Hydrogen bake out and PWHT requirement

By rajaramc Date 07-21-2003 15:51

Dear experts,

Due to heat transfer problem in our stacked feed /effluent exchangers(two nos elements) in Hydrocracker unit, it is planned to repair the exchanger in the exchanger vendor's shop. For this, the interconnecting nozzles are to be cut and after repair of the exchangers , are again to be rewelded. The nozzles are having the following MOC: SA 336 F22 CL3 with weld overlay of SS309L+SS 347. The original nozzle welds involved buttering the joints with 5mm layer of Inconel and PWHT of the buttered joints.The joints were then welded in the following sequence: Root run and first hot pass with Er 347 followed by Er NiCr-3 +E NiCr Fe3 for the remaining passes. Snce these exchangers have been in service for 9-10 years and are handling feed with very high Hydrogen partial pressures (160 bar) , I am of the opinion that for rewelding, the joint with the same sequence as mentioned above, a hydrogen bakeout is required to drive out any hydrogen diffused into the parent material followed by PWHT at 690 deg C .However, the welding people of the fabrication shop are saying that no hydrogen bakeout and PWHT is required . Are they correct? I have read a few literature which mentions that even for Cr-Mo welds in Hydrogen service, PWHT is required to avoid disbonding of the welds.

I will be grateful for any advice. The thickness of the nozzles is around 36 mm with 6mm weld overlay of SS 309L+SS 347

C.Rajaram
Gujarat Refinery
India

By MBSims

Date 07-23-2003 02:52

Not sure I understand the logic for ER347 root & hot pass when both sides are buttered with Inconel. Seems like that would be prone to cracking. Why not ERNiCr-3 for root & hot pass?

Hydrogen is normally not a concern for nickel-base alloys. It sounds like only the ER347 cladding and root/hot pass would be in contact with the hydrogen service. I read somewhere that nickel-base alloys have a hydrogen diffusion rate that is a factor of 10 slower than ferritic materials. So, it would take longer to diffuse through to the CrMo base metal. However, I'm not familiar enough with the service experience for hydrocrackers to say whether the bake out would be beneficial or not.

Marty

By rajaramc Date 07-23-2003 08:11

THe reason for using Er 347 for the root and hot pass may be to avoid the process fluid which is rich in H2S to come in contact with Inconel welds. Similar procedures were used by another fabricator (Belleli, Italy) for our Hydrotreater Reactor nozzle joints.

Rajaram

By waheedu Date 06-15-2020 12:58

Please Clarify PWHT (Annealing / Stabilizing) required for welding of A312 TP321 using ER347 filler

following are the manufacture statement.

Stabilized grades contain small
amounts of titanium (321), niobium
(columbium) (347), or a combination
of niobium and tantalum (347, 348).
These elements have a stronger
affinity for carbon then does
chromium, so they combine with the
carbon leaving the chromium to
provide corrosion resistance.