Stress Relieving heat treatment of Monel 400

[Sanjay Gandhi]

Hi,

 

Has any body come across requirement of PWHT or stress relieving for Monel 400? I have read some where that Monel 400 material that is to be used in HF acid service, welds must be given stress reliving treatment to minimize the risk of stress corrosion cracking. 

 

I am looking for following information.

1.) Whether post weld heat treatment is necessary for welds of Monel 400 operating in HF service. Any precedence of heat treatment or no heat treatment?

2.) If heat treatment is inevitable, how to carry out heat treatment of complete vessel? Conventional way by heating or any new technique, any specific furnace, any inert gas to be used, etc…

3.) Any cleaning treatment required after heat treatment? What is the effective method of cleaning?  

 

I would highly appreciate your inputs on the above. We have got two Monel 400 vessels to be used in HF services.

 

 

Regards,

 

Sanjay Gandhi

[pgoswami]

Hi Sanjay,

 

HF is an extremely corrosive acid, very few materials can withstand this service environment.Monel 400 is an excellent alloy against Hydrofluoric Acid. However in as fabricated condition it is prone to SCC.

 

Common causes for SCC are :-Stress,Corrosive environments,Susceptible microstructures. One can not avoid corrosive environments, nor susceptible microsturcre always (in this case austenitic alloys, e.g Monel).However the tendency to SCC can be minimized by lowering stress (weld residual stress in this case) by PWHT.

 

That's why for application of Monel in HF environments, either clad or full Monel construction calls for Stress Relieving.

 

I have attached two guiding documents for your info,NIDI publication & MATERIALS FOR USE WITH HYDROFLUORIC ACID, both talks about SR requirements for Monel in HF environment. Corrosion is severe in presence of oxygen.

 

In addition the attached extract  of Mobil Alloys-Tech Spec for HF Alkylation units also specify the same.

 

The complete vessel should be stress relieved. typically between 550-620 deg C (do not exceed the PWHT temperature requirement for C.S). It is preferable to inert gas purge while stress relieving (check your project/client's technical specification).

 

After PWHT  the weld joints shall preferably be chemically pickled (if required).

 

Iron dilution in Monel during clad restoration seriously harms corrosion resistance. AWS limit for "Fe" max in the weld is 2.5%, make sure this limit is not exceeded.

 

Hope this would clarify your queries.

 

Thanks.

 

Pradip Goswami,P.Eng.

Welding & Metallurgical Engineer/Specialist

Ontario Power Generation Inc.

Email-pgoswami@sympatico.ca,

pgos...@quickclic.net

 

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From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Sanjay Gandhi
Sent: Tuesday, June 01, 2010 7:16 AM
To: material...@googlegroups.com
Subject: [MW:5333] Stress Relieving heat treatment of Monel 400

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[Rajagopal Kannan]

Dear Sanjay,

As per UNF 56 of ASME sec. VIII Div.1, UNS N 04400 is not required to be Post Weld Heat treated.

Thanks and Best Regards,

K.Rajagopal

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[c sridhar]

Mr.Sanjay,

Welded MONEL alloy 400 components normally go into service as welded. If there is a concern that the
environment could induce stress corrosion cracking (e.g, steam containing ammonia), the structure could
be stress relieved by heat treatment at 620 ⁰ C to 760 ⁰ C. It could be done by using LPG pumping,
Electrical Resistance heating, Furnace  etc.

Sridhar.
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From: Rajagopal Kannan <rajagopa...@gmail.com>

http://www.linkedin.com/groups/MaterialsWelding-122787?home=&gid=122787&trk=anet_ug_hm

[pgoswami]

Hello Sanjay,

 

Are  these vessels intended for HF -Alkylation units in petroleum refineries?? The purpose of the HF alkylation unit is to convert butylene, propylene, and isobutene to high-quality motor fuel alkylate.

Due to their inherent resistance  against HF, Monel 400  is a popular alloy  for any HF service. However highly stressed Monel components are  susceptible to HF stress corrosion cracking. During  fabrication and operation, it's recommended that  care is  taken to maintain low stress levels by not overloading or by post weld heat treating the as-fabricated components. In the service environment the  presence of oxygen  increases the likelihood of cracking Alloy 400 containing high tensile stresses resulting from cold work or weld fabrication, particularly when exposed to HF vapor. A stress relieving heat treatment  should  be used to provide better resistance to cracking of Alloy 400 in HF service.

If you look at the industry documents it is pretty much across the board , that  a stress relieving heat treatment is advisable for fabricated monel components, either as clad  vessels  or  formed components in solid piping. This is again the decided by the process designer.

In your case if your organization is fabricating the equipments , then PWHT  yes or no should be as per the OEM/Project specifications' requirement.

Monel  clad vessels could be fired for PWHT  in normal gas fired furnace, however low residual sulfur in the firing  gas is recommended, otherwise model may get embrittled.

You would more details in  of API RP-751-Safe Operation of Hydrofluoric Acid Alkylation Units.

 

Thanks 

 

Pradip Goswami,P.Eng.IWE

Welding & Metallurgical Specialist

Ontario, Canada.

Email-p...@sympatico.ca,

pgos...@quickclic.net

 

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From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of c sridhar
Sent: Monday, December 31, 2012 1:09 PM
To: material...@googlegroups.com
Subject: Re: [MW:16396] Stress Relieving heat treatment of Monel 400

[s murali]

Mr.Sanjay, 

Monel is suitable for HF service and the process licensor of HF handling vessels will provide complete guidelines on how to weld and inspect Monel material. Post weld heat treatment is more relevant with respect to process requirement and process licensors like UOP technologies will provide the details.This could be due to SCC point of view. We have carried out heat treatment of monel equipments. The heat treatment cycle and hardness limits are specified by licensors.Conventional heat treatment can be done. For example, welds can be heat treated with resistant coil method. No cleaning is required after heat treatment.

Regards,

S.Muralidharan.

From: Rajagopal Kannan <rajagopa...@gmail.com>
To: "material...@googlegroups.com" <material...@googlegroups.com>
Sent: Monday, December 31, 2012 3:49 PM
Subject: Re: [MW:16392] Stress Relieving heat treatment of Monel 400

http://www.linkedin.com/groups/MaterialsWelding-122787?home=&gid=122787&trk=anet_ug_hm

[pgoswami]

 

Hello Sanjay,

 

Are  these vessels intended for HF -Alkylation units in petroleum refineries?? The purpose of the HF alkylation unit is to convert butylene, propylene, and isobutene to high-quality motor fuel alkylate.

Due to their inherent resistance  against HF, Monel 400  is a popular alloy  for any HF service. However highly stressed Monel components are  susceptible to HF stress corrosion cracking. During  fabrication and operation, it's recommended that  care is  taken to maintain low stress levels by not overloading or by post weld heat treating the as-fabricated components. In the service environment the  presence of oxygen  increases the likelihood of cracking Alloy 400 containing high tensile stresses resulting from cold work or weld fabrication, particularly when exposed to HF vapor. A stress relieving heat treatment  should  be used to provide better resistance to cracking of Alloy 400 in HF service.

If you look at the industry documents it is pretty much across the board , that  a stress relieving heat treatment is advisable for fabricated monel components, either as clad  vessels  or  formed components in solid piping. The    need for such requirements  are  decided  by the  process designer.  

 In your case if your organization is fabricating the equipments , then PWHT  yes or no should be as per the OEM/Project specifications' requirement.

Monel  clad vessels could be fired for PWHT  in normal gas fired furnace .However low residual sulfur in the firing  gas is recommended, otherwise monel may get embrittled , as Nickel based alloys are prone to embrittlement in High "S" environment at high temperature.

You would  get  more details in  of API RP-751-Safe Operation of Hydrofluoric Acid Alkylation Units  regarding the design, materials selection , and construction.

 

Thanks 

 

Pradip Goswami,P.Eng.IWE

Welding & Metallurgical Specialist

Ontario, Canada.

Email-p...@sympatico.ca,

pgos...@quickclic.net

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