Weld preparation best practice

[EngAddict]

Hi, I was wondering if there is any good source for weld preparation best practice. Things like prep angle, gap, root face, back gouging, backing runs, specialized first run, etc. I normally reference the pressure vessel codes and think I have a reasonable understanding of how it should be done but I am getting conflicting information from a fabrication shops 'welding engineer'.

As an example, what would be a recommended preparation angle for welding a set in nozzle into a pressure vessel? It is double welded double bevel with a cover fillet all the way around the nozzle. The material is about 60-65mm thick. Also for this thickness what would be the best root face. Should a large one be used to avoid having to back gouge or should it be small and back gouge always. Also they use a very small gap so i would recommend increasing the angle to ensure full penetration.. but I am not an expert on the subject.

Cheers for the help.

 

[metengr]

As you have found out, there are various ways to weld items for fabrication. In my years of conducting audits and having our own pressure parts fabricated, I found out it is best to have the fabrication shop do what they are comfortable in doing as long as they have a basis for implementing design code requirements and maintain quality hold points. If you start mandating what you consider best practices for particular weld joints, it might be best for some shops and marginal to other shops because of comfort level and experience.

 

[EngAddict]

I never mandate, we always let the fabrication shop follow their welding procedures. Provided they can produce an equivalent quality weld. This was just for my own interest really because in this case I don't believe the weld will be of equivalent quality. I believe their preparation angle is too small to adequately achieve full penetration. They will essentially create a partial penetration weld and I don't see how back gouging will help them unless they remove a heap of material. However I guess they must be able to because it would be picked up on the tests if they hadn't.

Then again this is being manufactured over seas and this weld preparation would never be used here, so I may not be that far off the mark?..

Either way I would still like some information just so I can design my own work correctly, not to stipulate how experience fabrication shops should weld.

 

[racookpe1978]

What angle are they trying to get by with?

Remember, smaller angle -> less weld material and less prep time and less waste material and much less welding time. More profit for them.

So if all the cost advantages are pushing them to scrimp on the angle, and the only penalty is to you for a bad weld, can you be sure they are actually welding correctly down inside the metal?

65 mm is deep - not unheard of, but very deep. (Make a full side of two pieces of wood with "their" angle and with the AWS/ASME limited angle, then try to wedge a rod down into the tip of their joint. Sobering results)

If "they" are getting all these apparent savings, are you really sure they are actually doing the back gouging (have you (your independent inspector) actually seen them? Have you measured the back-gouged depths and partitioned a test strip. Make them justify their "best profits" (er, "best practices") to you.

They might be right. Most weld shops (here) are good. Some are not.

 

[HgTX]

It's not the applicable code, but for fun you could look at the prequalified weld details in AWS D1.1. In essence, those are the details that AWS thinks ought to reliably give good results.

Hg

Eng-Tips policies: faq731-376

 

[HgTX]

Pretend I said "joint details" instead of "weld details".

Hg


Eng-Tips policies: faq731-376

 

[stanweld]

Joint design is also dictated by welding process. What process(es) are they using? Increasing the root face will generally increase the amount of material to be removed during backgouging.