Starting and Stopping Welds in the Field 6

[STrctPono]

I have some repairs that we are making to a steel plate girder bridge. We are welding new 1/4" steel plates to the outside of the girder web. We specified similar repairs to another area of this bridge last year and were told that the Contractor warped the 1/4" steel plates when they made the continuous CJP weld. Too much heat applied in one location all at once.

My question is this: Is there a difference in weld quality/strength if they were to weld continuously and completely around the web plate vs. stitch welding it 3" at a time @ 12" o.c. and then come back and weld in the areas to remain? In essence, does starting and stopping create issues at the point where you start/stop? They are most likely going to be using Shielded Metal Arc Welding but might choose to use flux cored arc welding too given the total length of the welds.

 

[TugboatEng]

Stitching is standard practice to reduce the amount of warpage in welded structures. After the stitches are made, the beginnings and ends should be gouged out with a grinder to provide even transitions between the stitches.

 

[STrctPono]

TugboatEng,

As always, I appreciate your input. I'm just wondering how the Engineer of Record (myself) can control this? I'm not sure if I can as this seems to be more of a means and methods thing. Hopefully the weld inspection would uncover any defects in the weld that may occur at the stop and start point. I have a note that warns the Contractor of warping the cover plates but not sure if there is anything additional I can do...

 

[TugboatEng]

For our application it's usually an air lance or vacuum chamber test. Soapy water one side and blow air from the other. A bubble means failure.

 

Almost everyone correlates heat input or 'heat' with distortion.
In many situations you want to go in heavy and fast with the first pass.

However this is a case of making a substantial, continuous weld on sheet metal. Major distortion is unavoidable, and mitigations in terms of technique and sequence offer minimal reduction.

As per previous comments, see if your designer can accept a reduction in the weld volume by stitching.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Tmoose]

I'd want to positively confirm the true nature of the "warping."
The proper corrective measures will depend on identifying the actual problem.

I was surprised that AWS D1.1 ( structural welding ) does not mention starts and stops in the SMAW section.

For reducing distortion "back stepping" can be useful.

Certainly welding over cleaned tack welds should be acceptable without LP, UT, back gouging etc.

 

[STrctPono]

Tmoose, I too would have liked to confirm it but it is all hearsay at this point. I too thought that tack welding every 4" or so around the perimeter prior to laying a continuous bead of weld down would have helped restrain the plate. Interesting note on the back stepping. I had never heard that before.

ironic metallurgist, Is 1/4" plate considered sheet metal!? I could go thicker but it makes handling and installation that much more difficult as the plates get heavier. They are going to bring them in with a small boom truck but will need to be maneuvered into place by hand as it is being installed underneath a deck overhang above the ocean. I need this weld to perform like a seam. It is on the ocean and takes a serious beating from salt spray. Any gaps left in the weld will be detrimental to the longevity of the structure.

 

[dik]

With all those rivets, I assume the steel is weldable... have occasionally encountered high sulphur content to make the steel 'hot short'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

ironic metallurgist, Is 1/4" plate considered sheet metal!?

In my world sheet metal is anything under 3/4 inch. But seriously, unsupported 1/4" steel will be very prone to distortion, the worst type of which is the tendency to potato chip.

have occasionally encountered high sulphur content to make the steel 'hot short'.

That would not be a factor in distortion.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

I could go thicker but it makes handling and installation that much more difficult as the plates get heavier. They are going to bring them in with a small boom truck but will need to be maneuvered into place by hand as it is being installed underneath a deck overhang above the ocean. I need this weld to perform like a seam. It is on the ocean and takes a serious beating from salt spray. Any gaps left in the weld will be detrimental to the longevity of the structure.

Go heavier and call it a 1/4" corrosion allowance. Sounds like you'll need it.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[dik]

It was just a 'heads up'... might make all the distortion concerns, moot...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r6155]

The 1/4" plate is too thin for marine service.
Preheating can be helpful to avoid distortion, depending on the ambient temperature.
Use small diameter electrode and 2 welders at the same time.

Regards

 

[dik]

For that environment, I'd likely use 1/2" min plate and use filet welds each side, not a prepared weld.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[weldstan]

While there are many techniques to controll distortion, the best ones are those initiated by a competent welding engineer/metallurgist for the specific application. I have made numerous such welding procedures but each was differnet based on the material, thickness, welding process, etc. Stitch welding followed by back step welding is just one of many employed. Good luck. Sometimes one learns much through failure.

 

Preheating can be helpful to avoid distortion, depending on the ambient temperature.
Use small diameter electrode and 2 welders at the same time.

Preheating - very marginal benefit
Smaller electrode - counterintuitive, but that is one of the myths of distortion management. In a multipass weld, more passes only makes things worse.

Distortion caused by welding is an unavoidable fact. One of the guaranteed mitigations is limiting the amount of weld, both in design and by overwelding.

Any WE who comes along and claims he can give you zero distortion must be fired on the spot.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

When making long welds on thin flat plate, the plate distorts from heat as it's being welding. When you pause, some of that distortion relieves itself. If you weld continuously, the distortion continues to grow and the welding fixes it in the distorted shape. Stitching allows the metal to return to close to it's original shape before being locked in place.

For example, if I have a long weld with no gap and I weld continuously I may have a 1/2 inch gap at the end of the weld. By stitching I can lock the gap down tight so it can't continue to grow.

I can't think of better words at the moment, I hope this is sufficient.

 

[STrctPono]

Thank you all for your help. I've learned a lot. I did end up increasing the thickness of the plate. I cannot, however, decrease the length/amount of the weld. Long term corrosion of the structure takes precedence. I have several notes to the welding Contractor that they need to prepare welding procedures that minimize plate distortion as much as possible for us to review. Ultimately, that's as much as is within my control...

 

[gtaw]

Call me stupid, but what is this "stitch welding" I see tossed around?

As for distortion control and communicating a specific plan to control distortion, it called a "distortion control plan". AWS D1.1 Structural Welding Code/Steel includes a clause that states (paraphrased) that if there is a concern about distortion, the Engineer should develop a distortion control plan, which similar to a WPS provides direction to the welder on how something is to be welded.

Several responses include suggestions on what measures can be taken to mitigate the problem. The back step technique can be used, the use of a specific sequence (intermittent welds isn't exactly the same thing) of where to place the welds can be effective, in some cases preheating and interpass temperature controls can be effective. Believe it or not, in some cases multiple beads deposited with small electrodes can be worse than a single pass weld deposited with large electrodes.

I have developed distortion control plans for bridge components and I almost always include a plan when developing plans for major repairs on forging presses. Don't leave it to the welder to determine the sequence of weld placement.

And I agree with the observation that the rivets should make one question the type of steel used for the construction of the built-up members and the weldability of the steel. It's all about the chemistry of the steel.



Best regards - Al

 

[dik]

intermittent?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

and as noted above, it has nothing to do with distortion...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik