800HT piping with no PWHT

[90harley]

I need some help in regards to 800HT piping. Last year my company welded several joints in 800HT piping using Inco82 filler 14” SCH..312. Now about ½” from the weld toe in the HAZ the plant is experiencing t/w cracking. The system runs over 1000°f during service. My question is our PQR and WPS for P45 list no PWHT. I am aware of the requirements noted in SEC VIII Div. III for PWHT of 800HT however the code of construction for this job was B31.1 and B31.3. Are we wrong for not PWHT the joints?

 

[metengr]

There is no PWHT required for this material used in piping/tubing systems because these are austenitic-based materials. There is a PWHT requirement for 800, 800HT for pressure boundary welds in pressure vessels operating above 1000 deg F - see Section VIII, Div 1, UNF-56.

With that said, you have followed the necessary code requirements for piping welds using B31.xx, as applicable. I am speculating that the cause of cracking could be from a fine grained region induced by welding. There are other possibilities - did you induce any cold strain in the piping system where the cracks are observed? This is critical because if so, the location of cold work in this material will result in premature failure from local creep damage.

Here is what I would do - contact Special Metals and consult with one of their materials folks that understands Incoloy 800, 800 HT. They should be able to provide you with technical guidance on this material.

 

[90harley]

Thank you, My thoughts as well however my engineering department is stuck on the requirements listed in Sec. VIII even though the code of construction was B31.1 and 31.3.
In regards to cold strain, I am not sure of the conditions at this time. I do know that in the area of cracking the plant washes down the floor with a water hose!! And I can recall see the pipe in services last year glowing red. I trust the pipe didn’t get wet at some point form the wash down causing this. The pipe is insulated but poorly.

 

[metengr]

The Plant absolutely needs to keep water off of this line, period. Thermal fatigue cracking could be a real possibility if portions of the line are subjected to step changes in metal temperature.

Regardless, you still need to contact Special Metals and provide all information on how this pipe material was supplied and welded in the field.

 

[MikeMet]

If you truly are 1/2" away from the weld toe, you should not still be in the true HAZ unless you poured the heat into the piece. I have seen cracking in that area due to thermal fatigue on a I800 pipe welded to an elbow and the entire fabrication was under restraint due to poor support. I suggest you are better off investigating operating conditions, pipe stress and do a little field metallography.

 

[blacksmith37]

As stated; cracking has nothing to do with PWHT. I agree , thermal fatigue or stress rupture. However, if you see deposits (eg. salts from the water) there is also a chance of caustic stress corrosion cracking.
I think you mean wall is 0.312" which , I guess is Sch 20 or 30.

 

[ijzer]

If the cracks are intercrystalyn and the hardness in the cracked area is high and your operating temperature is in the range 500 to 750 °C ,it could be relaxation cracking. To avoid this alloy 800H(T)materials shall be given a stabilizing heat treatment at 980°C in addition to the solution annealing. Further it requires PWHT at about 900°C after welding.