Not logged inAmerican Welding Society Forum
Forum AWS Website Help Search Login
Up Topic Welding Industry / ASME Codes / PWHT for clad pipe under B31.3
- - By hung7601 (*) Date 03-22-2012 03:45 Edited 03-22-2012 03:57
Hi Every one

I have non-ending discussion about the PWHT of the welding on clad piping, we are preparing the piping WPS for the thickness of 25mm A106Gr.B plus 3mm Inconel 625 cladding. The welding is by GTAW process using ER NiCrMo-3 tig rod throughout the thickness (A106 Gr.B & Inconel 625 cladding). 

I refer to Clause 323.4.3 d ASME B31.3 2008 and ASME VIII Div.1 UCL 34 & UCS-56, the PWHT is applicable in this case; when refer to table 331.1.1 ASME B31.3 the PWHT is required if thickness more than 20mm for P no.1 with weld metal A no.1; the electrode used in this case is ER NiCrMo-3 which is not fall in  A no. 1 so the PWHT is not required.

Can I say the PWHT is not applicable to this case? Please offer your expertise.

Thanks and Regards
Mike
Parent - - By MBSims (****) Date 03-22-2012 13:05
I think paragraph 331.2.3 addresses this situation clearly.  You have to do the PWHT required for the P-No. 1 base metal, even if the filler is not A-No. 1.

331.2.3 Dissimilar Materials

(b) Heat treatment of welded joints including both
ferritic and austenitic components and filler metals shall
be as required for the ferritic material or materials
unless otherwise specified in the engineering design.
Parent - - By jon20013 (*****) Date 03-22-2012 17:38
Given the nickel alloy cladding, I'd probably ask my engineer to allow a deviation as permitted by UCS-56; i.e., elevate preheat to avoid PWHT.
Parent - - By hung7601 (*) Date 03-28-2012 02:26
Hi Jon20012 and MBSims

I have same idea with MBsims in this case, but from metallurgy point of view do you think the PWHT temperature would impair the corrosion resistance of the cladding layer?

Regards
Mike
Parent - By jon20013 (*****) Date 03-28-2012 04:27
Mike, nickel HATES heat and when exposed to it (as I'm sure you're aware) she tends to protest and shows all kinds of nasty attributes; including decreased corrosion resistance. 

In my opinion, there might be an acceptable deviation (as I previously mentioned) by elevating the preheat as permitted by ASME VIII, Div 1, UCS-56 to avoid PWHT. 

Naturally this would have to be approved by engineering (and possibly the customer) since you'd be mixing apples and oranges to some regard (B31.3 and ASME VIII) however in my opinion the technical justification is worthy of approval.
Parent - - By js55 (*****) Date 03-28-2012 12:44
If I may repsectfully disagree with jon. One of the characterisitcs that alloy 625 was originally invented for is high temp. Its tensile strength under stress does not even start falling off until around 1400 degrees.
It is not uncommon to use NiCrMO-3 for buttering dissimilars between stainless and carbon or low alloys BECAUSE it can hold up under stress relief and has similar thermal expansion.
It is being considered for advanced high temp nuclear reactors as well.
Its allowable stresses at 1500 degrees are around 4,200 psi.
It has very good high temp oxidation resistance and good corrosion resistance at high temp as well.
This alloy is one of the best workhorses around.
And since we are talking about PWHT I would not be concerned.
Also, keep in mind it is Cb stabilized as well.
Parent - - By jon20013 (*****) Date 03-28-2012 18:24 Edited 03-28-2012 18:33
js55, I agree.  Must be the medicines I've been taking for my recent illness :confused:  Guess it just goes to show any of us can be wrong from time to time. 

In fact the thermal problems I've had with 625 were welding problems (overheated beads in "relatively" small diameter, very thick wall pipe, using SAW process) in which we had unbelievable cracking.  Again this is apples and oranges with the original post, but that's where my feable mind took me when responding!

Having said all that, I stand beside my original response; were it me personally, I would propose the elevated preheat to avoid PWHT.  Again, just in my opinion, I don't believe PWHT adds any value in the stated condition even though it's technically a B31.3 requirement.

EDIT:  Having now explained that my previous erroneous response was due to the pain of a very bad experience, will simply state that during the WELDING process nickel HATES heat, keep it cool or you'll be sorry.
Parent - By js55 (*****) Date 03-29-2012 12:01
jon,
The preheat suggestion is a good one. No disagreement there. And you're right, PWHT adds no benefit. The 625 sure don't need it.
Part of the reason, I would guess, that you had problems with cracking due to overheating is as with many things, assets become liabilities. Cb is added for stabilization and high temp strength. It will also segregate quite readily when high heat input regimes are used, and this can contribute to low temp solidification phases and hot cracking.
Also, if memory serves, this asset Cb will lead to long term high temp microstructural instability by contributing to the formation of Laves phase and creep ductility concerns. But this takes a long long time.
It also does not work well with nitrogen alloys due to the formation of Cb nitrides that can contribute to microfissuring. Though as I've stated in other posts tons and tons of it has been used for AL6XN with nary a problem.
Parent - - By ozniek (***) Date 03-29-2012 13:57
Hi Mike

I think you have received some good answers already, but just my thoughts also:

1) Inconel 625 has no problem being exposed to the normal heat cycles used for C/Steel stress relieve.
2) My biggest concern would be disbonding of the lining from the steel. For some clad steels, you will have no problems, but may pick up issues with others. This may have to be proven with some mock-ups using project materials, if you intend to PWHT.
3) The reasons for the PWHT on the C/Steel, is to ensure that hardened phases that are more common in thicker materials are tempered, and that residual stresses are relieved. This increases the fracture resistance of the pipe, significantly reducing the probability of brittle fracture.
4) If the designer can show that brittle fracture is not an issue with the particular piping system, and the absence of detrimental microstructures can be shown during WPS qualification, or other testing, then you can dispense with the PWHT. (See 331.2.2 - I will attach.)

Regards
Niekie
Parent - - By jon20013 (*****) Date 03-29-2012 18:42
Gents, on a "for what its worth".... I'm aware that there's been discussions amongst some technical groups (believing EPRI may be included?) to suggest eliminating PWHT for P1 materials, I'm sure there's deep feelings on both sides of this discussion but I would tend to support the notion to do away with it for a majority of applications...
Parent - - By hung7601 (*) Date 04-03-2012 06:47
Thanks guys for your valuable contribution.

Can I say that the PWHT doesn't harm to the 625 cladding layer but it also doesn't add any benefit? My concern here is to collect the evidence within the code requirements to avoid PWHT? As you know one of the reasons to do PWHT is to reduce the residual stress on the carbon steel caused by welding, Do you think elevating preheat as per ASME VIII Div.1 UCS-56 will be the best reason for not doing PWHT?

Regards
Parent - - By jon20013 (*****) Date 04-03-2012 10:02
js55 said it best, but having been the engineer of record on similar quiries I've approved based on UCS-56 with the bessing of my engineering management.
Parent - By hung7601 (*) Date 04-03-2012 13:44
Thanks Jon20013

Regards
Mike
Parent - By ozniek (***) Date 04-03-2012 13:44
Hi

Pre-heat would be a tool to achieve certain outcomes, (such as improved fracture toughness and reduced hardness) but stated in isolation does not give a reason for (in my opinion) waiving the PWHT, as allowed by 331.2.2. My approach would be to submit a technical query (or whatever the document is called in your system) with the following reasoning:

1) There is a good reason for not performing the PWHT. (What reason do you want to give? - This is the critical point! e.g. Schedule constraints; Disbonding of lining; Accessibility issues; Proximity to sensitive equipment such as welded in valves; Safety issues...)
2) The PWHT is primarily to increase the fracture resistance of the pipe installation. (By tempering brittle microstructures and reducing residual stresses.)
3) 25mm Is relatively close to the 20mm limit requiring PWHT, so for a material such as A106 that is not prone to welding dificulties, eliminating the PWHT should generally give acceptable results.
4) A WPS will be qualified with an elevated pre-heat (and no PWHT) to reduce the residual stresses and reduce the probability of brittle microstructures.
5) This WPS will be qualified with impact tests and hardness testing. (If you are going to weld the entire weld with Inconel filler, (which is the normal practice) the weld metal will have very good fracture toughness. The only issue will be proving the HAZ impact properties.
6) The production welds will be hardness tested as an additional QC activity to show that the outcomes of the pre-heats are giving the required results.

Most engineers would accept this line of reasoning, as long as your reason for wanting to eliminate the PWHT holds water.

Regards
Niekie
Up Topic Welding Industry / ASME Codes / PWHT for clad pipe under B31.3

Powered by mwForum 2.29.2 © 1999-2013 Markus Wichitill