P1-P8 Vessel with PWHT - ? Sensitization? 1

[EmilGies]

First - Just wanted to say thanks to all of the contributing and knowledgeable folks that help each other here. Lots of great info.

Onto my questions -
We are building a very thick SA-516 gr 70 vessel to ASME VIII (Div 2). We will use a stainless liner and all nozzles will be 304L pipe. Due to the thickness, PWHT is required. Am I missing an exemption regarding the P1-P8 joints? AF-402.2 states that the material with the Higher PWHT drives the treatment and since P8 has no requirements it requires me to PWHT the nozzle to shell joint at the P1 temp/time.

If I am not missing any way out of PWHT, how does Sensitization come into play? I have found limited information regarding Temperature/Time dependencies for stainless. I called a major stainless supplier and their metallurgist could not point me to a better source. Can anyone give me an idea of what 2 hrs at 1100°F will do?

What else am I missing?
Thanks for reading.
EG

 

[metengr]

I don't believe PWHT is required. If you intend to use a corrosion resistant overlay or liner that is seal welded, you can use preheat requirements and follow the PWHT exemption Note 2 (b) 4 in Table AF-420.1 of Section VIII, Div 2, Article F-4.

For the P-No 8 nozzles, I would butter the SA 516 base metal weld joint, local PWHT as required, and weld the P-No 8 nozzle to the butter layer in the weld joint.

 

[metengr]

EmilGies;
One clarification to the above, if your pressure vessel is intended for lethal service, the exemption I mentioned above for PWHT does not apply (see AF-402 about half way through the paragraph).

 

[EmilGies]

Metengr - thank you for the reply. I understand the liner of the shell itself does not require PWHT. I believe I was a bit unclear above - I mentioned it so that it was a little more clear (like mud, right) as to what we are trying to accomplish. We will install the liner after we have PWHT'ed the shell.

My concern has to do with the several nozzles that attach to the shell. Our requirement is to have all wetted surfaces be stainless. The nozzles in question have a reinforcement ring and so we are limited to essentially AD-612.1(a) or (b) where the nozzle penetrates the shell. If we could somehow use the nozzle attachment that is permitted in Div 1 where the nozzle sets ON the shell with re-pad we would be thin enough to not need PWHT. As it is though, I don't see how we can get around welding these nozzles prior to PWHT.

Perhaps, however, I did not fully understand your reply. If so, please pardon me. Thank you for taking the time,
EG

 

[EmilGies]

PS - no lethal service!

 

[metengr]

EmilGies;
The buttering that I referred to above is used to basically safe end the surface to accommodate a full penetration weld without PWHT. What you do is apply 309 weld metal over the entire weld prep surface on the carbon steel side of the weld joint (buttering) and locally PWHT. I would use about a ¼” thick deposit. The 309 stainless steel will not be harmed by exposure to PWHT because of the alloy content. Once this is completed, you can weld the P-No 8 nozzles to the butter layer with no need for PWHT because it would be like welding P-No8 to itself.

 

[metengr]

If I am not missing any way out of PWHT, how does Sensitization come into play? I have found limited information regarding Temperature/Time dependencies for stainless. I called a major stainless supplier and their metallurgist could not point me to a better source. Can anyone give me an idea of what 2 hrs at 1100°F will do?

Since you have an L-Grade 304, this is good because the lower carbon content reduces the severity of sensitization in 304/316 austenitic stainless steels. However, exposure to PWHT temperature at 1100-1200 deg F for 2 hours will result in some carbide precipitation, because it is inevitable with this alloy. The affect of sensitization can only be quantified by corrosion testing per ASTM A 262 to evaluate susceptibility to intergranular attack.

 

[EmilGies]

Metengr - thank you for your continued input!

We had thought of maybe decreasing our PWHT by 200°F per table AF-402.2 and welding the P1-P8 joint and then PWHT the whole damn thing but because we have impact testing I believe this shoots our current WPSs that are being used on the P1-P1 portions of the shell. As these WPSs were written with PWHT Temp of 1,100°F, and change in PWHT is an supplementary essential I do not think it is an option.

I have found some information stating that 304L sensitization temp range is 850 - 1150°F and the time required in this range is greater than 100 hours. Can anyone verify this sort of data? I have not been able to find via a web search or my stainless suppliers any additional information.

I like the idea of the A 262 test. I am not familiar with the test - how do I interpret the results - is there a definite quantitative value of carbide precipitation that I must be under to be ok? I believe I still need to qualify a P1-P8 with impacts, PWHT within the thickness range I need so I could have the A 262 run at this time...

Again, thank you for your input,

 

[metengr]

EmilGies;
In my reference material ("Corrosion of Stainless Steels" by J. Sedriks), there is a time-temperature-sensitization curve for 304 and 304L that was developed from the Strauss test (ASTM A 393), which is now discontinued, and replaced with the ASTM A 262 Practice E test. I would encourage you to obtain ASTM A 262 and review it carefully.

Now, regarding the time-temperature-sensitization curves, for 304L (0.03% C), because the curves are C-shape, the shortest time to achieve sensitization is about 10 hours at 575-600 deg C (1067 deg F - 1112 deg F).

 

[EmilGies]

We are now looking at possibly using 347 as nozzle material. Any comments on this material? The same sensitization data I mentioned above re: 304L lists 347 sensitization range of 925 - 1050°F for more than 1000 hours. Our PWHT is 1100°. Thoughts?
Trying to keep things as uncomplicated as possible. Thanks for the continued input.

 

[metengr]

The 347 stainless steel is considered a stabilized grade of austenitic stainless steel, where Nb and Ta are added to form TiC (titanium carbides) instead of harmful CrC (chromium carbides).

The stabilized grades of stainless have their advantages and disadvantages. The advantage is greatly reduced susceptibility to sensitization because the carbon is either in solution or tied up as TiC. However, the stabilized grades upon welding can suffer from knife-line corrosion attack under certain conditions.

 

[metengr]

Oops...I just noticed my own typo in the last post, this should be NbC and TaC, not TiC. The TiC would apply to the 321 stabilized grade of stainless steel, which has deliberate additions of Ti.

 

[EmilGies]

Doing some more research I still 'think' that 304L would be ok in the situation I have described above. The following is from Sandmeyer Steel's Spec sheet for 321 & 347
"While Alloys 321 and 347 continue to be employed for prolonged service in the 800 to 1500°F temperature range, Alloy 304L has supplanted these stabilized grades for applications involving only welding or short time heating... Alloys 321 and 347 stainless steels offer higher creep and stress rupture properties than Alloy 304 and, particularly, Alloy 304L, which might also be considered for exposures where sensitization and intergranual corrosion are concerns."
The do also mention a maximum use temperature of 304L limited to 800°F. Is my ignorance showing through thinking the 304L should have no adverse results due to PWHT at 1100°F for approx 2.5 hours?

 

[metengr]

EmilGies;
Based on what we discussed above, the 304L nozzle material should be acceptable. The low carbon material is not suitable for elevated temperature service in pressure containing applications because of the lower creep strength.

I would proceed with using 304L, and run an ASTM A 262 Practice E test on your weld coupon.

 

[EmilGies]

Metengr -
Thank you for your continued patience and input.
My test lab told me that their 262 Practice E incorporates 1 in² sample. Should I run two tests? One on the P8 HAZ and one on the P8 base? Or can I assume the P8 HAZ to be the most severe case?

 

[metengr]

EmilGies;
I would do both (base metal and HAZ).