Welding SA-106 Gr.B Utilizing a Stainless Steel Backing Strip

[iam42]

Hey Guys,

Hopefully someone can shed some light on a problem I have with a new customer. This problem is 2 fold, firstly it is a metallurgical issue and secondly it is a ASME code issue.

We routinely weld CS pipe to CS pipe utilizing a Stainless Steel Backing ring( it is not a backing ring but acts like one). We usually weld this joint using either a 309 consumable or inconel consumable depending on the service temps.

A customer has now asked us to make this joint using 309 to weld the root only and then the fill and cap to be welded using a carbon steel consumable. At the moment I am saying no to this on the basis of:

1. If this weld is made with mild steel electrodes to match the mild steel side of joint, the weld metal would be enriched by the wash-in of chromium and nickel from the stainless side. This intermediate chrome-nickel is usually hard and crack sensitive. (I beleive!!!)

2. The B31.1 code states that any welding between ferritic and austenitic steels shall have an austenitic structure. B31.1 127.2.1 (E)

Am I correct in the 2 points above? If so, how would you convince the customer of this?

I have attached a sketch of the joint.

Thanks for any help.

 

[iam42]

Sorry,

I should have added that the customer want the weld to be welded with CS due to

"The concern with the utilization of 309L/Inconel filler metal is the presence of high chlorides in the process flow. While the chlorides may attack the nozzle itself over a long term, we do not want to experience a rupture due to chloride attack on the filler metal."

This is a straight quote from his email to me.

Thanks again.

 

[weldstan]

I first must ask why are you using a 304 backing ring? I would absolutely prohibit you from doing so. If one desires to produce hard to detect weld cracks and cororsion issues in service, I cannot think of a better way.

 

[weldstan]

If you are welding into a 304 stainless clad or applied lined material using the backing as shown as essentailly part of the applied lining, weld metal could be 309, as shown, followed by Nickel based (Inconel) or Nickel based all the way out.

 

[iam42]

Weldstan,

It is not actually a backing ring but a weld-in flow nozzle. I did not have time to draw up the full nozzle!

Thanks

 

[cul8r]

iam42

I have seen many welds made with 309 root and a CS filler metal. What is the thickness of the base metal? What is the operating temperature?

The 309 root is better for ductility and a little more strength at high temperature then the CS metal. What NDE if any are you doing on the root pass?

The Carbon steel filler metal might provide improved corrosion protection in some environments.

Cheers

 

[iam42]

cul8r,

Thanks for the help.

The operating temp is 582F with a design pressure of 649F.

The completed joint will be x-rayed 100%.

Thanks.