Back gouging after 1 run 3

[Gem1]

Hi All,

Is it normal practise to back gouge after only one run in a single vee butt weld and then finish the other runs on the first side? This is for a 316L, 8 mm butt weld.

Usually I see procedures where the backgouging and run on the other side done last, so I'm not sure if this is a common thing?

Thanks,
Gemma

 

[Mr168]

Not particularly common in that you run a greater risk of blowing through the backing weld, but it's an acceptable practice.

 

[Weldstar]

Agreed, there is no real reason why you cant but I cant see why you would want to? As previously stated you do run the risk of burning through if you back grind on a sinlge run. The other thing to consider is that if the first run is background to thin enough section the 2nd run may subsequently heat the 1st run to a temperature where oxidization will occur (On your 316L material) meaning you may have back grind the first pass again from the other side ... make sense ?

Welding Engineer
MSc Weld Eng,IWE,CEng
Australia

 

[stanweld]

Gem1,
It is not common but might be used with specific welding processes and/or for other issues. You might provide further information regarding welding process, accessibility, hex chrome safety issues, etc.

 

[Gem1]

Thanks All.

It is a totall normal single vee butt weld on 8 mm 316L, using GTAW. Prequalified joint design to AS 1554.6, so no access problems. I'm not sure about their gas extraction issues.

I think I might ask the fabricator why they want to do it that way and go from there.

Thanks again.

 

[Guest102023]

At 8mm plate thickness you should be able to find a joint design and method that gives full penetration without gouging.

 

[gtaw]

The reason for depositing weld on side one, then back gouging from side two, followed by back welding on side two may be to better balance the residual stresses and to minimize the resulting distortion.

The weld would then be completed from side one, but the root bead and the back weld provide sufficient restraint so as to minimize angular distortion toward side one.

Think of it this way. First the weld on side one causes the plates to distort in the direction of side one.

Back gouge side two and reduce the thickness of the root bead. Deposit the back weld, the plate distorts toward side two.

Back to side one, more weld is required to provide sufficient residual stress to overcome the distortion and restraint from the back weld on side two.

Weld is completed from side one, the residual stresses of side one now are in balance with those of side two and the angular distortion is minimized.

Best regards - Al

 

[Weldmedic]

Well stated Al.

Steve

Steve Dodd

 

[tc7]

Hey Al, or anybody-
Been doing some research on backgouge reqt's, per AWS D1.1 and cannot find a lot of reading material.
For instance Al, you suggest that the reason for backgouging may be to balance the shrinkage stresses at the root, but why does D1.1 state that unless a CJP includes a backgouge on the second side, that it is then considered to be a PJP? Does it have more to do with not being absolutely sure that full fusion is achieved at the root and assuring complete penetration? Consequently unless we are including the backgouge as a requirement then the design must suffer a reduction in allowable strength, at least that how I read section 3.12.1.

Also why then does D1.1 make NO mention of an inspection of the backgouge?

Hopefully you'll see this post and offer you insight.
Thanks

 

[gtaw]

Read the requirements of D1.1 a little closer. BG is required if the double sided groove weld is specified as a CJP. A single sided groove welded with backing does not require back gouging.

The primary reason for back gouging the second side is to remove any weld in the root of the first side that might be compromised, i.e., incomplete fusion, slag, etc., before depositing metal from the second side.

The failure to back gouge the root before welding the second side may permit the welder to deposit weld over areas where the weld is not sound. Thus, the weld is considered to be a PJP.

Best regards - Al

 

[tc7]

thanks Al, this is a good answer.

But I cannot find where D1.1 specifically says CJP's must be backgouged. The only statement related is sort of an inverse requirement in paragraph 3.12.1 which says if a weld is not BG'ed then it is considered PJP.

Do you know of a specific, more direct statement that requires CJP's to be BG'ed ?

 

[gtaw]

It is a condition for prequalification.

Look at AWS D1.1-2010, clause 3.12.1.

The contractor always has the option of qualifying the proposed joint detail in accordance with clause 4, part B to demonstrate it will produce the desired results, i.e., CJP.

This requirement is not "new" and it can be found in every welding code, i.e., aerospace, ASME, military, etc., that I have worked with.

Best regards - Al

Best regards - Al

 

[Kiwi2671]

Tom,
When you welded the joints in question did you follow a WPS - either pre qualified or qualified by testing ?
If it was pre qualified it should have noted the joint designation as per Figure 3.4.
Was this a CJP ? If it was a pre qualified CJP and it was welded both sides it must have backgouging.

If it was qualified by testing was the PQR qualified with backgouging ? If so the deletion of backgouging is an essential variable so you can't not backgouge.
The only way you can get away with not backgouging is to qualify a procedure that gives you full penetration without backgouging.
Hope that helps,
Regards,
Kiwi