PWHT of Cladded Vessel 1

[massad]

Hello All ,
Carbon Steel Vessel : SA 516 70
Cladding : 316L (For corrision resistance)
Service : H2S
Thickenss of Vessel : 18 mm
CA for CS : .125" and SS : .0625"

Is PWHT is required for this vessel ? I know that the thickness is within limits , but since the service is H2S , may be PWHT is required , I have checked section UCS , but didnt find the solution . Can any one give suggestions.

BR
Assad

 

[metengr]

Is this service considered lethal by definition in ASME B&PV Code, Section VIII, Div 1 (review Part UW-2)? If it is not considered lethal service, PWHT is not required in accordance with the fabrication rules in Part UW and UCS. If is lethal service, PWHT is required, regardless.

I would check with the engineering specification for this contract to be sure that PWHT was not separately specified by the owner.

 

[massad]

Yes , the services is Lethal , but my confusion is that , if we are providing cladding to protect the internal surface of the vessels and the liquid inside will not be in contact with the CS vessels , then is there a requirement for PWHT or not ?

BR
Assad

 

[metengr]

YES. PLease review UW-2.

 

[Steven444]

I do not think you need to do PWHT because you are already covered by doing cladding.

 

[metengr]

Steven444;
I would have to disagree because the cladding only serves to provide a barrier. Lethal substance is what it is and if the vessel is categorized as containing lethal substance it must follow ALL of the rules in Section VIII regarding fabrication of vessels containing a lethal substance.

 

[jehan17718]

metengr,

I am from a non-metallurgy background and I concur with your views regarding PWHT.

Just for my curiosity, how does one take care of the grain boundary sensitisation problem if a CS vessel is having a weld overlay of SS 316L? Would'nt the SS weld overlay (WO) experience grain boundary sensitisation during the PWHT?

Let us consider a shel having a total thickness of 123mm [120 mm thk SA-516 Gr 70 vessel with a 3 mm thk SS 316L WO]designed as per ASME Sec VIII Div 1. Offhand, for this configuration, I can say that the PWHT temperatures will be in the 800-1600 deg F range.

You opinions and comments as always are greatly appreciated.

-jehan

 

[metengr]

jehan17718;
In the situation you describe, one would use a 309 ss to butter the entire surface of the CS shell. Since this is a dissimilar metal weld, you need a filler metal that can be welded directly to CS. The PWHT would be performed, and the final clad layer of 316L would be deposited over the butter layer with only a preheat (if the shell required PWHT because of lethal service).

 

[jehan17718]

Metengr,

Thanks for your prompt reply.

Am I correct to interpret from your above reply that SS 309 is immune to the grain boundary sensitisation problem? As per my rather elementary metallurgy knowledge, stabilised austenitic SS grades 321 (Ti stabilised) and 347 (Cb stabilised) are completely immune to sensitisation and are used in the 800-1600 deg F temperature range. SS grades 304L and 316L may be used in this temperature range but not for prolonged duration..

Please clarify my above views on SS used in the sensitising temperature range.

Thanks in advance.

-Jehan

 

[metengr]

jehan17718;
The 309L ss is less susceptible to sensization for this type of application, in comparison to 309. You need to use a compatible filler metal, like 309 or 309L when welding austenitic stainless steels to CS. In this case for welding cladding, the 309 or 309L filler metal will not be in contact with an environment where sensitization would be a concern because it will be overlaid with 316L.

The stabilized grades of stainless steel like 321 or 347 will eventually sensitize after prolonged exposure to elevated temperature service. If the stabilized grades are properly solution treated during fabrication, the dissolved carbon is tied up as TiC to avoid precipitating chromium carbides.

 

[massad]

Thankyou very much for everyone , finally this is decided after having discussion with client and senior engineers , cladding will be done on this vessel b/c if the base metal has the requiment of PWHT then it should be done .

Thankyou 4 all comments .

BR
Assad