigue strength in comparison to the all weld deposit of 6010. This is about as far as I could go with this because any further conclusions would be pure speculation.

 

[engineeringguy]

This was a very interesting project. Its been done for a while but I wanted to post for future questions.

The results that I found was that (according to RP579 - Fitness for Service) fatigue failure of the weld is directly related to the quality of the weld. Stress concentrations due to slag inclusion and poor welding techniques can cause unexpected catastrophic failure.

Because 6010 is a much easier rod to weld than 7018, the field personnel may have done themselves a disservice by choosing a higher strength rod.



"If experience was always the only factor, how would we get to the moon?"

http://www.piping-designer.com

 

[metengr]

Because 6010 is a much easier rod to weld than 7018, the field personnel may have done themselves a disservice by choosing a higher strength rod.

This is really an incorrect statement. I have seen many welders deposit either rod with no problems. Weld strength should be equal to or greater than the base material, in most applications.

 

[Mr168]

6010 is not only a much more difficult rod to run than 7018 (with the exception of the downhand variants that are only dragged), but typically lends itself to a much higher risk of defects. The deep digging characteristics of the rod makes undercut a common concern (particularly root bead "wagon tracks"), and further emphasizes necessary precaution against slag entrapment, which is already of some concern because of the whip and pause or circular technique necessary to run 6010 (referring to subsequent stringer passes, not a keyhole root technique). As long as the pipe welder doesn't switch from a drag to a push as he's coming around the 3/9 o'clock positions on a pipe with 7018 and has proper bead placement, slag entrapment is unlikely to be an issue, and undercut is typically not a concern.

Experienced welders can use either rod with no problem, but any rod that requires greater mechanical manipulation is typically thought of as the more difficult rod to run. If you took away 7018 rods from structural guys and forced them to run 6010 all the way out on site, your CWI would be in tears.