Welding & pwht time difference in P-91 Material

[pradip kumar Sil]

Dear All,

Can anybody inform code reference in B-31.1 welding & pwht time diference. Client is asking for 14 days.

Is it mandatory in code

[John Henning]

This is not a Code requirement. 

 

If I remember correctly, this originated from EPRI recommendations for 91 fabrication.  It typically appears in customer specifications/requirements.  If low hydrogen procedures&practices are followed and proper storage is maintained, the requirement is, in my opinion and experience, totally unnecessary.  If the forgoing are not properly attended to, 2 weeks maybe far too long a time period.

 

Others may disagree.  However my 20 years of fabrication experience have shown my statement to be correct.

 

Enjoy - John

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[pgoswami]

Pradip,

 

Delaying 14 days would be way too long. Surrounding environment, humidity, pollution level, salinity all needs to be considered. Original development and welding practices for this alloy called for immediate PWHT following welding. However over the years ,control of welding practices, e.g  using strictly H4 grade consumables for SMAW, H5 grade flux for SAW, have allowed  some flexibility to the fabricators through  deferment of PWHT  but not to the extent of 14 days.

 

If  fabrication and PWHT sequences makes it mandatory to defer  PWHT then dehydrogenation treatment is a must.Please see the extract from Alstom paper. 

 

Hardness levels of as welded Gr-91 steel would be easily 350HV or more. Leaving such components outdoor,without  performing PWHT for extended period  would aggravate the scenario, through cracking by SCC.

 

If at all the PWHT is deferred , it should not be more than 3-5 days at the most , subject to  environmental conditions as above  are deemed satisfactory.

 

Thanks.

 

Pradip Goswami, P.Eng,IWE

Welding & Metallurgical Engineer/Specialist

Ontario, Canada

pgos...@quickclic.net

pradip....@gmail.com

 

***************************************************************************************************************************************************************** 

From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of pradip kumar Sil
Sent: Monday, May 18, 2015 6:52 AM
To: materials-welding
Subject: [MW:23297] Welding & pwht time difference in P-91 Material

 

Dear All,

Can anybody inform code reference in B-31.1 welding & pwht time diference. Client is asking for 14 days.

Is it mandatory in code

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[pgoswami]

[John Siefert]

In EPRI Report 1023199, the following statement is made regarding this topic:

"The component should then be post-weld heat-treated within 8 hours of the completion of welding. If this is not possible, either:

1. The component should be maintained at a minimum temperature of 175°F.

2. The humidity of the environment in which the weld is stored should be controlled to guarantee that no condensation can occur at any time (e.g., due to changes in temperature) on either the OD or ID surfaces of the joint until the post-weld heat treatment can be initiated.

An agreement between client and contractor should be established on the best practice to minimize the risk of stress-corrosion cracking occurring on any Grade 91 components."

you can download this document for free here: http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001023199

The other option, other than storage in a controlled environment would be to protect the surfaces of the OD and ID with a water-impregnable coating, paint, etc. if the component could not be PWHT immediately. 

The issue, as others have stated, is that although there are as many experiences where components have been OK when left uncontrolled for extended periods of time there are as many instances where components have also cracked under nominally the same conditions. Many of the cheap manufacturing sites in the world where Grade 91 components are being fabricated, such as southeast Asia, China, Indonesia, Mexico, etc. are in hot and humid environments and also potentially near the ocean leading to additional concerns about specific measures that are likely to NOT be enforced or undertaken to ensure the integrity of the manufactured component through its entire fabrication route. The exact conditions and experiences are subject to heightened scrutiny because the issues and factors surrounding SCC are not well-understood and the fact that environment conditions are highly variable. Even in Beijing (the location of a large OEM fabrication facility for fossil fired power plants), where the climate is arid and rather dry for most of the year there exists a monsoon season where the humidity and risk to SCC increases for components in specific parts of the year. Thus, best practice will always be to mandate a short period of time between welding and PWHT since there is historically little attention paid to this important detail when welding CSEF steels. In your case, the 14 days is actually extremely generous.