Does anyone have a good information source for recommended steel backing in full penetration groove welds for joining say P-1 to P-3, or P-3 to P-4, or P-3 to P-5 piping? What is the appropriate selection? Can you use P-1 material for backing on any of these, or should you use backing from the higher P-no. material? B31.1 says to use weldable quality material, compatible with the base metal and a sulfur content not exceeding .05%. Maybe I'm trying to read too much into this, can anyone help straighten me out??
Thanx in advance,
DPW
By -
Date 06-21-2001 17:32
Theoretically I think that in most instances it would not make a difference which of the materials in the joint you use as backing. Remember that both the materials at the joint will be exposed to the same "in service" conditions. It is for this reason that the filler metal is usually chosen to match that of the lower spec material, as this usually gives the best weldability results. I believe that the safest approach is to have the backing material chemical composition match that of the filler metal. In most cases this will be a match to the lower spec material.
Any other ideas out there?
Regards
Niekie
Have you considered using an open root no backing in lieu of backing? I work in the power industry and my company doesn’t allow the use of permanent backing. This would eliminate your dilemma. If you are using SMAW, GMAW, FCAW and have a WPS qualified with backing the deletion of backing is considered a non-essential variable. Keep in mind your welders must be qualified to weld open root.
On the other hand if you are using GTAW the deletion of backing would be considered an essential variable.
In addition to your welders being qualified you must be aware that the procedure also may require qualification. The deletion of backing per ASME Sec IX is a non-essential variable however some codes of construction such as Sec I or B31.1 indicate that procedures for welding from one side only without backing must be tested. Thats not an exact quote as I don't have any code books here now.
ASME is by far the way to go if you have a choice for a code to qualify a procedure by if a procedure qualified by testing is required.
If I am doing my own fitting and welding I myself would much rather weld an open root as opposed to a backing ring joint. Especially of materials that have already been in service and may be slightly out of round.
Have a good day
Gerald Austin
Weldinginspectionsvcs.com
Thanx for the input everyone. All of our procedures are open root. I'm in the process of rewriting the WPS's. Very rarely we have needed to use backing in special repair situations and I would like to address that in WPS's. When entering "Backing: As needed", one must specify the P no. for the backing. So for lack of any source data I've been specifying the same P no. as the base material. WPS's are written to ASME Sect 1 and B31.1. Thus my question. Thanx again.
