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

 

[metengr]

No, not technically. Stress relief is typically performed at temperatures lower than post weld heat treatment (PWHT). The purpose being to reduce residual stresses from fabrication, exclusive of welding. Stress relief can also be achieved thru mechanical methods, like vibratory stress relief.

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.

 

[PEHasan]

Stress relief can also be achieved by applying a preheat at a minimum temperature of 200 degree F during welding.

PEHasan.

 

[JStephen]

I've never heard of preheat being considered stress relief. Yes, it may accomplish that, then again, using different size rods or welding in warmer weather reduce stresses, but that's not what is normally meant by stress relief.

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.

 

[OwaisGillani]

@JStephen:

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

 

[metengr]

Certainly. During heavy machining or cold forming operations on carbon or low alloy steels, a stress relief could be performed at temperatures BELOW the reported post weld heat treatment temperature to relieve only residual stresses. No welding was performed.

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.

 

[JStephen]

As for the ASME code differentiating them, does it ever use the term "stress relief"? If not, no need to differentiate them.

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.

 

[metengr]

If stress relief = PWHT in API applications so be it. PWHT in ASME Code space is exactly what it is - a post weld heat treatment. If no welding is performed on the item, there is no PWHT.

 

[Xce]

Hehe pretty good points, but lets make this easy for all of you.

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.

 

[johnnymist2003]

Hi all,

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

 

[OwaisGillani]

If I consider following interpretations for the terms on my drawing, is it okay?

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

 

[metengr]

OwaisGillani;
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.

 

[abba7114]

What an interesting posting, I think what is missing is to differentiate between heat treatment and stress relieving.
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.

 

[panduru]

...and, returning to the original point,
"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.

 

[metengr]

We have already answered this question.

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.

No, incorrect - see above.

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

No, incorrect - see above.

 

[panduru]

So, that leaves the original question still unanswered: "what do you make of "Stress Relief: YES; PWHT: NO"? What must have been the intended meaning of the spec, in the said drawing?

We have managed to establish that they are not the same.

 

[OwaisGillani]

For "panduru" who joined us late in the discussion.

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?

 

[panduru]

@OwaisGillani:

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.

 

[OwaisGillani]

Well punduru, your explanation has made it easier for me to answer you.

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.

 

[vesselguy]

I don't know about you guys, but this thread is getting long and really screwed up.

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.