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B31-1 requires a 400F preheat be achieved before welding and maintained during welding. If, on 1 1/2" wt material, the first half of the weld was done with only a 200F to 250F preheat, and the second half of the weld was done with the required 400F preheat and the weld was subsequently PWHT and radiographed and no cracking was evident; can I be certain I will experience no future cracking in this weld?
Typically, hydrogen cracking can still occur for up to 72hrs after the component's temperature is reduced. If this time period has expired, without H2 cracking, then you should not get further H2 cracking. If this time has passed, again perform radiography and MPI.
I would recommend that you perform some hardness tests just to check the hardness achieved. If it is too high, there are other problems that you could pick up.
Thanks Niekie, I know they didn't wait 72 hours after PWHT to radiograph the weld. I suppose I could make them do another shot and some Brinnells; is 241 the limit?
My main worry is that we will be hitting this weld with 1525 PSIG at 1005 degrees F. about 4 months down the road. They are in violation of their procedure and the applicable code; Why should I not just reject the weld on that basis?
The PWHT should have removed any left over Hydrogen in the weld, and erased any areas of hardness.
How long after the welding was done before the PWHT was performed?
Did they preform an Hydrogen bake out at the end of the first half of the welding process?
Another RT wouldn't hurt anything.
Because we do not know exactly what happened, there is a possibility of problems still arising after PWHT.
Often, especially in these thicknesses, the weldment is not cooled down before PWHT. In this instance there could be a lot of retained austenite present in the weld and HAZ. The PWHT cycle would not temper this. It would merely turn to martensite on cooling after PWHT. This is why the hardness tests are adviseable.
Regarding the PWHT removing the H2, I would agree that this is almost certainly the case. "Almost certainly" being the operative phrase. If this was always the case, then hydrogen cracking following PWHT should never occur. Unfortunately this does occur, up to 72 hours following the PWHT. Again, it depends a lot on the microstructure and initial amount of H2 present.
Theoretically you could reject the weld, and should there be any reasonable doubt that the weld will be able to perform its intended job, you SHOULD reject the weld.
On the other hand, if there is no reasonable doubt that the weld will perform, then why reject it? What you have to ask yourself, is what is all the pre-heats and PWHT's there for? In the case of the pre-heat, it is there to:
1) Keep the temperature above that at which hydrogen cracking could take place should you meet all the conditions required for H2 cracking.
2) Help to control the microstructure to that which would not result in H2 cracking. (One of the conditions for H2 cracking is a susceptible microstructure.)
If you have completed the weld, and the cracking did not take place, and the hardness is OK for the intended service, then you achieved what you set out to achieve with the pre-heat, so there is no problem.
Regarding the maximum hardness, I would suggest you try to find a value in a relevant code or specification. (Often the end-user will have such a specification.) In the ascence of all else, check the material specification. It often has a maximum value. (241 Hb sound good to me.)
Whether you accept the weld or not, I would make sure that the organization performing the welding receives some sort of a non-conformance report. This is just to make sure that all actions surrounding the incident is recorded (Including your additional requirements to ensure that the weld is suitable.) and also helps this organization to sharpen up their systems so that it does not happen again.
Thanks Niekie and you too Dale,
There was no hydrogen bake out after the first half of the weld... The crew on nights, untrained in the use of the resistance heating equipment, put the ceramics on one side of the weld about 18" away and attached the controlling thermocouple at the ceramics. They did this because the Deerman clamp was in the way. I noticed the problem because half the weld was completed at shift change and there were no bumps in temperature on the chart to correspond with the welding that was performed. That's when I went to the temp sticks and, sure enough, it wouldn't melt 300. The contractor is quite independent-minded and he placed new coils closer to the weld, brought it up to temp. and finished the weld. They went directly into PWHT and radiographed it 48 hours later.
I wonder if it is possible that the hardness of the first half of the weld is unacceptable as well as inaccessible. More to the point; I harbor grave doubts about this weld.
Along with your doubts about the weld, I'd also have some doubts about the contractor.
>>The crew on nights, untrained in the use of resistance heating equipment<<
Have you thought about the use of out side contractors for the pre and PWHT, to eliminate this problem?
With the heating coils 18" from the weld zone, what was the temperature at the weld joint?
Unfortunately, this is a turn-key, EPC contract, and it's not mine until commercial operation is achieved. So I can't tell the contractor how to do it. I can scream and yell and jump up and down, but that mostly just entertains them.
When you asked the question "what was the temperature at the weld joint?", you struck right to the heart of my problem. I can't know what the temp. was; the chart is not correct. The thermocouple was attached 18" away with the resistance coil. I know what the temp. was there. I do know the 400F temp. stick would not melt on either side of the weld and the 300F temp. stick would not melt on one side of the weld. I don't have a 200 or I would have checked with that.
To satisfy yourself, you may require another PWHT cycle. If there are any brittle areas in the inaccessible root, this should sort it out. Following this, again do radiographs and hardness tests.
One thing to keep in mind is PWHT of this material is performed to temper the Martensite that has formed in the weld. Proper tempering of the martensitic microstructure is essential for obtaining reasonable levels of toughness. I would suggest that you perform hardness checks on the welds. I would take 5 readings throw out the low and the high readings, then average the rest. You should see hardness readings in the range of 225-280 HB. These hardness values will give you toughness values of 20 ft-lbs or better. Another important point is that the time specified in B31.1 for PWHT is inadequate. A minimum of 2-3h in the range of 750-760C (1382-1418 F) is required, or longer for thicker sections to provide sufficient tempering.
Also, your welding rod should have a Ni+Mn content of <1.5%. This will ensure that the Ac1(Transformation Temperature) Will be high enough so that your PWHT temperature will not exceed it. It has been shown that when the Ni+Mn is = 1.5% the Ac1 temperature is 1436 F. This is extremely close to the PWHT of 1400 F, and exceeding the 1436 F temperature would not be uncommon, it is very possible that the AC1 could or would be exceeded! If the Ac1 temperature were exceeded, some austenite would form which in turn transforms to fresh untempered martensite on cooling.
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