NACE MR0175 and PWHT Requirements

[zakk]

We are welding A516-70 plate for a vessel that will be exposed to sour service. Are we required to PWHT, even though the hardness tests on the weld procedure (WPS) used for construction does not exceed 22HRC in the as-welded condition?

 

[strider6]

No, it's not required by NACE / ISO (see para A.2.1.4 "Welding" of NACE / ISO part 2).
You've also to look at the code, ASME VIII for example, if the PWHT is required or not.

S.

http://www.corrosionist.com

 

[zakk]

Thanks, Strider6.

PWHT is not mandatory under ASME Sec VIII Div 1 in this case.

Our Customer stated that regardless of the hardness to qualify the WPS, the actual production weld may exceed 22HRC, and therefore PWHT should be done regardless. I will pass the info in your post along to them.

 

[stanweld]

If your Customer only specified welding per NACE 0175, you are not obliged to perform PWHT. If the Customer now wants to add PWHT to the requirements, inform your Customer that this represents a Change to the Contract and provide the added cost for compliance.

Depending on how you sequence the weld and the carbon equivalent of the base metal and welding filler metal, high hardness may occur on the side exposed to the process, regardless of what the hardness tests on procedure qualification indicated. It is adviseable to weld the process side first (one or two passes max), followed by welding the bulk of the weld from the oposite side to take advantage of the tempering effect on reducing hardness on both the HAZ an deposited weld metal exposed to the service.

 

[rustbuster]

Most companies do specify PWHT for sour service applications. In my experience this is a very good practice even if it falls below the NACE curves.

 

[zakk]

Just to clarify:

NACE MR0175 Section A.2.1.4 first two paragraphs state welding and weld hardness shall be determined in accordance with 7.3.3 and acceptable hardness shall be per Table A.1. These two paragraphs only pertain to the Weld Procedure Qualification, not the actual production weld...correct?

If so, if I have as-welded carbon steel base metal less than 22HRC prior to welding and use a WPS that qualified the test weldment for less than 250 HV hardness, I am meeting the requirements of A.2.1.4 and do not required to PWHT or need to perform any type of hardness survey on the production weldment...correct?

Thanks for the great input so far!!!

 

[SJones]

Your second paragraph is correct - you will need to perform a post fabrication hardness check in order to verify compliance with ISO 15156.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[strider6]

SJones,

Why is necessary to perform post fabrication hardness check if the WPS are qualified and hardness values test are below the NACE / ISO requirements (22 HRC & 250 HV) ?

S.

http://www.corrosionist.com

 

[SJones]

Strider,

If you have Corrigendum 1 to Part 2 of 2005, look at the replacement text for Page 18, A.2.1.4 Welding; specifically paragraphs 3 and 6. Both paragraphs use the term 'as welded'. A PQR test weld is not an 'as welded' pressure vessel for example. Of course, the applicable clause for hardness testing, 7.3, only deals with PQR hardness testing and then goes on to completely omit any controls on applying qualification hardness to a change in steel chemistry or the like. So, my recommendation would be to perform post fabrication hardness testing as a measure of verifying compliance particularly if the parent material is not from the same heat of steel as that used to make the PQR test weld. I think that this would be a good question for the ISO 15156 Maintenance Panel (I'll send it on to Derek Milliams).

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[zakk]

How would you perform hardness testing on the production weld that would meet MR0175? Figures 2,3 and 4 depict a hardness survey of the weldment cross-section (for WPS qualification purposes), not the inner and outer surface accessable on an actual vessel plate. Would I need to make a production test coupon at the same time as the production weld, section it and do the survey? I don't know of anyone that does this to ensure NACE compliance nor do I see a procedure in MR0175 for testing production welds.

What would be the purpose of qualifying a weld procedure for sour service if you need to PWHT and/or post hardness test anyway?

 

[gr2vessels]

zakk,
Are you worried that any hardness test on the "as welded" vessel might contradict the PQR? Why don't you try after hours, a quick harness test at the surface of the parent metal, HAZ and weld cap, to see if the test complies with the 22 HRC? I hate to think of the consequences to the completed vessel and the associated costs if it doesn't. If the Purchaser is determined to get to the bottom of this issue, he could do the test himself...Perhaps the PWHT would solve all the problems, don't you think?
gr2vessels

 

[strider6]

i think that gr2vessels is right.
The NACE /ISO tells you what are the minimum requirements and the WPS / PQR tells you that if you do the weld following the WPS you'll have always hardness within the limits.
Then if your customer is not satisfied and wants other tests and a PWHT even if not required is his choice and you have to do it.

S.

http://www.corrosionist.com

 

[SJones]

Sorry to take issue - but the PQR in no way guarantees that the final weld has a hardness that meets ISO 15156-2 requirements unless it has been made in material from the same heat of steel and welding parameter variation was totally negligible.

Zakk - The only suggestion that I can make is that you agree some form of production coupon testing with the Purchaser. I can tell you that Shell require it as part of their pressure vessel specifications. Alternatively, if the Purchaser can be satisfied with an Equotip type, surface hardness measurement programme, that could be a way to proceed.

Your arguments are completely valid but, unfortunately, so are the Purchaser's!

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[zakk]

SJones - agreed! The Purchaser pays the bills, so it will be pretty obivious how this will get resolved. I just need to know if I'm interpretating the NACE Spec correctly.

We can do a surface survey, but there is nothing in MR0175 that deals specifically with this method (only cross sectional for WPS qualifiaction purposes), so it will be up to the Customer to agree.

One last general thing regarding the WPS and PQR...if I can't guarrantee that my production welds will be acceptable when performed within the parameters of the WPS, what good is a WPS and the supporting PQR? It has been mentioned a few times in this post that you can't relate the production weld to the WPS test coupons due to variations in heats, etc, so how can I be confident that any welding performed anywhere by anyone will be acceptable?

 

[stanweld]

ASME IX and other Code welding procedure qualification requirements have been made to assure the strength and ductility of the welded materials. When hardness testing is needed to assure freedom from in-service, stress corrosion cracking, it is up to the Owner or Owner's Engineer to determine the best approach with regard to procedure qualification and testing of production welds, or when to mandate PWHT or classify as the equipment as being in Lethal service. Some Owners/Engineers require hardness test qualifications from actual production materials and limit the maximum carbon equivalent to 10% > than that of the production material used in qualification or further limit the minimum and maximum thickness range qualified. Others specify hardness testing per NACE RP0472. Others require the use of temper bead techniques as now defined in QW-290 and the additional qualification essential variables listed therein.

 

[strider6]

zakk

The meaning of the WPS and PQR is to have a procedure qualified that produce the same quality, in terms of hardness ...., of welding.
In the WPS there are limits for every parameters and if you are not within the limits you have to make another WPS.
If as you said "f I can't guarrantee that my production welds will be acceptable when performed within the parameters of the WPS, what good is a WPS and the supporting PQR?" then you have some problems with your welders..

S

http://www.corrosionist.com

 

[zakk]

strider6 - I agree with your statement re: WPS. In an earlier post, you stated:

"... WPS / PQR tells you that if you do the weld following the WPS you'll have always hardness within the limits".

However SJones stated:

"the PQR in no way guarantees that the final weld has a hardness that meets ISO 15156-2 requirements unless it has been made in material from the same heat of steel and welding parameter variation was totally negligible".

I tend to agree with Strider6 because you are always going to get variations of the heats and welding parameters between the PQR and production. The essential and nonessential variables of the WPS should address those variations.

The point I was trying to make, regarding SJones, is if I can't guarantee conformity of my production welds by following my WPS, then what is the purpose of the WPS and the PQR?

According to ASME Section IX: "The purpose of the Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) is to determine that the weldment proposed for construction is capable of having the required properites for its intended application" Therefore, why would I need to perform post hardness if my PQR results come in well under 250HV?

If I can relate this to impact testing, our WPS addresses notch toughness to -50F. If my base material is charpy tested to -50F and I use this WPS, I do not have to perform production impact testing on the weldment. My hardness on the PQR came in at around 160HV (<250HV), therefore, by the same reasoning, I shouldn't have to perform production hardness testing.

 

[SJones]

Look at all Stanweld's posts - the concept is contained within those.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[stanweld]

zakk
I must contradict you with regard to Charpy testing and hardness testing. Production impacts are required under API 620 Section Q, ASME Section VIII {UG-84 (f),(g),(h,(i)} and Div.2. ASME B31.3 further limits testing to the thickness range T/2 to T+1/4" because of the well known affect of thickness on toughness. B31.3 also mandates production hardness testing on specific materials.

It is not the purpose of ASME IX to "be all things" regarding hardness requirements or toughness requirements in production weldments. The design Codes and Engineer/Owner Specifications trump ASME Section IX in ths regard.