PWHT

[Ani18]

Dear all,
While reviewing a PQR of thickness 22mm i found the PWHT time range is mentioned as 300 minutes and in WPS it is mentioned as 375 minutes.
The Pqr tested with impact test.
My question is that from whre did they get the soaking time of 300 minutes in PQR according to ASME SEC 8 UCS 56 the soak time doesnt matches with the mentioned one in the PQR?
If by using ASME SEC 9 then how? Explain the calculation pls?

How the soaking time in WPS changes to 375 from 300 as in the PQR? Is it correct

 

[Jovi Zhu]

It must be a typo as I can hardly imagine a 300+ minutes (5+ hours) holding time for 22mm thk carbon steel. It would be more reasonable that the PQR was done with a 30.0 miniutes holding time and the WPS specifies 37.5 minutes. The welding engineer who approved the WPS should be able to explain.

UCS 56 requires 15 minutes as a minimum. Sect 9 is not a construction section and it does not give any welding parameters but just rules of qualification.

If nothch toughness is not applicable, as per QW-407.1, the change from one of the PWHT conditions of (1) to (5) to another requires re-qualification. Practically QW-407.1(a)(2)is applied and increasing holding time is not an essential variable so to the experience of a welding engineer, he/she can increase a little bit for a better stress relief.


QW-407.1 A separate procedure qualification is required for each of the following:
(a) For P‐Numbers 1 through 6 and 9 through 15F materials, the following postweld heat treatment conditions
apply:
(1) no PWHT
(2) PWHT below the lower transformation
temperature
(3) PWHT above the upper transformation temperature
(e.g., normalizing)
(4) PWHT above the upper transformation temperature
followed by heat treatment below the lower transformation
temperature (e.g., normalizing or quenching
followed by tempering)
(5) PWHT between the upper and lower transformation
temperatures

However, there is one possibility that the carbon steels are to be welded with austenitic or other materials calling for necessarily lower PWHT temperature than the miminum specified by Table UCS-56-1. For this case the Code allows alternative PWHT as per Table USC-56.1 with increased holding time. Again, The welding engineer who approved the WPS should be able to explain.

 

[Ani18]

As per UCS 56
For 44 mm-The PWHT Temperature shall be 60minutes+45,6Minutes for additional 22mm? So total 105.6 minutes minimum
Am i rIght?

 

[weldstan]

The soak time of the PQR may be greater than the minimum Code required. With impact testing, additional time at temperature may be used to add for repair welding and additional PWHT.

 

[Ani18]

Can you please explain in detail

 

[Jovi Zhu]

For P No.1, Section VIII-1, Table USC-56-1 requires for thickness "up to 2 in. (50 mm)", "1 hr/in. (25 mm), 15 min minimum".

This should be simply interpreted as:
For thk < 1"(25mm), go with 15 minutes as a minimum;
For thk = 1"(25mm), go with 1 hr(60 minutes) as a minimum;
For thk > 1"(25mm) but < 2"(50mm), also go with 1 hr(60 minutes) as a minimum;
For thk = 2"(50mm), go with 2 hr(120 minutes) as a minimum;

And for thk Over 2 in. to 5 in. (50 mm to 125 mm), 2 hr plus 15 min for each additional inch (25 mm) over 2 in. (50 mm)

This should be simply interpreted as:
For thk > 2"(50mm) but <3"(75mm), go with 2 hr(120 minutes) as a minimum;
For thk = 3"(75mm), go with 2 hr +15 minutes=135 minutes as a minimum;
For thk > 3"(75mm) but <4"(100mm), also go with 2 hr +15 minutes=135 minutes as a minimum;
For thk = 4"(100mm), go with 2 hr + 2x15 minutes=150 minutes as a minimum;
For thk > 4"(100mm) but <5"(125mm), also go with 2 hr + 2x15 minutes=150 minutes as a minimum;
For thk = 5"(125mm), go with 2 hr + 3x15 minutes=165 minutes as a minimum;

......
IMHO, the Code is giving the user "stairs with 15 minutes in between" to stand on, not a "slope".

If one is good at math and wants to calculate with additional thk to proportionally get an additional holding time before reaching to the next "stair", it is normally acceptable and exceeding the minimum Code requirement but the Code never requires such a calculation.

If there is indeed such a requirement in the Section VIII that I have overlooked, I'm happy to learn from anyone on this forum who can point it out.

 

[david339933]

300 minutes is definitely not a typo.

When supplementary variables are applicable, the PQR Coupon shall be PWHT to at least 80% of the time at temp of the production welds, per QW-407.2.
If you only do the minimum soak times, you will not be able to repair any welds down the road, as you haven't PWHT your coupon long enough the first time. You need to show the weld can take the multiple PWHT times and still have the necessary impact properties. This is commonly done.

Weldstan's comment, although only one sentence says a a lot.

 

[Ani18]

That means my Wps is qualified with a max thickness Of 44mm and the soak time in pqr shall be 80% of soak time required for 44mm.
Am i right?

 

[david339933]

At a minimum, yes.

 

[weldstan]

You are partly correct. Your PQR enables you or the Purchaser to PWHT multiple times or one time for as many as 375 minutes in total for all thicknesses qualified (16 mm to 44 mm) when impact testing is required for the production part.

You truly need to acquaint yourself with ASME IX requirements before you review any welding procedures in accordance thereof. It is obvious from your post that at present you are not qualified to do so.

 

[DekDee]

Weldstan,
Was just in the process of posting something similar.
It is a sad indictment on our industry in this day and age.
If you are reviewing WPS/PQRs you should have a sound knowledge of the applicable code/standard.
3 x basic questions (not including others on the AWS forum) in 4 days is not a great reflection on the OPs knowledge / experience.

 

[Jovi Zhu]

Ani18,
My appology for my post saying "If nothch toughness is not applicable" as I was revewing another post at the same time on this forum for Section I topic without impact test issue and I partly mistook your post with it.
Now it is clear that QW-407.2 is applicable for your case as a supplementary essential variable.

With this consideration, the 300 minutes holding time on the PQR (it is definitely too long for a stress relief purpose if you are not forced to go with a PWHT temperature lower than Table USC-56-1) can be INTENTIONALLY made to support any or repeated/additional PWHT. With this PQR you can have an aggregate holding time of 375 as per QW-407.2 for the joints of your vessel as already pointed out.

It may be wise to use a PQR with a short holdng time meeting the minimum Table USC-56-1 requirement, together with that "300 minutes" PQR, to prepare a WPS specifying a short but enough time, i.e. 60 minutes as a minimum for your case with a note/remark saying "Aggregate holding time at the temperature such as due to any additional PWHT shall not exceed 375 minutes."

Again, calculation based on Table USC-56-1 with additional thk "between the stairs" to proportionally get an additional holding time giving more heat input is NOT required by the Code and not necessary.

David and Weldstan,

My curiousity for a further disscusion: for repair welding I believe it should be treated separately as it is not only the repeated PWHT, but also the repeated preheat and welding heat are giving additional heat input. Or can this be controlled by proper defect removal or limiting the times of repeated repair? Not sure any manufacturers would gouge a test coupon to simulate a repair welding with repeated heat input. Any ideas or comments.

 

[weldstan]

Let's take a look at some other types of repairs. A mislocated nozzle or nozzles found after PWHT which requires removal and replacement and all other welds or one or two other long or circ seams will be subject to additional PWHT and this has certainly happened in my experience. Or a process change requiring a larger nozzle or nozzles to be added and/or replace existing nozzles requiring an Alteration and PWHT fully encircling the vessel and subjecting long and possibly circ seams or parts thereof to additional PWHT and I have a number of such Alterations that I have addressed. Similarly repairs to defects found after initial PWHT may subject other non repaired welds to additional PWHT.

 

[Jovi Zhu]

Weldstan, thanks and yes for those cases defective welds and HAZ are completly removed and the WPS is used to make "new" joints for the repair.

For repair of partly defective welds after defect removal when new welds need to be made on exsting weld beads, I'm thinking twice and trying to figure out what would happen to the existing weld beads by a review of the Section IX wording for another two supplementary essential variables, QW-406.3 and QW-409.1.

For preheating, QW-406.3 only limits the maximum preheat/interpass temperture and no mention on time of preheat/interpass temperature. If one can consider the repeated preheating for this repair as an increasing of the aggregate time at the preheat/interpass temperature, this is definitely not condisdered by the Code as a supplementary essential variable.

Theoretically there would be a new HAZ created by the repair welding on the existing beads. Within this narrow HAZ there would be a band of weld metal heated above and cooled from the upper transformation temperature, which makes QW-409.1 not applicable. For the reamining part of the newly created HAZ, if one can also consider it as no more than an increasing of time at the temperature created by the welding heat, the time during the passing of the welding arc would be very short and negligible even it turely exists.

I'm refreshing my idea with above thoughts: the repeated preheat and welding heat input will casue increasing of heat input but may not to the level of calling for a procedure re-qualification as per Code rules.

Again thoughts and ideas are welcome.

 

[weldstan]

Jovi Zhu, For the welds stated, not all of the welds subject to additional PWHT are even repaired.

In answer to your question about a local repair, you must understand the purpose of welding procedure qualification per ASME IX, which is to prove that the minimum mechanical properties of the base metal are met by the weld. When the repair is done per the WPS used to make the original weld, the mechanical properties of the weld are expected to meet the minimum mechanical properties of the base metal.

Owners, in their contract specifications, have required my company to qualify an additional PQR with a localized repair a couple of times. In all such cases, the repair area also met the minimum mechanical properties of the base metal as required by the Code or the Owners additional requirements.

Regarding your concerns with preheat and interpass temperatures, the original weld may have been preheated multiple times with stops for a number of reasons and restarts to complete the weld. There is no question that localized repairs can induce localized weld residual stresses that can be detrimental to weld service life, especially when the weld repair is not subject to PWHT.

 

[Jovi Zhu]

There should be a repair procedure giving specific requirement taking precedence over the original WPS for a localized weld repair as for such case not all the variables of the original WPS can be kept such as the joint configuration.

A mock-up is sometimes deemed necessary beyond the minimum Code requirement to serve the similiar purpose of the additional PQR you mentioned.

If the original WPS qualified with PWHT is used for the repair weld I do not understand how the weld can be put into service without PWHT, or maybe you were refering to some site repair cases where PWHT can not be practically done and specific technique such as temper bead is used so that the joint can be exempt from PWHT, or cases other than the OP's such as thin wall joints which are originally exempt from PWHT.

Anyway thanks for the disscusion. Back to the OP's post it is not clear that the WPS is specifically for repair or it is generally for new joint welding. If it is the one the welder/production people are reading when making a new joint, there should be no need to indicate any wording about the 375 minutes which is probably for unepxected additional PWHT. The 300 minutes PQR can be kept by the welding engineer and he should be able to judge if there is still enough PWHT time margin for a repair calling for a repeated/additional PWHT.

 

[weldstan]

Weld repairs to stamped vessels which were subject to PWHT may be made, in some instances with Owner acceptance, without PWHT in accordance with NBIC mandates. I did not intend to indicate that weld repairs requiring PWHT could be made without, only to state that localized repairs made to welds without PWHT are known to produce adverse effects that may be greater than the adverse effects of the defect if left unrepaired. EPRI has been looking at these types of repairs for some time now. Because I have been retired for the past 4 years, I am unable to know the results of EPRI's studies.

Localized repair procedures are often required by Purchasers but are not required by ASME I, VIII, B31.1 and B31.3. They usually state the method of excavation to remove the defect, verification of its removal, the WPS to be used to make the repair and the inspection method(s) to verify repair integrity.

 

[Jovi Zhu]

Weldstan, I share the same understanding and my "latest" EPRI Carbon Steel Handbook 2007 still reads "The ASME Post Construction Committee is generating guides on repair techniques (not yet published as of this writing)."

There would be still years ahead before my retirement but I have experienced enough cases that people at various levels, especially engineers, are not willing/able to make objective judgement for an "accept as is" decision for weld imperfections found by common NDE methods. Mechanical properties which are vital to the integrity of the joints are not practically detectable non-destrucitvely hence people without a certain depth of knowledge tend to overemphasize soundness which is economically detectable. For most of such cases the biggest difficulty is to educate and convince the purchaser/customer.

For repair procedure yes there is even no such term in Code books but it's necessary to at least state the repair requirement by something in written, approved by responsible engineer and document it as a written evidence(record) of Code/contract/QMS compliance. No dispute between a good practice and Code minimum requirement.

 

[Ani18]

Dear all,
Refer QW-253 when CVN require, We "change" Holding Time of PWHT is Essential Variable. Refer QW 407.2 i know that maximum holding time of PWHT of WPS = 1.25xPQR.

So How about Minimum Holding time? We afollow record in PQR or follow ASME section 8,1 hours/in.

 

[Jovi Zhu]

As my opinion in my previuos post:

It may be wise to use a PQR with a short holding time meeting the minimum Table USC-56-1 requirement, together with that "300 minutes" PQR, to prepare a WPS specifying a short but enough time, i.e. 60 minutes as a minimum for your case with a note/remark saying "Aggregate holding time at the temperature such as due to any additional PWHT shall not exceed 375 minutes."