PWHT of Rolled Nozzle prior to welding on shell (UCS-79) 2

[LPZ74]

Hello fellow engineers

Is anyone out there familiar with paragraph UCS-79(d) which states that "pressure boundary parts of carbon and low alloy steel plates fabricated by cold forming shall be heat treated subsequently (see UCS-56) when the resulting extreme fiber eliongation is more than 5% from the as rolled condition..."

We have a vendor who is currently planning on rolling some heavy barrel nozzles from plate instead of using heavy forgings. As a result, the extreme fiber elongation exceeds 5%.

My question is, does the final PWHT performed on the vessel satisfy the stress releif required for the rolled nozzles due to fiber elongation, or do these nozzles require PWHT stress relief prior to being welded onto the vessel, PLUS the final PWHT?

Thanks in advance for any responses.

 

[metengr]

LPZ74;
My 2007 Edition of Section VIII, Div 1 UCS-79 states heat treatment, not PWHT. So, to answer your question, a separate heat treatment shall be performed prior to welding this nozzle into the vessel. After welding, PWHT requirements would apply.

 

[LPZ74]

Hi metengr,

Thanks for your response. I am just trying to get the correct interpretation of UCS-79. My interpretation was that since UCS-79 refered you back to UCS-56 requirements[Requirements for Post Weld Heat Treatment], that the PWHT satisfied the nozzle stress relief requirements due to extreme fiber elongation.

I do not specifically have a problem with forcing the vendor to perform the additional stress relief, but want to be sure we have the correct interpretation as this could impact their costs and schedule.

Are you aware of any code cases which explicitly defines these requirements?

Thanks...

 

[doct9960]

LPZ74,

I agree with metengr's reply. Maybe this ASME interpretation will make things clear. Although the interpretation applies to a head, it may apply to your rolled nozzle.

Interpretation: VIII-1-95-50
Subject: Section VIII, Division 1 (1992 Edition, 1993 Addenda); UCS-79(d)
Date Issued: March 13, 1995
File: BC94-684

Question (1): A cold formed head has to be heat treated to fulfill the requirements of UCS-79(d) in Section VIII, Division 1. Does the term “heat treated subsequently” mean just after forming and prior to any other operation?

Reply (1): No.

Question (2): If Reply (1) is no, then will PWHT in accordance with UCS-56 satisfy the requirements of UCS-79(d) following welding of the head to the vessel?

Reply (2): Yes.

 

[metengr]

LPZ74;
I don't know if there is a specific interpretation on this item. I can offer advice based on my expierence with the Codes and material performance for pressure vessels. The reason for a subsequent heat treatment in UCS-79 (d) (either re-normalize or subcortical) is to restore ductility into the material as a result of cold forming operations.

Welding a nozzle that has exhausted some ductility could result in cracking from residual welding stresses before PWHT. I would highly recommend you heat treat (either re-heat treat or perform a subcritical PWHT using UCS-56 requirements) this nozzle before welding.

 

[LPZ74]

Hi metengr / doct9960

Thanks for your responses. According to the interpretation [VIII-1-95-50] provided by Doct9960, it seems as though you could proceed to weld on the formed head prior to a stress relief heat treatment and then use the final PWHT in accordance with UCS-56 to satisfy the requirements of UCS-79(d). If you extend this interpretation to a rolled nozzle, I would interpret this that the final PWHT is sufficient also.
However, even with that being said, I think metengr convinced me that a separate stress relief prior to welding the nozzles into the shell is a good idea, even if not explicitly required per the interpretation.

Thanks for your posts.

 

[metengr]

LPZ74;
I have run into this many times with Code requirements and their application. One has to understand material limitations and what is good engineering practice to avoid potential problems during vessel fabrication.

 

[stanweld]

Is the manufacturer cold forming these nozzles? Most often they are hot formed, in which case thickness will dictate whether PWHT is required. Depending on the plate material specification, the nozzles should be heat treated to the base material specification requirements after hot forming.

 

[vesselfab]

Just a quick question

how thick are we talking about and what diameter?

if it is really thick and small diameter as suggested by the elongation....

I think I would look at heat treating in the normalizing range and then perform a surface MT

 

[LPZ74]

In response to vesselfab and stanweld, the thicknesses range for different nozzles. 30mm thick is the thinnest nozzle rolled to an ID of 387mm. Thickest nozzle is 70mm rolled to ID of 1372mm. Material is SA-516 Gr.70N, and the nozzles are cold rolled.

 

[stanweld]

It is quite common to cold roll and weld the larger bore nozzle and include the UCS-79 (d) required heat treatment in the full vessel PWHT per UCS-56.

I am somewhat more concerned with the smaller bore nozzle. If the elongation exceeds 10%, excessive grain growth at the highly strained area could easily occur at the 1100 F to 1200 F tempering temperature resulting in degraded impact toughness properties in that area. Depending on actual cold stain, I would recommend normalizing after forming and tacking, prior to welding.

 

[LPZ74]

Hi Stanweld;

According to the formulas provided in UCS-79(d), the extreme fiber elongation is 7.19% for the small bore nozzle, and 5.1% for the large nozzle (with other nozzles ranging in between). As indicated in a response above, I think I've convinced myself that the stress relief prior to welding into the vessel is a good idea, even though not required accoding to interpretation VIII-1-95-50. My plan is to discuss with the AIA, and have them make final recommendation, since they will be the ones stamping the vessel in this case.

Thanks to all for the posts...

 

[kwaliteitshenk]

LPZ74,
In my opinion there is a great difference between the Normalizing Heat Treatment required by UCS 79-(d) and the Post Weld Heat Treatment required by UCS-56.

 

[stanweld]

Based on the extreme fiber elongation reported, a tempering heat treatment or PWHT is recommended. Remember that if you heat treat the completed/welded nozzles prior to PWHT of the vessel, the additional heat treatment will have to be taken into account regarding ASME VIII and IX impact test requirements (if required).

 

[metengr]

I am going to throw an additional 2 cents of advice. For nozzle fabrication from rolled and long seam welded plate, as the OP mentioned, I would use the material specification requirements of ASME SA-671, which is electric-fusion welded steel pipe for atmospheric and low temperature service. The fabricated nozzles could be certified under ASME SA 671, with reference to Class 20 or higher and with impact (if required) prior to installation in the vessel.

 

[vesselfab]

for a-671, wouldn't that force the fabricator to do a bunch more physical testing, PER LOT, which in many times is one piece of rolled and welded cylinder, which would include a flattening test (is this a waste?), extra tensiles, properties, etc which is not required if you call it a common rolled and welded cylinder. Kind of hard to imposes these kind of ""material spec"" changes on the fabricator after the fact.