Number of times you can PWHT P91
Number of times you can PWHT P91
(OP)
Does anyone have any experience or preference on the number of PWHT cycles that may be performed on Grade 91? We have a CC HRSG that has had multiple repairs (multiple PWHT's) performed on a P91 stop valve (repair of Small Bore by-pass line, replacement of same, removal and plugging of by-pass with manufacturer's blessing, and lastly repair of a cracked valve seat). I'm feeling the cracked valve seat is indicating we should be looking into replacement at this point.
I understand that chemistry, stress, PWHT temp and process are all significant factors when dealing with P91, but assuming everything has been done properly to this point and hardness ranges are still acceptable, is there an industry standard or general consensus that would limit the time at PWHT temperature? Is there any NDE method recommended or would a boat sample sent out give me any further insight? My refinery experience has me stuck on three repairs and replace, and we've gone that far already.
I can find no references in Code or technical literature and could not find anything in the forum. As a first time user, not sure if this thread would be better located here or somewhere else? Thanks for any assistance that may be provided.
I understand that chemistry, stress, PWHT temp and process are all significant factors when dealing with P91, but assuming everything has been done properly to this point and hardness ranges are still acceptable, is there an industry standard or general consensus that would limit the time at PWHT temperature? Is there any NDE method recommended or would a boat sample sent out give me any further insight? My refinery experience has me stuck on three repairs and replace, and we've gone that far already.
I can find no references in Code or technical literature and could not find anything in the forum. As a first time user, not sure if this thread would be better located here or somewhere else? Thanks for any assistance that may be provided.





RE: Number of times you can PWHT P91
I would not remove a boat sample because subsequent weld repair and PWHT will be required.
If you really want to determine the maximum number of times PWHT can be performed where properties are degraded, you would need to demonstrate this via multiple PWHT cycles and destructive examination to determine mechanical properties and microstuctural evaluation of a Grade 91 weld coupon.
RE: Number of times you can PWHT P91
RE: Number of times you can PWHT P91
Apart from the issue of Ni+Mn content and its effect on lower transformation temperature, some have reported that P91 welds can be overcooked at the factory or during initial field construction, with little or no allowance for possible further PWHT in future. Properties can only decline with repeat PWHT, but the question is how much and after how much total PWHT. Also, hold temperature in the field is less controllable/consistent than we would like.
RE: Number of times you can PWHT P91
RE: Number of times you can PWHT P91
see online presentations by superheatfgh and also some recent papers by Bill Newell.
RE: Number of times you can PWHT P91
RE: Number of times you can PWHT P91
RE: Number of times you can PWHT P91
Assuming that parts are long seam welded, the completed part should be normalized and tempered after welding to avoid Type 4 Creep failure in the HAZ with the permitted design allowable stress.