hydrogen induced cracking (HIC).
For HIC to occur; the following variables must be present. a sensitive
microstructure, a sufficient level of hydrogen, or a high level of
stress (e.g., as a result of highly constrained connections).
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 postheat temperature and held at
this temperature for some minimum amount of time. Although various
code and service requirements can dictate a variety of temperatures
and hold times, 450°F(230°C) is a common post heating temperature to
be maintained for 1hour per inch (25 mm) of thickness. Post heating is
not necessary for most applications. The need for post heating assumes
a potential hydrogen cracking problem exists due to a sensitive base
metal microstructure, high levels of hydrogen, and/or high stresses.
Post heating, however, may be a code requirement. For example, ASME
Section III and the National Board Inspection Code (NBIC) both have
such provisions. The Section III requirement for P-No.1 materials
is450 to 550°F (230 to 290°C) for a minimum of 2 hours, while the NBIC
requirement is 500 to 550°F (260 to290°C) for a minimum of 2 hours.
Furthermore, post heating is often required for critical repairs, such
as those defined under the Fracture Control Plan (FCP) for Non
redundant Members of the AASHTO/AWS D1.5Bridge Welding Code. The FCP
provision is 450 to 600°F (230 to 315°C) for "not less than one hour
for each inch(25 mm) of weld thickness, or two hours, whichever is
less."When it is essential that nothing go wrong, post heating can be
used as insurance against hydrogen cracking. However, when the causes
of hydrogen cracking are not present, post heating is not necessary,
and unjustifiable costs may result if it is done.
Jaspal Singh, CWI-AWS, CSWIP-3.1
Quality Assurance
Process Equipment Division
ISGEC, Yamuna Nagar, Haryana -135001
Phone: 911732307251
Ext: 251 Mb: +91 9996624244
On Oct 7, 12:36 pm, muthusrinivasan muthuselvam <msm...@gmail.com>
wrote:
> Hydrogen embrittlement occurs when the weldment temperature is below 300 C
> not close to ambient temperature.
>
> br,
> msms
>
> On Wed, Oct 6, 2010 at 7:35 PM, Jignesh Desai
> <jignesh.desai....@gmail.com>wrote:
>
>
>
> > 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 <
> > msm...@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 carrried
> >> out immediately after completion of welding.
>
> >> best regards,
>
> >> M.S.Muthu selvam.
>
> >> On Tue, Oct 5, 2010 at 10:42 PM, senthil raja <
> >> senthilrajabara...@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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