Re: [MW:18255] PHWT for P91

Hydrogen bake is specially important in monsoon due to high humidity. It can initiate microscopic

hydrogen cracks if not baked in time, so very important in wet climate. These minute cracks can

be revealed by electron microscope.

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From: kannayeram gnanapandithan <kgpandithan@gmail.com>
To: materials-welding <materials-welding@googlegroups.com>
Sent: Tuesday, July 23, 2013 10:13 AM
Subject: Re: [MW:18253] PHWT for P91

 These has been discussed by experts in earlier forum. Aim of pwht is not only to reduce residual stress but to bring down the Hardness also.

After completion of welding, put it on Hydrogen bake out( time according thickness) at temp more than preheat+100deg Celsius for 3 to 4 hrs.

After completion of Hydrogen bake out,  cover with glass wool to make it slow cooling till 70 degree Celsius and take it for PWHT

Pandithan

welding consultant

On Mon, Jul 22, 2013 at 6:25 PM, rishikesh shahi <rishikeshshahi@gmail.com> wrote:

Dear Binoy,

 

 after completion of welding allow to cool the weld metal at 90 deg. centigrade for the formation of complete martensite. in actual the  90 deg. centigrade temp. is called Mf Temp. (Mf = Martensite finished temp.). after this we have to do the Post Weld Hydrogen Baking of Welded joint for 04 Hrs.  at the Temp. of 350-400 deg. centigrade for diffusion of hydrogen from the weld metal.

 

After this you have to allow to cool the weld metal minimum 90 deg. centigrade till completion of PWHT. but the cooling rate should not be more than 100 deg. centigrade.

 

you have to maintain this temp. (90 deg. centigrade) before PWHT.

 

as per code it is only telling about the minimum soaking time that is 30Min., means understood that you are not restricted to maximum.

 

 

Regards.

Rishikesh Shahi

Welding Engineer 

Doosan Power System India Pvt. Ltd. 

 

 

On Tue, May 7, 2013 at 11:38 AM, Binoy Soman <binoy.spp@daelim.co.kr> wrote:

 

Dear experts,

 

Kindly explain following queries

(a)   PWHT holding time for P91 mentioned in B 31.1 is 1 hr/25 mm; minimum 30 minutes. Does it mean that if the thickness is 6/10/12.5 mm the holding time is 30 minutes & for 15 ,20,30,40 mm the holding time is 36  48,72,96 minutes respectively.

(b)    I got some special heating curves for P 91 from our group. But Kindly explain about the reference /standard used for preparation of this heat curve ( eg- why the preheat maintenance is 4 hours & again free cooling).

I raised these doubts earlier in [MW:17585], but I didn't receive any response. Please give your comments.

 

 

S.Binoy

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