Stress Relief Vs. PWHT 7

[OwaisGillani]

Guys,

I have come across a pressure vessel drawing which says:

Stress Relief : YES
PWHT : NO

which seems a bit confusing. Aren't these the same?
Please clarify.

Regards

Owais Gillani

 

[vesselguy]

I forgot to mention that although the intent of PWHT is to reduce hardness, for some metal, the code specified min temp may not be high enough to give that hardness reduction. So consult a metalurgist.

 

[panduru]

@OwaisGillani:

I suppose the problem lies in applying the theory in a pragmatic manner. The Code seems to have taken good care of that. If only the engineer appreciates that.

Ultimately, the equipment is a product of all the processes that it undergoes.

@vesselguy:

Thanks, vesselguy. And pardon me for further lengthening the thread.

 

[robsalv]

Never apologise for lengthening a thread if it adds value!

So with that intro...

I have a material glossary of terms (I can't remember where I photocopied it from) which says:

Stress Relief: A thermal cycle involving heating to a suitable temperature, usually 1000F-1200F, holding long enough to reduce residual stresses from either cold deformation or thermal treatment, and then cooling slowly enough to minimise the development of new residual stresses.

Post Weld Heat Treatment: Also referred to as stress relieving, this process is used to soften the heat affected zone and relieve residual stresses created during welding.


So though they are interchangeable [and it's certainly how I've used the terms], one has a bias towards welding and the other doesn't.... which I think is the consensus of the discussion.


PEHasan mentions an equivalent to stress relieving using preheat. 200F seems a bit low, but he is right - if you slow the cool down rate you give the weld more time to sort the residual stress situation out. When we do pad weld repairs on tanks or localised repairs on PWHT/Stress relieved vessels, we'd ideally like to carry out PWHT to reduce residual stresses and hardnesses, but often it's not practical [and some times damn well not recommended - stress relieving a whole vessel for the sake of a small weld repair??]. In that case, we employ a "high" preheat, minimum interpass temperature, "high" post heat, slow cool down, temper bead procedure. According to proprietary and industry literature I use, it's the next best thing.

API RP577 and RP582 has some good guidance on PWHT and interestingly, since they're welding guidance documents, they use the terms PWHT and stress relief interchangeably.

Cheers

Rob

 

[SnTMan]

OwaisGillani, now that we have defined the two terms every which way, I'm curious about your vessel: is there any indication in the documents about how the "stress relief" was carried out, i.e. thermally or some other means?

Mike

 

[unclesyd]

Stress Relief
Low temperature heating for removing internal stresses, such as those resulting on a metal from work hardening or quenching.


Stress Relieving
Heating to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly enough to minimize the development of new residual stresses.

This is out of an old brochure from one the heat treating companies explaining their capability.

We used the term "stress relief" when we would put parts in an low temperature (normally 300°F-400°F) oven after austenizing when there wasn't but one oven or furnace capable of higher temperatures for tempering.

We used "stress relieving" on all parts fabricated for our polymer process, operating at 600°F and cleaned at 900°F, prior to machining. We cycled them through the cleaning cycle at 900°F. Our Fabrication shop generally referred to PWHT as stress reliving. Early on our best fabricators were exshipyard employees where PWHT was unknown, but a lot of piping fabrication was stress relieved.

Anecdotal:
My boss for many years had accomplished a lot of work on the time at temperature it actually takes to relieve stresses. It started as a doctoral thesis but he never finished it or published it. According to his research the time to relieve 90% of the stresses is very short. During his early work it was hard to measure residual but later on as better methods became available he verified most of his results on the lower P numbers.

 

[gr2vessels]

Guys,

The answer could come from a normal practice in developing the datasheet/schematic drawing;- usually the datasheet has to specify the stress relief requirements for the Process reasons and is always indicated on the Process datasheet (Yes / No). The Mechanical datasheet developed on the basis of the Process datasheet, carries accross the statement of NO Stress relief for process reasons, however, additional PWHT has to be specified as per the Code requirements (if any).

In the above case, the corrosive or other similar service requires the material (Carbon Steel?) to be stress relieved to control the base metal hardness, to prevent any specific type of corrosion cracking;- the material thickness or other conditions however, do not require PWHT.

I hope that I'm not trivialising the thread above. I'm only pointig out a possible missunderstanding.

Cheers,

GRVessel

 

[DavidRibeiro79]

At risk of adding to the length of this thread, something I noticed which has not been mentioned.

OwaisGillani (Mechanical), you mentioned: "Please comment on these interpretations with respect to SA-516 Gr. 70 plate of 44 mm thickness intended to be rolled into 1200 mm ID shell."

Section UCS-56 of the ASME BPV Code requires one to PWHT vessels greater than 38 mm thickness. If your vessel is 44mm, PWHT would be a requirement.

Furthermore, one would need to check the percentage elongation (PE) of various formed sections in the vessel in accordance with the requirements of UCS-79 of the code. South African industry specific standards require stress relieving above 5% to below 10% and normalising above 10% PE. I'm not sure if this is defined specifically in the ASME BVP code however, suspect it is not.

Happy arguing!

D

 

[djack77494]

Maybe I should start a new thread, but what about "normalising"? How does it relate to stress relieving and/or PWHT?
Doug

 

[metengr]

Does both, and eliminates the base metal HAZ.

 

[OwaisGillani]

@ SnTMan
Well the guys at fab. shop did what was best suited for this scenerio (as in metengr's post).

They placed the entire vessel into the furnace and in one go completed requirements of stress relieving and PWHT.

@ unclesyd

Yes I have also read about this theory in some technical literature. (90% stress relief in very short time). But I wonder how important are those remaining 10% which are probably more difficult to relieve. Shouldn't there be a clause in the code covering this alternate approach of 90-10% stresses and allowing for a FACTOR for reduction in service limits that be applied in such cases? Plausible, nonsense or otherwise, please comment.

@ gr2vessels and DavidRibeiro79

The PWHT=NO was technically wrong as I mentioned above.

Regards

Owais Gillani

 

[GenB]

I use PWHT temp as stress relief, that being covered by the QC, even if no welding was done or PWHT was nor required
genb