Local PWHT of SS321H Pipe 1

[Engdoitbetter]

Good Morning to you all,

I'm an italian Mechanical Engineer and I've been working in my present Company as Technical Department Engineer for almost three years. We operate in Oil, Gas & Power sector and do business with Companies all around the world.

I'm writing to you since we're facing some problems in defining PWHT for a 2 in sch.80 (thick. 5.5 mm) in SS321H pipe.
This pipe is going to be used as a thermowell in a reactor vessel, so it will have to withstand external pressure, not mentioning a design temperature of 560°C. In order to seal the end of the pipe, we joined it with an end cap (same material) by the means of a circumferential butt weld which underwent successfully a 100% radiographic examination.
Now, our Customer is asking us to perform Post Weld Heat Treatment on this item at 1095°C, i.e. a solution anneal treatment typical for Austenitic Stainless Steels. I googled a lot and found out that this treatment is performed in order to restore the original internal structure of the material, eliminating carbides, sigma phase etc. etc. Strange to say, I also read that stabilized grades, as SS321 or SS347, as well as low carbon grades (eg SS304L), are usually employed in order to avoid subsequent PWHT, since in both cases there is less carbon free to form carbides, therefore less risk of sensitization and intergranular corrosion.
Anyway, Customer justifies this PWHT requirement writing that operating temperature is greater than 538°C (1000°F); this claim and the information I found led me to think that Customer isn't actually concerned with corrosion (in fact operating temperature falls within the carbide precipitation range, about 400 to 800°C, thus the use of a stabilised grade) but with creep resistance of the welded zone.
Since we are forced to perform only local PWHT (the pipe is 4 m long), I have two questions:

a) is this PWHT really useful? i.e. local treatment would only heat the soak band to 1095°C, but some other areas of the pipe will be at lower temperatures, thus modifying material structure elsewhere.
b) is a slow (instead of a fast) cooling rate harmful to the material? I don't think so as SS321 doesn't undergo sensitization easily.

What do you think?

Thank you for your attention and best regards.

Stefano

 

[metengr]

I don't know of any thermowells that are 321 stainless, and after installation subject to post weld anneal in the field. Seems rather risky and impracticable to perform on installed equipment and could harm adjoining material/equipment.

I would argue the following; if the 321 stainless steel was originally supplied in a solution treated condition, the local welding performed on a stabilized grade of material will not harm the material performance at ambient or in elevated temperature service. Over time even the stabilized grades 321, 347 stainless steels will sensitize. The risk of damage to base material and adjoining equipment in performing an in-situ post weld solution anneal to attempt to achieve a uniform microstructure is much greater than leaving this in the as-welded condition.

On our Power Generation equipment (coal-fired) we have never performed post weld anneal after welding, and never needed to perform this activity for thermowells. The thermowells we installed were 321 stainless welded to Cr-Mo base material and following proper welding guidelines using Inconel filler metal.

 

[Engdoitbetter]

Hi metengr,

Thank you for your reply.
Perhaps my previous post was not clear (sorry!), the PWHT will not be performed in situ but during manufacturing stage. We opted for a local treatment since the pipe is long and we want to avoid as much as possible to descale the pipe.
My concern is that local treatment will only "move" the problem from the weld zone to another one. If this is true, I'm still wondering why Customer is asking for PWHT; furthermore, if, as you point out, material performance is not going to be harmed in as-welded condition, this PWHT seems to be at least useless, if not harmful.

Stefano

 

[metengr]

Engdoitbetter;
Engdoitbetter;
Most construction codes do not require thermal treatment of austenitic stainless steel material after welding (field or shop fabrication) because the material is not air hardenable. Now factor into this austenitic stainless steel material which has been stabilized by the material specification heat treatment, thus further reducing the need for thermal treatment after welding.

I agree that corrosion resistance is probably not the issue based on stated service conditions. From data I have seen on the code committee I am involved with 321 stainless steel performs well without any post weld thermal treatment of connecting welds. I would use the comment I made above about the construction codes as a significant discussion point. Sometimes, just a little prodding in the right direction is all that is needed.

 

[unclesyd]

Adding to the above respose concerning PWHT of 321SS I cannot recall any PWHT on the material used in Nitric Aid manufacturer. I don't recall the temperatures but they were in your range around the burner basket and catalyst pack. All the 321 SS was welded with 347SS, both during fabrication and repair. Our lrgest mode of failure was vibration and expansion joint failure on the hot gas line .Again I don't recall any PWHT ofthe 321SS components.
.

 

[Engdoitbetter]

Googling I found this, do you think it suits my case?

"The material specification requires all of these materials to be provided in the solution-annealed condition. That is, the final heat treatment is performed at a temperature of 1900-2000oF, depending upon the particular grade. For the 321H grade, there is a further requirement: a grain size of ASTM No.7 or coarser is specified to insure adequate creep strength. A solution anneal at 2,000o F minimum is usually sufficient to meet this specification requirement.

After the high-temperature solution anneal, the microstructure will be equiaxed austenite. The word "equiaxed" means that the dimensions of an individual austenite grain will be essentially the same, regardless of orientation or direction. The material is in the fully annealed condition and will be a single-phased material with only some non-metallic inclusions inherent to steel making, apparent within the microstructure."

Thanks for the help.

 

[metengr]

This suits your case for the original product form to be supplied. As I mentioned, once the product form has been received solution annealed, subsequent welding can be performed with no re-solution anneal required.

 

[Engdoitbetter]

Ok, thank you again, my thoughtis the same.

Regards,

Stefano