[MW:14158] RE: 14154] PWHT OF P 91 ALLOY STEEL MATERIAL.

Stress corrosion cracking has been noted in as welded 91 material when it has been wetted.  The concern is that the welds may have dew formation, or in some way be exposed to water.  91 in the as welded condition should be held indoors in temperature and/ humidity controlled conditions.  An alternative is to keep in indoor storage but with the welded material maintained at a temperature above the expected dew point (by electric resistance pads most likely).  The seven day thing is bit arbitrary and my personal take is that it is just to make sure that PWHT is performed in a timely manner.  My experience with large amounts of welded 91 is that there has been no cracking even after extended time (a couple of months)  in the as welded condition but we hold all as welded 91 in climate controlled conditions.  I have not seen any empirical evidence that 7 days, 14 days, or 30 days makes any difference provided storage conditions are appropriate.

 

Note that function of the preheating and post heating you are doing is to prevent hydrogen assisted cracking.  The preheat and post heat allow hydrogen to diffuse out of the material.  Entirely different worry than SCC in the presence of water. 

 

Also note that any water; pure or otherwise has been demonstrated to allow SCC in as welded 91 and it only needs be a film of water.

 

John A. Henning

Welding & Materials

 

From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of manoj john
Sent: Thursday, April 26, 2012 3:24 AM
To: materials-welding@googlegroups.com
Subject: [MW:14154] PWHT OF P 91 ALLOY STEEL MATERIAL.

 

Dear All,

 

 We are going to do P91 piping fabrication,   as per client specification the stroage time after welding is limited to maximum 7 days for PWHT.  Is there any problem if it delays 10 days when  we are  following preheating procedure for welding as mentioned below:

 

 Preheat at 200 degree C.  Min. 300 degree C. Max.

 Inter pass temperature  250  to 300 degree C.

 Cooling 20  to 80 degree C. after wleding

 Post heating (extended heating) after cooling 300 degree C.   

 for four hours or 400 degree C. for three hours.

 

Thanks in advance for your response.\

 

Manoj.

 

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