308 flux core welds cracked after PWHT

Date 09-06-2007 23:46

We have just recieved a 5" thick 304L weldment back from heat treatment (1900F solution anneal with forced air cool). It has two branch connections. Each branch connection has a cast neck with a forged flange (around 24" ID). The cast neck is joined to the flange with a sub arc weld. Standard groove weld backgouged and welded. The casting to forged case (around 3' ID) was welded with flux core. How we got to this point is a long story and has taken many months. The first attempt at stress releiving this yielded the same results however it was done at 1675F for some reason and the results were much worse.

The sub arc welds have cracking in three areas about 5-6 inches long. It looks like these areas were repaired at some point with flux core. The major cracks are between the beads with some minor cracks perpendicular and scattered about. The dye penetrant test bleeds all over the place so I know they are deep. The rest of the sub arc welds are clean. The cracking appears to be restricted to those areas that appear to be repaired.

The flux core welds also have cracks that run with the beads and in some areas across. I'll have more info on the extent of those tomorrow, however, the dye penetrant shows cracks as long as a foot. The cracking seems to be restricted to the end of the branch that runs down the side of the case (connections are oval). I checked the ferrite numbers before heat treat and the welds were around 8-10 and after heat treat they were .5-2. We also performed RT, MP, and PT before heat treat and everything was clean.

We used one heat of flux core for this case.

If you have more questions, let me know and I can piece more history as questions are asked.

Any idea on the cause of the cracking (significantly long cracks where two beads meet)?

By GRoberts (***)

Date 09-07-2007 02:29

Jon,
When you heat treat something that big with high restraint, it probably doesn't take much reduction in ductility to help start cracking.

I think you could have 2 possiblities.

1. If an flat-horizontal flux core wire was used, it most likely had bismuth added (you may want to check anyway-some manufacturers may add it to all-position wire, but not the few I've checked). Bismuth reduces ductility at high temperatures. The only time I have seen it (I think), is on repairs that have been solution annealed a second time, which makes the problem even worse, but it could happen on the first time to high temperature too. However, the cracking between the beads make some sense in this regards since the high temperature HAZ has already seen the high temperature once, and the solution anneal make

2. With the very thick sections and air cooling, if you have much ferrite at all, it can form intermetallic phases such as sigma since it takes so long to get through the embrittlement ranges. This could be one of the reasons your ferrite is cheking lower. If it transforms to sigma, it won't be magnetic anymore.

I'll send you a personal e-mail as well so you can give me a call if you want. Say hi to Angel for me.

Greg

By MBSims

Date 09-10-2007 01:23

Is it possible that an incorrect electrode was used (carbon steel?) and local repairs were made to remove the incorrect weld?

Also, you stated "We also performed RT, MP, and PT before heat treat and everything was clean." What is "MP"?

By ctacker (****)

Date 09-10-2007 01:27

Probably meant MT magnetic particle

By MBSims

Date 09-10-2007 03:10

On stainless?

By ctacker (****)

Date 09-10-2007 04:43

dont know what else he could have meant by MP!

By Tommyjoking (****)

Date 09-10-2007 05:26

"It looks like these areas were repaired at some point with flux core."

Sounds like you have your own line of reasoning here....what filler was used for each process? If repairs were made why were they made.....at any rate you have some arc gouging to do and then a reweld. You have a limited amount of tries on this same part so ......find out if they actually were repaird by wire and why.

By jrupert (*)

Date 09-10-2007 22:23

Sorry all - PT and RT only.

We used 308L for both the flux core and sub arc. I don't know why the repairs were made, because they were done back in Jan or Feb and I wasn't working there at the time. Know one seems to remember why, but they do recall making a few repairs.

The second connection was worse than the first one. 80% of the weld with significant indications up to 3/4" deep. The entire suface of one side of the connection looks like the rails in a train yard. All the cracks following the edge of the beads. Since the top was ground off for another PT, it was hard to see where the crack originated at the surface in the crossection. It looks like the toe of each weld was located in the center of the next bead. The crack starts at the toe (or center) of the welds and follows what looks to be the grain boundary. Since it was a solution anneal, I was expecting to see a fairly evenly dispersed grain structure. It appears that we had elongated grains extending from the surface into the part about 3/4" where it met up with a refined structure. The cracks followed those boundaries.

By js55

Date 09-17-2007 14:36

Even though I'm gettin in on this one a little late I'd like to add:
At a 1900 anneal the microstructure will homogenize, including the existence of any sigma (though Chuck may be able to help us out with sigma dissolution temps for this alloy) so it seems to me that you had some pre-existing problem. Perhaps microfissures in the SAW welds from too high a heat input that while fine as welded became stressed from the lower stress relief and/or the repairs. I'm still a little fuzzy on the sequence of events.
From what it sounds like its really more HAZ (in the prior SAW weld-due to bead edge cracking) than it is related to liquation (though the more random pattern of cracking away from the bead edge could argue for liquation). Though this alloy is generally quite tolerant of such problems.
Having said that, if the FCAW was heat treated at the lower temp then the bismuth issue could cause a grain boundary liquation that could manifest in the random cracking pattern you descibed, though the bead edge issue is strange.
If I've misunderstood please clarify.

By jrupert (*)

Date 09-17-2007 22:22

Is there a way to upload a sketch? It would be much easier to explain and show some detail.

By MBSims

Date 09-18-2007 00:06 Edited 09-18-2007 00:09

Liquation cracking may be a possibility. Especially if the base metal grain size is large, as in a forging. We had this problem in a 10" thick 304L forging that had a ASTM grain size larger than 00. I would expect to see small microcracks show up along the toe of the weld during liquid penetrant testing. Not likely to show up between weld beads.

By CWI555

Date 09-18-2007 03:22

well, this one is interesting. You mentioned Having done PT and RT, but before you came along there had been repairs to the sub arc. I also gather that the problem is in the sub arc and repairs to the same. Making the assumption that RT didn't see anything, and given SAW, previous repairs for "unknown" reasons, center-line cracking under the next succeeding pass, your description of of it appearing to follow a grain boundary, I am leaning towards a dendrite problem, probably exasperated by other factors. Given that they are in fact there, and RT didn't see them, nor did PT, they were probably just below the surface. If in fact it is the problem, then you should have some more of it extending into either edge of the now visible cracks underneath the surface.

Rapid solidification of 308 can create center-line cracking problems. that same problem could have been highly stressed, especially if it also was subjected to the the stress of being in the succeeding passes heat affected zone. If in fact it was a columnar / dendritic solidification problem to begin with, they are problematic to repair. Some people will stop to earlier on the excavation of the crack, and weld over it, causing it to crack again underneath, not to be found until an attempt at annealing.

For more specifics on the testing, PT must have an open surface. I'm willing to bet that at some point in the parts life, a pt test showed a problem hence the unexplained repair.

If in fact it is the problem I've mentioned, RT sucks for finding it. Especially in thickness range you describe. Not only is the flaw orientation a problem, your going to have a problem with diffraction. At normal pipe thicknesses it's never a really an issue (< 1"). Getting into 1.5 and 2 inches and up, you run through enough of a cross section containing 9-11 percent nickel that it becomes an issue. Anything such as a fine crack in the upper surface oriented nearly parallel with the plane of energy propagation (opposite film as opposed to in contact with the film) will be muted potentially to the point that you can lose enough subject contrast to make it nearly hidden.

If the part matters enough, ship it out to or bring in a group familar with round trip tandem UT, or phased array. Either could dial it in via refracted Lwave inspections.

In short it could have been there all along, but not seen until it dropped anchor and cracked out to the surface during annealing.

Therein is my two cents worth,
Gerald