PWHT of a cladded pressure vessel

[Paulettea]

Dear All

I have a problem with the PWHT of a vessel when it comes to its overlay.

CAUTION: Postweld heat treatment may be in the carbide‐precipitation range for unstabilized austenitic chromium–nickel steels, as well as within the range where a sigma phase may form, and if used indiscriminately could result in material of inferior physical properties and inferior corrosion resistance, which ultimately could result in failure of the vessel

I want to know the following in the above cautionary note from ASME.
What grades of stainless steel are stabilized and which ones are unstabilized?
What are the temperature ranges of carbide participation? Can I find a standard specification in which for each grade of stainless steel it introduces a range of in which the material is kept safe from sensitization?
There is a vessel that I want to start designing for which the shell thickness is 40mm SA-516 70 and there is a cladding of 3mm SS 304. The plate has to be impact tested also. I want to know if SS 304 is stabilized or not and if it is not stabilized can I choose a different grade for cladding so that I can get rid of thinking about sensitization?

 

[metengr]

What grades of stainless steel are stabilized and which ones are unstabilized?

The stabilized grades of austenitic stainless steel are 321 and 347.

Can I find a standard specification in which for each grade of stainless steel it introduces a range of in which the material is kept safe from sensitization?

No. A materials engineer should be involved to assist you because you are unfamiliar with the topic based on your questions.

304 stainless steel is not stabilized for exposure to PWHT of the carbon steel shell material during fabrication. There are ways to work around this issue but you really need assistance from a material engineer who knows or has pressure vessel experience.

 

[SnTMan]

Paulettea, for some further information see Appendix HA & Sec II, Part D, Appendix A, A-360.

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Paulettea]

Thank you for your replies.

metengr, do you think that if I use 347 or 321 as the cladding material there will be no problem with the PWHT?

 

[metengr]

Paulettea
I gather from your post you are either considering welded strip cladding of 304L or are using a corrosion resistant weld overlay (CRO) of 308L on the carbon steel shell material? Or are you intending to roll or explosive bond 304L on CS plate and fabricate into a pressure vessel?

If you are applying a CRO, my suggestion would be to use Inco 82 as a butter layer, PWHT the CS shell, and then apply the weld overlay on top of the butter layer using 308L weld deposit with no PWHT. Make sure you qualify a proper CRO weld procedure to ensure a minimum thickness of 308L composition for corrosion resistance.

If the CS has roll or explosive bonded cladding applied before fabrication, you need to use 347 which is a stabilized grade of austenitic stainless steel to reduce risk of sensitization during exposure to PWHT from fabrication.

 

[dig1]

Paulettea,

metengr has good advice. In continuous kraft pulp digesters good performance with 304L rolled/explosion bonded clad which had the carbon content limited to 0.020 max has proven to perform well in most cases (there are always exceptions)where PWHT was performed. I have seen some people specify 0.015% max. Nozzles were fabricated from SA312-TP304L or plate SA240-304L w/ the same limiting carbon content. Material availability has never been a real issue in the overall project schedule provided everyone knew the requirement during the quoting stage.

I don't know your application so the above may not work or be applicable.

BR,

Patrick

 

[Paulettea]

Thank you metengr.

Is it OK to put an overlay on a vessel which is already gone through PWHT? I thought no welding is permitted after PWHT unless applying overlay is not considered a welding at all.
Is it not better to choose the second method (i.e. bonded cladding with 347) because if any repair is required we can PWHT the vessel without the concern about the cladding.

SnTMan, thank you but in Appendix HA it is referred to article A-310 to A-360 in II-D. I could not find these articles exactly (where is A-360?). What has happened? Did they change the naming system of these articles?

 

[SnTMan]

Paulettea, sorry about he mixup, it looks like Apx A, Sec II, Part D has been re-numbered, not sure when. Section on "885F Embrittlement" is now A-207.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[weldstan]

Stainless steel overlayed/clad vessels are often subject to PWHT and have been for decades. For the most part, low carbon or stabilized grades of stainless steel are used dependent on the corrosion service.

In some cases weld overlay can be done after PWHT of the vessel. In most cases that I have dealt with over the past 48 years, PWHT has been done after the vessel is overlayed due to costs (closure weld considerations} or Code requirements.

 

[SJones]

http://www.eng-tips.com/search.cfm?q=clad+vessel+PWHT&action=search

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.