Stress Relief Vs. PWHT
Stress Relief Vs. PWHT
(OP)
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
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





RE: Stress Relief Vs. PWHT
Post weld heat treatment is a higher temperature thermal treatment to reduce residual stresses from welding AND to temper or soften the heat affected zone of the base metal.
RE: Stress Relief Vs. PWHT
PEHasan.
RE: Stress Relief Vs. PWHT
API specifies "stress relief" for certain items on oil tanks. The stress relief times and temperatures are exactly the same as specified for PWHT in ASME Section VIII. So the terms do get used interchangeably. PWHT is a more general term, and could mean things beyond stress relief, but in many cases, they'd be identical. If the drawing mentioned gives no more detail than that, it is very ambiguous.
RE: Stress Relief Vs. PWHT
The reason why I asked this question is well explained in your reply.
Does the Code differentiate in two of them? If yes, then how?
@metengr:
Can you mention any typical case where you would like to have stress relief only and No PWHT.
Looking forward to replies.
Regards,
Owais
RE: Stress Relief Vs. PWHT
I have seen on periodic occasion stress relief at or below 700 deg F performed for austenitic stainless steels after certain heavy machining or cold forming operations. I don't believe this is effective, however some still do it.
RE: Stress Relief Vs. PWHT
The definitions are not as trivial as you might suppose. For example, weld qualification under API-650 is done to ASME B&PV. Well, per code, PWHT is an essential variable, and that PWHT is exactly the same operation as the "stress relief" required by API-650. So if you assume that stress relief is not PWHT, you misapply the qualifications.
RE: Stress Relief Vs. PWHT
RE: Stress Relief Vs. PWHT
PWHT applys only to items that have been WELDED
Stress Relief applys to all items. (ie Mechaincal Stress, Heat Stress, etc etc.)
Codes of Construction apply these in different ways.
RE: Stress Relief Vs. PWHT
we've all come across these confusing terms. At the company I worked at, we used the term "post forming heat treatment" and "post weld heat treatment". We used these because post forming heat treatment could be a stress relieve, or normalising (eg. on a dished end). We would then use post weld heat treatment for when the vessel was "stress relieved" in an oven after fabrication.
john
RE: Stress Relief Vs. PWHT
Stress Relief = Post Hotwork/Coldwork heat treatment i.e. heating below the A1 line followed by natural convection cooling. Intended to be done on individual parts of a vessel after they have been shaped for assembly to remove the residual stresses.
PWHT = Post weld heat treatment i.e. tempering to modify the grain structure of HAZ to avoid the risk of brittle fracture.
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.
Regards,
Owais
RE: Stress Relief Vs. PWHT
Your interpretations look OK with the exception of one slight modification to PWHT;
i.e., tempering to reduce the hardness of the base metal HAZ to avoid the risk of brittle fracture.
RE: Stress Relief Vs. PWHT
Heat treatment is a term applied to metalls for several purposes including stress relieving after machining, other popurses are tempering to remove brittleness and even quenching to hardn it.
RE: Stress Relief Vs. PWHT
"I have come across a pressure vessel drawing which says:
Stress Relief : YES
PWHT : NO"
The way I look at it,
the treatment you give the pressure vessel in the name of stress relief is enough; no further heat treatment is necessary as the same also serves the purpose as post-weld heat treatment.
Generally, it is assumed that these are indicated in the drawing in the order in which the processes are to be carried out.
By the way, being a pressure vessel, welding is involved in any case, so, the question of PWHT being relevant only where there is welding is anyway not relevant.
Please correct me appropriately. Thanks everyone.
RE: Stress Relief Vs. PWHT
No, incorrect - see above.
No, incorrect - see above.
RE: Stress Relief Vs. PWHT
We have managed to establish that they are not the same.
RE: Stress Relief Vs. PWHT
Please have a look at the posts dated Feb 18, where metengr and I have concluded with consensus that Stress relief and PWHT are two entirely different things. We had also established the proper statements defining each of them separately.
Can you repeat your question with an explanation as what caused you to think that when stress relief is performed then PWHT is not required?
RE: Stress Relief Vs. PWHT
Thank you, OwaisGillani.
I've been attentive in the discussion all along.
My post that "We have managed to establish that they are not the same" is the same as your "metengr and I have concluded with consensus that Stress relief and PWHT are two entirely different things". Only the wording is different.
"when stress relief is performed then PWHT is not required" - that is how everyone understood my statement, which is not so. What was said is, to quote along with the context,
"the treatment you give the pressure vessel in the name of stress relief is enough; no further heat treatment is necessary as the same also serves the purpose as post-weld heat treatment.
Generally, it is assumed that these are indicated in the drawing in the order in which the processes are to be carried out. "
Now let me explain my point more clearly.
By stating "Stress Relief: YES; PWHT: NO" it was apparently intended to say that in this specific case, you must carry out the stress relief procedure. Thereafter, no heat treatment is required. It was necessary to say so because as per your practice you may decide that the two procedures are required, independently of each other.
If we ask ourselves another pertinent question, the issue may become more clear. Let's say the process carried out on the material is such that the Code prescribes stress relief. Let's also say that we carry out welding operations on it which necessitates PWHT as per the Code. We first subject it to stress relief. Do we now forget all about the stress relief done and do PWHT or do we factor in the effect or stress relief done? Should we do it anyway again for Code compliance? I think, not.
I appreciate the chance you've given me to explain in more detail, OwaisGillani and metengr.
RE: Stress Relief Vs. PWHT
quote
"Let's say the process carried out on the material is such that the Code prescribes stress relief. Let's also say that we carry out welding operations on it which necessitates PWHT as per the Code."
unquote
When stress relief is required after the process, it should be performed before welding. After welding, if PWHT is also required then it has to be done regardless of whether stress relief was done or not. Moreover the PWHT requirement is a consequence of welding, so what ever was done prior to welding is out of context.
The temperature vs holding time requirements for PWHT are stringent then those for stress relief. Thats why some times, localized PWHT is also performed.
RE: Stress Relief Vs. PWHT
The terms PWHT and stress relieve are used interchangablily in the Code to mean the same heat treatment; which is to heat treat a welded joint below the lower transformation temperature. "Purpose" and "result" of this heat treatment is:
1. to release locked in stress caused by welding or deformation at a joint. Thus, call "stress relieve" by mechanical engineers. Although stress relieving can be achieved by mechanical means (like peening, ultrasonic vibration, etc.,)as well but is generally not permitted as an instrustry practice.
2. to restore some ductility in the weld metal and lower the hardness in the HAZ. That's why metallurgist calls this a "heat treatment".
Hence, the two terms used in the PV industries and in Codes/Standards (see Sect VIII, API-650, etc) are one and the same with respect to actually doing it. You get both benefits in one shot.
Because tensile and other mechanical properties degrade when subjected to high temperature, you only want to do a PWHT (or call it stress relieve if you feel like it) once in the shop only, not counting a repair. Don't do it twice. Now, when we get into 1.25Cr and 2.25Cr steel then there more intermediate heat treatment terminologies, but let not get into that. If I were your Client, I will want you to explain to me why you doing 2 PWHT in the shop.
Hope this clears it up.
RE: Stress Relief Vs. PWHT
RE: Stress Relief Vs. PWHT
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.
RE: Stress Relief Vs. PWHT
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
RE: Stress Relief Vs. PWHT
Mike
RE: Stress Relief Vs. PWHT
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.
RE: Stress Relief Vs. PWHT
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
RE: Stress Relief Vs. PWHT
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
RE: Stress Relief Vs. PWHT
Doug
RE: Stress Relief Vs. PWHT
RE: Stress Relief Vs. PWHT
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
RE: Stress Relief Vs. PWHT
genb