Lower temperature of PWHT

[Abanamat]

Hello,
I have a question about PWHT:
Vessel base material is SA-516 Gr.70N, vessel will be exposed to alkanolamine and wet H2S.
Our vendor informed us that PWHT was performed slightly below 620°C. However, results of hardness test were below the maximum value of hardness required in H2S specification - 200 HB. Testing was provided on the base material, HAZ and weld material.

PWHT procedure shall meet NACE SP-0472 specification (3.4), which says:
“when PWHT is being performed for prevention of ASCC (Alkaline Stress Corrosion Cracking), lower temperatures (lower that required 635±15) shall not be used.

It means that we cannot accept PWHT performed below 620°C.
On the other hand, repeating of PWHT can negatively affect impact toughness of base material.

What do you think will be better to apply in this situation?

 

[metengr]

There are two parts of PWHT - reduction of residual stresses from welding and tempering of the weld and base metal heat affected zone. It appears you have been able to satisfy the hardness (tempering) aspect of PWHT. The other unknown is residual stress reduction.

If the client insists on ensuring the lowest PWHT is 635 deg C, you have not met that requirement, which means a second application of PWHT is necessary.

 

[pradipgoswami]

Few questions:
Is SP-0472 referred as one of the applicable document for the project, or just is to be referred for guidance?.
Note:- PWHT procedures as described in Paragraphs 3.4 and 3.5 of SP-0472 can negatively affect the tensile strength and impact toughness of the base metal. Each situation should be evaluated with the steel supplier to determine whether this specified thermal stress relief is adequate for all other considerations in addition to ASCC prevention.
What above clause tells that inputs from steel mills should be taken into consideration with respect to suggested PWHT @ 635 ± 15 °C(1,175 ± 25 °F) in SP-0472.

On the other hand API RP 945- Avoiding Environmental Cracking in Amine Units is an industry wide document, which is referred in SP-0472,Acceding to RP-945,
Clause 5.2.3 PWHT is an effective method for improving the cracking resistance of carbon steel weldments in amine service. An effective procedure consists of heating to (593-649 Deg c OR 1100-1200 Deg F) and holding in this temperature range for1 hour per 25 mm (1 in.) of metal thickness, or fraction thereof, with a 1-hour minimum holding time. PWHT below 593Deg C(1100 Deg F) is not considered effective for crack prevention; therefore, it is not recommended.

The advice would be to read between the lines in both the documents and decide the appropriate PWHT cycle. The main criteria is to control hardness below 200HB(after PWHT), which is possible by various means.
Thanks.


Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[Dhurjati Sen]

Dear Abanamat,

What do you mean by slightly below 620 deg C? How many thermocouples were present? Were all thermocouples attached to recorders?

Taking into account recorder tolerances etc. you can safely accept the PWHT as repeating it could lead to unnecessary complications.

However, make it a point to "BLACKLIST" the vendor who had performed PWHT slightly below 620 deg C.

Regards

DHURJATI SEN

 

[Abanamat]

Dear pradipgoswami,

SP-0472 is referred as one and only applicable document for this project. We do not refer to API 945... Do you know why there is such a big difference between API and NACE recommendations of PWHT procedure? Why the NACE is stricter?

As metengr mentioned above, tempering is not the only criteria of PWHT
API 945 (A.5) says:
ASCC has occurred in a variety of steels. Field experience to date has not indicated any significant correlation between susceptibility to ASCC and steel properties. Hardness of the steel has virtually no effect on ASCC. Susceptibility to ASCC increases with increasing tensile stress level.

Thank you for your advise

 

[Abanamat]

Dear Dhurjati Sen,

Unfortunately, I don't have data about the number of thermocouples, I know only, that PWHT was performed generally below 620°C.

Yes, I think we will accept PWHT, because I am also concerned that repeating of PWHT could lead to complications (softening of base material, I suppose?)

Thank you

 

[metengr]

Abanamat,,
Make sure you understand that residual tensile stress can cause increased susceptibility to ASCC. If you believe there was not enough stress relief at the lower PWHT temperature, regardless of hardness, you may have problems. I doubt a second PWHT is going to result in less than minimum specified strength. The concern is notch toughness. Even then I doubt one additional PWHT cycle is going to cause a problem. Think carefully about this .

 

[SJones]

It is an interesting question as to why the NACE standard is so prescriptive, particularly as it appears to completely ignore issues that could arise with TMCP steels subjected to such a cycle. Not that the subject steel is TMCP, of course. There seems to be two approaches to moving forward here:

1.
Repeat the PWHT after running some test pieces to confirm no adverse effects of a second cycle

2.
Find some technical justification to support an argument that stresses have been sufficiently relieved.

For point 2, work by Leggatt (R H Leggatt, Stress Relieving Heat Treated Welded Steel Construction, pp 247 - 256, 1987)suggested that significant stress relief is obtained when the Holloman-Jaffe Parameter is 18 or higher. Perhaps, the end user might accept that, or a similar, argument?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[weldstan]

Agree with Steve. For the test pieces, use cut outs/drops from the original SA-516-70 and subject to a double stress relief cycle mimicking the original PWHT and the proposed second PWHT.

 

[pradipgoswami]

Abanamat,
If NACE SP-0472 is a binding specification, then the PWHT recommendations of this document should be followed to mitigate ASCC. Suggested PWHT @635 ±15°C(1175±25°F),with a short soaking cycle may not harm the mechanical properties significantly. However longer soaking time @ 640-650 Deg C, may cause drop in mechanical properties(tensile & impact).It's better to check with the Mill upfront for simulation tests on SA 516/70 steel in such cases involving higher soaking temperature.

It would be appropriate to inform the vendor about the inadequacy of PWHT and repeat the PWHT to conforming to SP-0472, should you wish to do so. However before enforcing an additional PWHT @640-650 Deg C,a material simulation test coupon result should be done as recommended in the earlier response by Steve Jones & Metengr.

There're two-part aspects in NACE SP-0472:-hardness control(less then 200 HB) and control of residual stress, both of which could be controlled to a great extent by chemistry (CE) control, control of weld volume(better groove design), control of welding parameters, reduction of heat input... so on. In act all or many these are advised to be followed in SP-0472.

Recommendations of SP-0472 are the results of NACE industry wide survey, published in report: NACE International Publication 34108, Review and Survey of Alkaline Carbonate Stress Corrosion Cracking in Refinery Sour Waters.
My understanding is that API is also trying to bring the PWHT recommendations in API-RP-945 at par with SP-0472.
:-

https://becht.com/blog/updates-and-...sues-due-to-corrosion-and-degradation-issues.

I'm not involved with API committees, inputs from those involved would be welcome.

Thanks.




Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[Abanamat]

Dear all,

In the end we decided to repeat the PWHT.

Thank you for your good advise.
Regards

 

[metengr]

A very wise choice.

 

[Dhurjati Sen]

Dear Abanamat,

Do keep posted about the results. Also ensure that the PWHT is carried out without any glitch.

Keep your fingers crossed.

Regards.

DHURJATI SEN

 

[Abanamat]

Dear all,

Production test coupons were used simulating the whole situation (first incorrect PWHT, and then second correct). Mechanical test report (hardness and impact) showed appropriate results. After that, PWHT on vessel was performed at temperature 620°C min.

Thank you all for your help

 

[SJones]

Good to hear the result

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.