NDE before and after PWHT

[humbug]

In an effeort to cut out costs our estimating dept. wants to eliminate the NDE budget for performing radiograhpy before PWHT. By B31.1 NDE is required after PWHT for the P4 material group. I am not a big fan of this but in an effort to win more projects I can understand the thinking. We normally perform RT, PWHT, final RT for Code compliance.
We do have some real good welders in the shop with a reject rate hovering around 1%.
We have discussed the addition of a second PWHT cycle and an allowance for rejectable welds and the costs are still less than full RT prior to PWHT.
I'm just looking for various thoughts on the subject.
Thanks

 

[rustbuster]

It sounds like an economic desicision, if you maintain the quality of welding you have, there should be minimal re-work. Have you considered Phased Array UT?

 

[weldtek]

Be sure to consider the effect that multiple PWHT cycles will have on the materials. It may be negligible for most or all of your applications, it wouldn't be for ours.

 

[Mr168]

If you're in a shop and you do PWHT in a furnace, or only do one weld at a time, your situation may be more feasible. In the field, it's a bit of a different situation. We pay right around $125/shot negotiated rate with our NDE sub. However, look at what's involved for PWHT in the field.

-Materials (insulating blankets, power leads, ceramic heaters, TC wire, etc)
-Installation (workers running those leads, wrapping it, etc)
-Acutal PWHT (time for ramp rates in both directions, soak time, and typically two guys - one to monitor the controller, the other to do repairs when someone knocks off a TC, one burns out, etc)

Man hours and costs add up rather quickly.

 

[0707]

I THINK THAT IT IS AN ECONOMIC QUESTION, IF YOU ARE CONFIDENT OF YOUR WELDERS MAYBE YOU SAVE MONEY IN PERFORMING NDE ONLY AFTER PWHT.



LUIS

 

[thgirw]

I am imagining a situation where you have a problem with one welding detected by NDE after heat treatment, that would be a mess. Some materials can not be heat treated again and again, so you will not have a second chance with the hardware. If you are making heat treatment to expensive materials, i would not risk it. Also consider the expenses of removing the weld (if any problems detected) and performing a heat treatment again.

 

[HouseHark]

our customers also take into consideration delivery time.

if you have the time to take a chance then usually it would be better to skip the prior NDT.

if you are on a tight schedule reject PWHT can cost you quite a lot of time.

 

[magmetallurgy]

The PWHT is done to relieve residual stresses, improve mechanical properties, and reduce hydrogen-cracking susceptibility. The steels with P-N°4 (for example)have a stress-relief susceptibility cracking in multiple-pass welds case. This is due to a combination of more than metallurgical factors. For that, performing radiograhpy after PWHT is very recommended.

In this situation the problem is not in money and time but in safety.

for more metallurgical details you can contact me at :
mechri1973@yahoo.fr

 

[MachinaMan]

Hi Humbug,

We make that decision on a job by job basis.

I work in a power plant where during an outage we have many contract welders. So we look at the welder's performance on past RT jobs and the job schedule to make the call.

Like it was said above, when you have time to deal with potential repairs is when you would skip the RT before the PWHT.

Personally, there are only a few times where we had the luxury of skipping the first RT but it did pay off when we did.

 

[stanweld]

humbug,
With a shop reject rate of 1%, it will be more economical to delete the RT before PWHT. Simply compare the cost differential between 99 radiographs and one additional PWHT.

 

[gtaw]

The ideal situation is to remove unacceptable materials from the production stream as soon as possible so time and money is not wasted on bad product. NDT can help achieve that goal if the NDT is selected wisely and applied appropriately.

I assume the PO or code requires the RT has to be performed after PWHT. The RT before PWHT is performed to reduce the risk of spending time and money on post weld heat treating a weldment that doesn't meet the applicable acceptance criteria to begin with.

This would be the perfect time to use UT in place of the initial RT. UT is faster and less expensive than RT. If the weld is found to be unacceptable it can be repaired and reexamined with UT before PWHT. Once the weld is accepted by UT, it can be PWHT'd and RT performed as the final NDT as per the PO or code.

Best regards - Al

 

[ndeguy]

I consider it a dangerous practice to examine welds pre and post PWHT with different examination methods as each method has its own strengths and weaknesses.

Take porosity as en example. RT will easily pick up an isolated out-of-Code pore which UT would never condemn. Similarly lack of inter-run fusion can be very difficult to detect with RT but UT will discover it easily.

The 1st example would give you a repair after PWHT even though examined before PWHT and the 2nd you may repair LOIRF before PWHT unnecessarily.

My advice would be to stick to the same NDT method before and after PWHT.

 

[metengr]

For this very reason, ASME B&PV Code, Section I was revised in 2009 to allow volumetric examination (RT or UT) of welds provided the same NDT method is used throughout fabrication.

 

[humbug]

An update on the progress of this project.
The first section had approx. 94 - 3" XXH Buttwelds, 3-2.5" XXH BW's and 2 - 2" XXH BW's all of which was 2.25% Cr.
We RT'd the first 5 from each welder (5 welders) and then RT'd 5% thereafter from each welder. This preliminary radiography resulted in zero defects.
We then sent the fabricated assemblies off to PWHT.
Upon return all of the welds were then RT'd for Code acceptance and ALL welds passed.
In this case there was 314 pc's of film with zero defects.
There is a round two coming though and in that section there are approx. 300-3" XXH 1.25% Cr welds. We will see how we perform.
Congratulations to the welders. These are the guys who took the time to make it right and should be proud of there accomplishment.

 

[t1t2t3]

Totally in agreement with Stanweld. Little more, we can select critical welds by positions of welds, difficult access after welding (Scaffoldings), weld degree of material (5A B for example)..... Sure that NDT before and after PWHT it is not the best desición. A curve showing the cost/ risk for different situations could be more suitable.
But who assumes the responsibility?