3 Things Never Discussed IRT Weld Strength 4

[tc7]

3 Things that are never discussed in texts, forums or expert web sites:

1. depth of weld penetration into the base material, (depth of fusion is the correct term). It is well published that amp settings, type of process, type of shielding gas will all vary depth of fusion into the base metal, but I cannot find data on what those depths might be! And how much is sufficient to develop the intended weld strength?
2. bevel angles on V-grooves – is a CJP groove weld with a 45 degree included angle (subjected to tension only) equally as strong as a CJP with a 60 degree bevel? Stress equations NEVER account for this angle or any weld proportion execpt height and length.
3. size of root opening – considering the same CJP V-groove weld, the pre-qualified weld joints of D1.1, or AISC and even Mil-Std-22 are very specific about the minimum size of the root opening. Why is this? I used to think root opening had more to do with access and electrode size only, but why is the “minimum” opening size so restrictive and requiring requalification if varied?

Any thoughts/advice is welcome as these three issues (particulary the fusion depth) have bothered me for a long time. The literature does not address them very well.
Please and thankyou.

 

[DarthSoilsGuy]

great questions, let me put on my cliff craven hat (i'm talking out of my @$$!)..

1- depth of fusion is impractical to inspect, and fillet weld design accounts only for the deposited size of the weld
2- a 45 and 60 deg bev groove welds are equally strong since they both went through bend testing and qualified.
3- improper root spacing is the a heavy culprit for failed UT's on moment welds.

ok, even my geotech side thinks the 3rd one is weak...

 

[metengr]

tc7;

I can address the first question.

depth of weld penetration into the base material, (depth of fusion is the correct term). It is well published that amp settings, type of process, type of shielding gas will all vary depth of fusion into the base metal, but I cannot find data on what those depths might be! And how much is sufficient to develop the intended weld strength?

Fundamentally, fusion welding requires the local melting of base metal with weld metal. The depth of fusion is really not that important as is assuring you have demonstrated local melting, which is based on amp settings, and travel speed, etc. Yes, the depth of fusion can be varied but all this does is results in local dilution that can be calculated, but again, in the grand scheme of things it is not an essential variable.

Your concern regarding weld strength should be based on weld joint design assuming that you have 100% fusion between the base metal and weld metal. This can be demonstrated by several mechanical test methods - bends and tensile and by volumetric test methods - RT and UT, and by visual examination, macroetching.

 

[metengr]

2. bevel angles on V-grooves – is a CJP groove weld with a 45 degree included angle (subjected to tension only) equally as strong as a CJP with a 60 degree bevel?

In some specific service applications, no, despite the fact you can demonstrate that with complete joint penetration and fusion the weld region (WM and FZ) will be as strong as the base metal with either weld joint configuration. Where this argument breaks down, however, is in elevated temperature service where dissimilar metal weld joint design is critical to assure that thermal/mechanical stresses do not result in premature creep failure. There can be orders of magnitude of difference in service performance between a narrow versus very wide weld joint angle for certain base materials.

 

[Dinosaur]

Metengr,

I never thought of temperature service range in this context. Thanks. My welds are only designed for typical outdoor service temperature extremes.

 

[TVP]

(1) Depth of fusion is indeed a difficult parameter with which to find information. I have seen automotive standards that specify either a minimum value or a minimum percentage based on the thickness of the thinnest joint member. The following summarizes this information: depth of fusion to be a minimum of 5% of the thickness or 0.3 mm (whichever is greater). One standard requires a minimum of 20% of the thickness for structural applications.

One of the biggest factors that is interrelated with depth of fusion is residual stress in the toe. If the depth of fusion is low, it is possible that the weld parameters were optimized for welding speed, not for creating a good metallurgical joint. All of this can mean that the residual stresses are tensile at the surface, leading to poor fatigue life.

 

[SJones]

Root gap variations do have implications for such defects as centreline cracking and excessive penetration (too wide); or, incomplete penetration (too small). From a mechanical property point of view, there may also be implications for strain ageing effects in the root pass.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/