[MW:5342] Stress Relieving heat treatment of Monel 400

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,

pgoswami@quickclic.net

 

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

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

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