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NDE before / after PWHT? 2

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nur12

Mechanical
Jan 28, 2002
36
Hi,

I have a question regarding performing NDE on the vessel joints.

The Code requirement is to do NDE (RT & UT) before PWHT. Out Client's spec requires it before and after PWHT. The Vendor (due to extremely, uncompromisingly tight schedule) offered to perform it as per Code, before PWHT, and mag particle test after. Our Client mentioned that they doesn't care if NDE was performed before PWHT, but they definetely want them to do it after, just to make sure that they provide "QUALITY PRODUCT WITHOUT DEFECTS". Manufacturer is very reluctant to do NDE after PWHT, their argumentation is "what if we will find some defects, and will have to repair and re-PWHT again, and what if we will find defects again..."(??!!). I assume that if Code required NDE only before PWHT, it means that the possibility of the appearence of the new defects is very low. Am I right?

Please, share with me your considerations.

Thanks!

Inna
 
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InnSK:
I personally would not view this situation as a probability.

It is quite common to perform NDT before and after the PWHT. If the client has specified post - PWHT testing, then I'm not sure how the fabricator would negate this process simply due to schedules.

 
This view from someone at the "customer" end of the spectrum (though this particular job clearly isn't one of mine):

an early NDI is obviously of benefit to the vendor, because it helps him identify and fix problems before he's undertaken too much nugatory process - but doesn't offer the customer a huge amount of comfort.

As a customer, the thing I want above all else is assurance that any critical weld is still in good condition when it's delivered to me - and that might well mean NDI after the vendor has completely finished mucking around with heat and funny atmospheres.

Since my first run-in with hydrogen cracking (the welder did a nice weld, with a beautiful NDI - only to come in the next morning to discover that the joint had self-destructed overnight), I've become a fanatical believer in having a last chance NDI done as late as possible with the weld in its final state.

If your vendor is genuinely worried that post-treatment NDI might find defects, then I feel your client might have a point anyway!

Ultimately, I guess it comes down to whether your client is prepared to pay you and allow you enough time to humour his foibles. If he is, where's the shame in taking his money?

A.
 
Thanks for reply. The Client is not prepared neither to pay nor wait. The situation with this vessel is quite complicated - delivery was tight from the beginning; after getting a PO, the first Vendor notified me that he can't comply to this delivery (they've got a big job and didn't want to bother themselves with our vessel anymore); I withdrew the PO from the first Vendor and quickly moved this job to another shop; they had only couple of days to read the specs, but had a great desire to work for our Client, so accepted our PO and started to work; so obviously they missed couple of things, which started to appear afterwords, and the after-PWHT NDE is one of them. This shop usually don't do it. They already are working overtime and weekends to get it done in time... I realize that based on your expierence I have to accept schedule slippage and do the NDE afterwords... but Client will not understand delay (least to say...).
 
My previous comments are based on that I used to be on the client end of pressure vessels.

If for some reason something does go wrong down the road, no one can deny knowledge of the original project specifications.
 
In my opinion some kind of NDE is essential after PWHT and to add to the timing woes this should not be done until at least 36 hours after PWHT. The reason is to detect hydrogen cracking. Your vendor's suggestion to do Mag Particle after PWHT, if 100%, should be acceptable.

My experience is with other highly stressed, high integrity weldments so I am not familiar with the pressure vessel Code but doesn't it spell out what is acceptable?

Also I have been prepared to accept limited repair after PWHT without repeating the process provided the area is small and the specified repair procedure is unlikely to introduce significant in-built stresses. However again I would think that the Code has something to say about this.
 
We have similar decisions to make in UK power generation and we have had similar arguments in the past. Our logic for PWHT before and after on SOME components is this:

Welder/process induced defects and material defects (and I include delayed hydrogen cracking here) that can occur if should dictate that NDT is done before any PWHT is carried out. It is clearly better from a commercial point of view to carry out these repairs before the expense of PWHT is incurred. There is also a strong argument to avoid putting the base material through two PWHT cycles as well in some cases.

Of course the PWHT is not going to induce welder or welding process defects such as lack of fusion or lack of root pen etc. but, the heating regime may induce metallurgical changes that can manifest themselves in either the weld metal or the parent material - stress relief cracking in low alloy ferritics is the obvious example here. This is why PWHT is specified before and after PWHT.

Many British codes require or recommend that PWHT is carried out after PWHT (BS 2633 for CrMoV welds require UT for instance).

Yes you might find defects that weren't present before PWHT because you are testing a different surface for surface inspection (due to perparation) but any volumetric inspections carried out before PWHT should have identified any out of code defects before PWHT.

Here's a scenario which makes life difficult though..

Suppose you have a sub surface defect detected by UT before PWHT that is acceptable to code. Then after NDT, a defect is located by a different operator and he measures it 2 mm longer ('cos his tape measure slipped) the defect is now outside the code. What do you do?

Structural analysis, fitness for purpose argument..??

By the way, surface inspection only after PWHT does not necessarily guarantee freedom from defects in the remainder of the weld or parent. Sub surface defects can arise depending on the material and whether the weld metal overcaps onto the parent - these situations produce situations of hiogh stress triaxiality and in materials susceptible to stress relief cracking sub surface defects can materialise.

Do the NDT before and after - you know it makes sense.

Hope this was useful.

Andy
 
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