RE: [MW:18290] PHWT for P91

This necessity for a hydrogen bake out is basically an "urban myth". If proper preheat is applied and all other good low hydrogen practices are observed, this bake out is unnecessary. In our shops we have had no problems with hydrogen cracking. We assiduously follow our low hydrogen protocols including wrapping any 91 weldments with insulation after completion of welding to reduce the cooling rate. All welding is done indoors. Any completed weldment is kept indoors in a temperature controlled environment until PWHT can be performed. In 20 years of welding this alloy we have had no instance of hydrogen assisted cracking.

If practices to control hydrogen are potentially compromised then, yes indeed, a post weld hydrogen bake out is necessary. This would include field work, welding in high humidity conditions, questionable filler metal maintenance etc.

This alloy requires strict control of welding. Most problems due to welding are the result of improper PWHT (or other heating). Other problems have been the result of improper design.

John

-----Original Message-----
From: materials-welding@googlegroups.com on behalf of Shomenath Bagchi
Sent: Mon 7/29/2013 10:59 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:18289] PHWT for P91

Yes, but sometimes PWHT may get delayed due to unforeseen circumstances,
then hydrogen baking becomes very important.





>________________________________
> From: rishikesh shahi <rishikeshshahi@gmail.com>
>To: materials-welding@googlegroups.com
>Sent: Monday, July 29, 2013 3:31 PM
>Subject: Re: [MW:18277] PHWT for P91
>
>
>absolutely right, if you are having facility for the immidiate PWHT
>after completion of Immidiate welding then no need to go for the Post
>weld Hydrogen Baking.
>
>On 7/25/13, kannayeram gnanapandithan <kgpandithan@gmail.com> wrote:
>> *We have also done lot of P91 and what i expressed is my experience with
>> this alloy. If u take it for PWHT  immediate after welding ( Cooling down
>> to less than 80degree Celsius) it is not require for Hydrogen bake out.
>>                      Otherwise it is require before PWHT*
>> *Pandithan*
>> *Welding Consultant
>> *
>>
>>
>>
>>
>> On Wed, Jul 24, 2013 at 9:06 PM, John Henning <jhenning@deltak.com> wrote:
>>
>>> No its not.  Adequate hydrogen control will prevent any cracking issues.
>>> We have welded a lot of this material without any hydrogen bakeout and
>>> have
>>> never experienced any HAC.  Problems with this alloy are oversold - with
>>> good up front preparation and reasonable care during fabrication and this
>>> alloy behaves itself.  ****
>>>
>>> ** **
>>>
>>> John A. Henning****
>>>
>>> Welding & Materials****
>>>
>>> ** **
>>>
>>> *From:* materials-welding@googlegroups.com [mailto:
>>> materials-welding@googlegroups.com] *On Behalf Of *kannayeram
>>> gnanapandithan
>>> *Sent:* Tuesday, July 23, 2013 11:26 PM
>>> *To:* materials-welding
>>> *Subject:* Re: [MW:18257] PHWT for P91****
>>>
>>> ** **
>>>
>>> *Hydrogen bake out always require for P91 welding with  flux containing
>>> welding process irrespective of climatic condition.*****
>>>
>>> *Pandithan*****
>>>
>>> *Welding Consultant*****
>>>
>>> ** **
>>>
>>> On Wed, Jul 24, 2013 at 9:16 AM, Shomenath Bagchi <snbagchi@ymail.com>
>>> wrote:****
>>>
>>> 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.****
>>> ------------------------------
>>>
>>> *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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