SA213 T9 tubes welded to SA387 Gr.91 plate 1

[LSUME]

I have what I believe to be a predicament.
516-70N tube sheet clad with 3/8" SA387 Gr.91 plate. The cladding is by a reputable manufacturer and all seems to be in order. The tubes for this TS are SA213 T9.
Tempering temps for the tubes and cladding are 1250°F & 1350°F respectively. The tube to TS joint will be a seal weld therefore will require PWHT & a possible bake off. The code requires 1350°F min PWHT. Anyone have any suggestions or have come across a similar issue?

thanks

 

[DG72]

I don't have much to offer, except be very careful with heat treatment of 9 chrome material--I understand that proper heat treatment is essential to success with this material. Many fabricators will not touch 9 chrome due to liability concerns.

Maybe some of the materials guys on this site can offer more useful info.

 

[metengr]

You have got to be kidding on this one. Why the heck do you have a clad CS tubesheet with Grade 91 material, when the tubes are Grade 9??? This makes no sense. Is this a roll clad TS? Also, please review ASME Section VIII, Div 1, Table UCS-56, Note 2, for dissimilar metal welds, the lower PWHT temperature of 1300 deg F is permitted for P-No 5B, Group 2 materials.

You will need to use an E9018 B3 or ER90S-B3 filler metal and locally PWHT the seal weld joint staying below the lower critical transformation temperature of the CS substrate. I would recommend 1300-1325 deg F PWHT at 15 minutes at temperature. Unfortunately, this will not be in the comfort zone for Grade 91 material, but this is one of those poor designs that someone forgot to ask a materials engineer for advice.

 

[LSUME]

Thanks for the info Metengr. I had to shake my head myself on this one. I was dealt the hand and am trying to deal with it best I can. Apparently SA387 Gr 9 is not available so someone got the bright idea to go with modified 9 Cr. The tubesheet is explosion bond clad. Thanks for the info.

 

[LSUME]

Metengr
I read UCS56 note 2, does this apply to my case due to filler metal?
It does say "welds made between a P-No-b Group 2 and another lower chromium ferritic, austenitic, or nickel based steel" then it mentions the filler metal.

 

[stanweld]

I would not recommend the use of the "B3" welding filler metals. You are welding 9Cr materials and they have been selected most likely for corrosion resistance, most probably sulfidic. Recommend using "B8" or "B8L" welding filler metals. PWHT per metengr.

 

[metengr]

stanweld
UCS-56 limits the chromium content of the filler metal to less than 3% to take advantage of the lower PWHT temperature (see Note 2). In this case, one could use a Nickel-base or austenitic filler metal, again per Note 2.

 

[adrianviorel]

Dear Lsume, we had a lot of trouble using Gr 91 material. For me, it does not seem to be a good idea to use martensitic structure on pressure vessels, simply because such structure is mainly used for wear resistance...whatever! Moreover, welding of P91 requires a lot of attention regarding welding regime. Material should be preheated at about 270 deg C, during all welding process. Imagine what happens for a thick material: where do you measure actual temperature: on the inside, or on the outside? which is the variation between the two surfaces?
Supposing you have solved this issue, you must take care at post baking temperature, meaning that after welding material's temperature should be raised to 315 deg C, for at least 30 min. All that requires, imperatively, electrical heating of material, together with a very tight insulation of it.
Above all, the material cracks very easily, even two days
after welding, and repairs are requiring another heating cycle, which means that it could not respond to intended structural transformation at final HT. Another risk is to make X-ray examination too early after welding, the film to be OK, and cracks could appear later.
The picture is not very encouraging, isn't it?
As Metengr said, using this material is not a good idea, at all

 

[stanweld]

metengr,
Thanks, I used an earlier edition. Per Note 1, the minimum PWHT temp is 1325 F and one could define a range of 1325 F to 1340 F using B31.1 as reference for the lower critical temperature.

Because this is cladding and assuming that the Grade 91 is not a pressure part, UCL-34 governs; PWHT is based on base material (pressure part)requirements.

In any event, LSUME has a tiger by the tail with this insanely accepted fabrication.

 

[vesselguy]

Lsume,
You got a bad case on your hand. I share the other guys concerns about using the Vanadium enhanced 9Cr; brittle. But since the P91 is used a cladding (which is not a pressure retaining compoent) then Stanweld is right.

Be aware of the concerns for tube bundle support during PWHT. Differntial thermal expansions on the tubes and carbon steel t/s is a concern. Have you consider welding several samples and do hardness test of the welds and HAZ and if the hardness is less than 230 BHN then you may justify no PWHT using your welding quality as a way out?

Or, perhaps you can change the design by using weld overlay tubesheet then you can put down more easier and compatible material for your tubes to weld to; perhaps Inconel. Best to have an experience welding/metalurgical engineer to work with you on this sensitive piece. Good luck and keep us inform on what you end up doing and the results.