Time between Welding and PWHT

[engrshanas512]

Hi All,
Can any body tell me to where I can find time between welding and PWHT for P91 materials especially for Power Piping. Thanks

Regards
Muhammad

 

[pennpiper]

The answer to your question "time between welding and PWHT", is as soon as practical.
However,there are a lot of factors here.

- How big are the spool pieces?
- How many spool pieces are involved?
- How big is the PWHT oven?
- How full is the oven?
- What does it cost to heat that oven up and run it at required temperature for the required duration?
- What day of the week is it?



prognosis: Lead or Lag

 

[metengr]

There is no code or standard requirement, this is a process detail. PWHT should follow welding as soon as possible to avoid delayed cracking or more important, stress corrosion cracking. However, if the weld region can be kept dry, you can delay PWHT.

 

[DekDee]

Gents,
Not sure if this is applicable (I only have the 2004 edition of B31.1 and P5B is now P15E)

131.6 Interruption of Welding
131.6.1 After welding commences, the minimum
preheat temperature shall be maintained until any
required PWHT is performed on P-Nos. 3, 4, 5A, 5B,
and 6, except when all of the following conditions are
satisfied.
(A) A minimum of at least 3⁄8 in. thickness of weld is
deposited or 25% of the welding groove is filled, whichever
is less (theweldment shall be sufficiently supported
to prevent overstressing the weld if the weldment is to
be moved or otherwise loaded).
81
(B) For P-Nos. 3, 4, and 5A (with a chromium content
of 3.0% maximum) materials, the weld is allowed to cool
slowly to room temperature.
(C) For P-No. 5B (with a chromium content greater
than 3.0%) and P-No. 6 materials, the weld is subjected
to an adequate intermediate heat treatment with a controlled
rate of cooling.
(D) After cooling and before welding is resumed,
visual examination of the weld shall be performed to
assure that no cracks have formed.
(E) Required preheat shall be applied before welding
is resumed.

Regards,
DD

 

[davefitz]

Meteng is correct.
Most shop purchased P91 filler has more nickel than does the parent metal which implies the weld must be cooled down to 200 F prior to PWHT ( in order to form 95% martensite prior to PWHT).The only exception may be welds thicker than 4" thk and true matching filler. The higher nickel filler is more ductile than matching filler and thus reduces the likelihood of cracking in the shop prior to PWHT due to handling stresses ( ie pushing the bundle into the oven).

If the shop uses space heaters that burn nat gas which has suphur bearing odorant, then condensation of the heater's combustion products leads to concentrations of H2SO3 on the weld , which can lead to SCC if the weld is not kept hot enough to avoid nightime dew condensation. So keep the weld s hot enough to preclued nightime dew condensation.

"Whom the gods would destroy, they first make mad "

 

[engrshanas512]

Many thanks for your of your comments, as per my study,

According to Babcock & Wilcox, the time between completion of welding and PWHT should be limited to 14 days maximum to minimize the risk of SCC:
Gold M, Hainsworth J, Tanzosh J M, ‘Service experience with design and manufacturing approaches with T/P91 materials’, Proc. EPRI Conf. ‘9Cr Materials Fabrication and Joining Technologies’, 10-11 July 2001, Myrtle Beach, South Carolina, USA

According to Vallourec & Mannesmann, the time between completion of welding and PWHT should be limited to 7 days maximum: Vallourec & Mannesmann Tubes, ‘The T91/P91 Book’, 1999

More recently, I have heard some people refer to a limit of 3 days or so, but I’ve not seen this in any document.

But these all I did not study in any quality procedures or in any standard/Code.

 

[OSIRIS7]

I am not a welding expert, but I have some experience.

From that experience it is advised to preheat, then weld and imediately after welding cool down to between 80 and 100 degree Celsius for martensite forming, and after that the heat treatment will be followed by a predetermined time which can be calculated according Larson Miller Parameter at a temperature approximately 20 degree Celsius below the heat treatment of the pipe material itself. After that cool down slowly. After complete cooling down + 2 days do a traverse hardness test across the weld, the HAZ and the base pipe material.

I do not know the exact details, but this is what I remember.

 

[OSIRIS7]

Oh, is this of any help?

http://www.psimedia.info/Industry Alert.pdf

 

[engrshanas512]

Dear Colleagues,
I am aware all of above techniques. What I am asking, there is some time limit between P91 welding and PWHT, say 3, 7 or Max 29 days, if we do not organise PWHT within 29 days, it will lead us SCC crack. I am wondering to where I can get aforementioned info i.e time between Welding and PWHT. Thank you

 

[OSIRIS7]

I would advice to do it immediatelly. From electrical pre-heat down to between 80 and 100 degree Celsius for martensite forming and then the PWHT right after that. This way you keep the absorbtion of hydrogen to an absolute minimum.

This is how we do it here, but then again: I am not a metallurgist and do not know all the details.

 

[engrshanas512]

Since one year we did not perform PWHT

 

[OSIRIS7]

Do you mean you did not make systems with P91 since one year, or did you not PWHT these systems made?

 

[engrshanas512]

We welded P91 one year back, but till we did not perform PWHT.

 

[metengr]

Dear Colleagues,
I am aware all of above techniques. What I am asking, there is some time limit between P91 welding and PWHT, say 3, 7 or Max 29 days, if we do not organise PWHT within 29 days, it will lead us SCC crack. I am wondering to where I can get aforementioned info i.e time between Welding and PWHT. Thank you

We answered your post - NO there is no time limit.

 

[weldstan]

It is advised to check for cracks. OD surface with wet fluorescent MT, ID with RT or phased array/TOFD, UT before and after PWHT.

Did you use a dehydrogenation heat treatment after discontinuing the weld?

 

[davefitz]

There is a variety of experiences regarding the wait period. At one plant in florida, the PWHT occurred 60 says after welding , with pipes unprotected in the rain and sun, with no SCC issues. At another site indoors in the winter time, SCC occurred when only 3 days lapsed between weld & PWHT, but it was found that the space heaters burned a natural gas with odorant ( Sulphur containging) and the tubes cooled down to a tmeperatrue that allowed condensate to collect ( along with H2SO4).

"Whom the gods would destroy, they first make mad "