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.
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of c sridhar
Sent: Monday, December 31, 2012 1:09 PM
To: materials-welding@googlegroups.com
Subject: Re: [MW:16396] Stress Relieving heat treatment of Monel 400
Mr.Sanjay,
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 <rajagopal.kannan@gmail.com>
To: "materials-welding@googlegroups.com" <materials-welding@googlegroups.com>
Sent: Monday, 31 December 2012 3:49 PM
Subject: Re: [MW:16392] 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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