Hi senthil,
This is the statement given in paragragh 331.2.4 of ASME B 31.3 regarding Delayed heat treatment,
If a weldment is allowed to cool prior to heat treatment, the rate of cooling shall be controlled or other means shall be used to prevent detrimental effects in piping.
So, as per code there is no time frame from completion of welding to start PWHT, it is up to us to decide. Even after welding, if it is wrapped with asbestos wool will lead to slow cooling. But in our company we use to do post heating at 300degC to 400degC, if PWHT is delayed.
As muthu selvam told, basically post heating is done to remove hydrogen (if any) from weldment. But Low alloy steels are prone to delayed hydrogen craking. So its a good engineering practice to do post heating, if PWHT is postponed.
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With Regards,
Sudharsan
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 reffered for this.
Kindly share your experience sirs & friends,
regards,
Senthil Raja.
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