Alternates to PWHT after Repair 3

[bob330]

Hi All,

The NBIC code refers to alternate techniques allowing the avoidance of PWHT on pressure vessels such as the half bead technique or controlled deposition techniques but fails to explain exactly how these techniques are to be done. Can anybody here elaborate on the details of these techniques. For example, on the half bead technique, are all passes touching up against the original base metal to be ground down to half width or just the very first pass? How should weld beads be sequenced?

Thanks,
Bob

 

[metengr]

bob330;
Yes, that is correct the NBIC provides alternatives to PWHT, and now states that the procedures are to be qualified to ASME Section IX, QW-290. The alternative welding methods were completely re-written to follow Section IX qualification, which by the way finally endorsed the temper bead method and essential welding variables in 2004. Everything is all there for your reading pleasure.

Please review the 2006 Edition of ASME Section IX, and the 2004 Edition/2006 Addendum of the NBIC. By the way, the half bead technique was dropped from the NBIC during the re-write.

If you need further help, please post back.

 

[bob330]

Thanks Metengr,

This is very helpful adn you are a fountain of good information. I wonder if most common R-stamp validated repair/fabrication shops have welders qualified for the temper bead technique. Basically we opened up a can of worms with a PWHT operation today. We welded some new nozzles into a very old 1966 vintage SA-212 pressure vessel and performed 1125F PWHT with blankets around the nozzles. After this and amazingly, a circumferential head to shell weldment about 6 inches from the nearest PWHT pad (there for the nozzle weld) started to light up like a Christmas tree upon WFMPT even though it checked out clear before PWHT and was never touched durign this outage. When we went to grind on some of the indications (all confined to the weldmetal iteself only), we found very long subsurface defects as much as 0.75 inches deep going right down the middle of the weld. The vessel never had PWHT performed on it and doign so evidently opened up some pre-existing weld related defects that have been there all along. The moral of the story for me is that new vessels should get this not only for stress relief but to help expose defects and as for used vessels, try to avoid it if its never been done before....we have an open can of worms now. I am considering alternates to PWHT since we already did it and it caused problems.

Another question: The vessel was built in 1966 and says 100% joint efficiency on the U-1 form. What was required back then to get this rating? I cannot believe that the vessel had 100% radiography based on what we are seeing now. I was told that general requirements were not stringent back then (for example, I was told that impact testing was not required for pretty much any situation and certainly not ours [1 inch thick SA-212).

Bob

 

[metengr]

Bob;
Local PWHT on any existing pressure retaining item needs to be performed with extreme caution, as you just experienced. My best guess is the weld defects were either hydrogen cracks that formed during or shortly after fabrication welding was completed or the weld metal exhibits marginal ductility. It is highly possible the hydrogen (or delayed) cracks that formed during original vessel fabrication remained dormant (subsurface) until enough of a thermal stress, as you suggested from PWHT, enabled the cracks to propagate or bust through to the surface.

I have seen some rather sorry looking girth welds on one of our old deaerator tanks that still amazes me today that the head was able to remain on the vessel.

I would continue with the local PWHT and make repairs to the fabrication welds, as required. For Section VIII, Div 1 vessels, PWHT is not necessary for a nominal thickness 1.5" or less in thickness with a preheat. So, even if the weld defects extend 0.75", a local preheat can be used to repair these defects w/o PWHT.

I believe, back in earlier Code days, if the welds were confirmed as full penetration, credit would be given for a joint factor of 1.

 

[bob330]

Thanks MetEngr,

I was thinking hydrogen damage or lack of fusion type defects myself. However, I think the hydrogen delayed cracking issues are much much more likely to be in the semi-hardened HAZ as opposed to the weld metal and all the defects we are seeing are confined to the weldmetal only. Much of it is right in the center of the weld also making me suspect the possibility of hot tearing or solidification tearing which is much less common in carbon steels than austenitic SS where it is much more likely.

Thanks,
bob

 

[stanweld]

Bob330
We have seen subsurface delayed hydrogen cracks, some of which appear to mimic your case. In 1966 carbon equivalents in low carbon steels and weld filler metals were considerably higher than today. Manufacturers also tended to use active fluxes (also not low hydrogen) in submerged arc welding when they should not have. Manganese buildup in the weld metal plus the higher carbon contents could readily have led to martensitic structures susceptible to delayed hydrogen cracking which may not have been generated prior to radiographic testing.

 

[bob330]

Thanks Stanweld,

This is good information Satnweld, the weldbeads on the outside of the vessel appear to be from sub arc as they are huge. Does anybody have the chemical composition of SA-212 grade B by chance?

Also, I did look at QW-290 for temper bead welding and it still does not describe the basic technique? Can somebody explain in plain terms how this technique works and waht is required. Do you have to increase amperage with succesive passes to pump more heat in to temepr previous beads? I take it no material removal is required as with half bead techniques.

Thanks,
Bob

 

[metengr]

bob330;

If your vessel is carbon steel, all you need to do is to follow Method 1 in the NBIC. This is an elevated preheat weld procedure that we use for weld repair to drums or headers made of P-No 1 material with no impact testing by Code. There is no temper bead requirment for this method. All you need is a qualified WPS for use on P-No 1 material with no PWHT.

Background info for the other Welding Methods in the NBIC;

The premise behind the temper bead technique is to temper or soften the previous weld pass by subsequent weld passes deposited on top of each other. Follow the Table in the NBIC for coupon thickness and repair depth.

I would suggest obtaining a plate, and prepare for a v-groove weld joint procedure qualification using one of the welding methods in the NBIC. The plate should be similar in CE or chemical composition and heat treatment to the original pressure vessel steel.

The technique for temper bead requires buttering the entire weld groove using a smaller diameter weld electrode (3/32" for SMAW). Deposit the butter layer using stringer beads with about a 50% overlap in beads. For the second layer increase the electrode diameter to 1/8" (SMAW process) and continue using stringer beads for the second weld layer while always remaining on top of the butter layer. For the 3rd weld layer, I would use conventional deposition techniques. Never allow any of the weld layers to touch the BM.

When you fill the weld groove, add reinforcement to temper the last weld pass. This layer will be removed by grinding. Grind the added weld reinforcement substaintially flush.

Send the plate out for mechanical testing using ASME Section IX guidelines. You are all set.

 

[stanweld]

I think that if you read QW-290 and further review QW-290.4 essential and nonessential variables and review the Figure in QW-462.12, you can determine appropriate methods. Whether you use 1/2 bead technique is up to you but it is an essential variable as defined by QW-410.65.

Another comment regarding SAW; deep but narrow penetrating weld passes are also more susceptible to cracking.

 

[jte]

Does anybody have the chemical composition of SA-212 grade B by chance?

From the 1962 Section II for SA-212 Grade B:

Carbon max (<=1" thk) 0.31
(<=2" thk) 0.33
Manganese max 0.90 (unless low temp service per A 300 which allows 0.85 to 1.20)
Sulfur max (firebox) 0.04
Silicon (ladle) 0.15 to 0.30
(check) 0.13 to 0.33

jt

 

[bob330]

Much thanks to MetEngr and StanWeld. You guys are great and I owe you both a beer next time you are in Texas!!!

Bob

 

[bob330]

Thanks JTE for the composition. Does anybody know if this material is a P1 type? Hard to find any information on it.

bob

 

[metengr]

This steel was discontinued. It is classified as P-No 1.