PWHT of Head before welding to shell 2

[Paulettaa]

Dear All,

Our factory cannot fabricate an elliptical head with especial dimensions and we have outsource it.
They have sent us a contract in which they have included all prices and one of them is PWHT of the head after forming.
The whole vessel is going to be PWHT'ed so I thought this PWHT of head after forming is not necessary and can be omitted.

So, the question is that is it necessary to PWHT one component of an equipment before fit-up and welding to the other components of the equipment which is finally given a PWHT as a whole?

Warm Regards

 

[racookpe1978]

Depends on the exact heat treatment they are going to perform.

A post weld heat treatment is a specific sequence of temperature rise-hold-soak-cooldown series at particular times and temperatures to treat the new welds. Obvious by its name, right?
But normalizing, stress relief, etc. are different for different purposes before, during and after extreme plastic deformations (blacksmithing, rolling, forging, and forming). They not a PWHT, but these cycle also heat, hold-and-soak, and cooldown to different (NOT PWHT!) temperatures .

Could be that what the "PWHT" charge is for. Could be a bad "name" for a properly needed process. Could be an excess charge they thought they can get away with. Get an independent check from a separate metaluralogical group.

 

[david339933]

Standard Designation: BPV Section VIII Div 1
Edition/Addenda:
Para./Fig./Table No:
Subject Description: Section VIII, Division 1 (1992 Edition, 1993 Addenda); UCS-79(d)
Date Issued: 03/13/1995
Record Number: BC94-684
Interpretation Number : VIII-1-95-50
Question(s) and Reply(ies):
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.

 

[Don56]

If the cold formed head requires PWHT due to fiber elongation, and the completed vessel will also be PWHT'd there is no need to duplicate this heat treatment of the head. However if this condition exists and the head manufacturer does not PWHT, the head manufacturer cannot certify the head as meeting the requirements of UCS-79 or UG-79.

 

Don56,
There's the rub.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[david339933]

Ultimate responsibility lies with the vessel manufacturer, so regardless of what the COC states, you need to verify UG-79/UCS-79/UG-81 are satisfied. Not having a COC certifying UCS/UG-79 would be the least of my worries.

 

[racookpe1978]

A single important addendum to all the above.

IF (big "if" there! SINCE WE DO NOT KNOW IF STRESS RELIEF HEAT TREATMENT IS NEEDED) the field weld is PWHT at the secondary fab shop as indicated, that shop may not be able to get the whole vessel into a single furnace for adequate whole-vessel preheat-heatup-hold-controlled cooldown sequence. But that fab shop may "easily" be able to heat treat the welds of each joint with heat blankets and local heaters. If so, the head(s) may not be able to be stress-relieved unless the original shop does the whole head in one furnace at one controlled rate.

 

[r6155]

Paulettaa
Can you tell us data of your "elliptical head with especial dimensions"?

Regards
r6155

 

[Paulettaa]

Thank you all for your replies.

david339933, that was a very relevant post thanks so much.
Actually I took another look at the technical offer by the head vendor. It was no mention of PWHT, it was stress relief. By "especial Dimensions" I mean it is especial because our company and our previous head vendors were not able to fabricate this one so we gave it to a new head vendor.
Material: SA-516 70
Thickness before forming: 50 mm
Shell ID: 6950 mm
Crown radius: 6255 mm
Knuckle radius: 1195 mm
With these dimensions, UG-79 and UCS-79 does not force a PFHT. So the stress relief cannot be due to excessive fiber elongation. Due to the large diameter of the head the blank has weld lines on it and since the thickness is 50 mm there will be a need for PWHT. However, I do not want to pay extra charge for PWHT since the whole vessel will be given a PWHT after fabrication is complete. Furthermore, as far as I know, doing PWHT may cause mechanical properties deterioration and is better to be minimized.

Warm Regards

 

[r6155]

Normaly this head is made with central cap and segments, then welded at the head vendor or at pressure vessel
fabricator
I prefer welding at the head vendor and no PWHT, but welding procedure shall include PWHT.

Regards
r6155

 

[Paulettaa]

Hello again

I talked to the head vendor and they say the heat treatment is not actually a PWHT but it is a PFHT. However, the procedure for both is the same as per the code. They claim that it is required to do the heat treatment after the head is formed into the crown radius and before starting to form the knucle radius. They want it to be performed in order to prevent development of cracks on the knuckle region. However, I argued that the fiber elongation as per UG-79 is only 3 per cent and it wont be a problem to do the job without stress relief.
They insist on doing the heat treatment even without asking for extra charge for it.
What should I do? I have heard that exposing the material of construction to temperatures in PWHT range could be detrimental to mechanical properties.

Warm Regards

 

[r6155]

Accept and go ahead, no problem with PWHT.

Regards
r6155

 

[racookpe1978]

Accept their fabrication experience.

You are attempting to "follow the book" by citing "3% elongation" and they are attempting to "make a product that will not fail" so they can "keep their business in business."

 

[SnTMan]

I have heard that exposing the material of construction to temperatures in PWHT range could be detrimental to mechanical properties.

This is not generally seen by ASME as being a problem in carbon steels, see UCS-85(f).

Regards,

Mike

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

 

[Don56]

50mm SA-516 must be normalized. It's mechanical properties are based on the material and do not come from any work put into it from rolling. PWHT should not reduce the mechanical properties.

 

[SnTMan]

The vendor may be buying "green" plate, i.e. plate that has not received the heat treat required by the plate spec, and heat treating after forming. This would be fairly common in that industry I believe.

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

 

[Don56]

SnTMan,
Normalizing after forming will distort the H out of the vessel. Unless adequately supported it will collapse under its own weight.

 

[SnTMan]

Don56, right, normalizing a vessel is asking for trouble. I was speaking of the head manufacturing only. A vendor may buy (so called green) plate that has not had the specified heat-treat, i.e normalized for the subject 50mm SA-516-70. The head may be formed at the heat treat temperature or formed then heat-treated as per its particular specification. Again, fairly common I think.

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