Hi Senthil,
Post heating is termed as DHT(dehydrogenation heat treatment). This involves heating to a temperature (preferably above interpass temperature) immediately after welding to cause sufficient dissipation of hydrogen from the weld pool. This prevent any localized entrapment or build up of hydrogen pressure which may cause delayed or hydrogen induced cracking. EIL specification should mention the post heating temperature.
Otherwise based on the intended application you may follow API guidelines:-
· As per API Recommended Practice 934-E- Recommended Practice for Materials and Fabrication of 1¼Cr-½Mo Steel Pressure Vessels for Service above 825°F (441°C)- The DHT shall be performed at a minimum metal temperature of 570°F (300°C) for duration of one hour minimum.
· API RECOMMENDED PRACTICE 934-C-- Recommended Practice for Materials and Fabrication of 1 1/4Cr-1/2Mo Steel Heavy Wall Pressure Vessels for High Temperature, High Pressure Hydrogen Service-- The DHT shall be performed at a minimum metal temperature of 570°F (300°C) for duration of one hour minimum.
However the fabricator always have the option of extending the DHT time beyond API recommendations.
Thanks
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Email-pgoswami@sympatico.ca,
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Jignesh Desai
Sent: Wednesday, October 06, 2010 10:05 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:7397] Post Heating for Alloy Steel
In ferritic steels, hydrogen embrittlement only occurs at temperatures close to the ambient temperature. Therefore, it is possible to avoid cracking in a susceptible microstructure by diffusing hydrogen from the welded area before it cools. After welding has been completed, the steel must not be allowed to cool to room temperature; instead, it should be immediately heated from the interpass temperature to the post heat temperature and held at this temperature for some minimum amount of time.
Also did you see UOP Spec 8-12-6, para 2.4(a) - Fabrication of piping?, where UOP asked for post heating of carbon steel? It may be carried out for some case of CS where thickness>50.0 mm). I am sure this spec. must be part of your project.
On Wed, Oct 6, 2010 at 11:47 AM, muthusrinivasan muthuselvam <msms2k@gmail.com> wrote:
Mr.Senthil..
The main purpose of doing immediate PWHT without post heating is to avoid hydrogen induced cracking.. To my knowledge there is no code requirement or specification mentioning about the time after which you can do the PWHT.
But please note that delaying the PWHT after welding will improve the condition for hydrogen induced cold cracking. So the PWHT has to be carried out immediately after completion of welding.
best regards,
M.S.Muthu selvam.
On Tue, Oct 5, 2010 at 10:42 PM, senthil raja <senthilrajabarathi@gmail.com> wrote:
Hi,
I m Senthil working in L&T. I m presently in MRPL project, Mangalore. In our client(EIL) specification it is given that post heating shall be carried out for the alloy steels (Cr-Mo) where PWHT cannot be carried out immediately.
I need to know what is the time limit within which PWHT shall be carried out without post heating. What is the standard or code which
can be referred for this.
Kindly share your experience sirs & friends,
regards,
Senthil Raja.
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