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Up Topic Welding Industry / Technical Discussions / What is different PWHT and post heating?
- - By tigerlee Date 10-23-2009 14:30
What is different PWHT and Post heating?
Accoring to AWS D1.1, table 4.5 37)PWHT is essential variable that means include post heating or not?
Generally after welding we had been post heating for protection of hydrozen crack.
But heating time and temperatue is little different due to base metal thickness.
I request post heating about over25mm thickness carbon steel to welder.
But third party inspector asked all work stop, because that(post heating) is not written on WPS.
Is that right?
post heating information need to written on WPS?

Please help me!!!!!
Parent - - By 3.2 Inspector (***) Date 10-23-2009 15:03
PWHT and post heating, or hydrogen bakeout as it's sometimes refered to is two different things, with different purpose.
When post heating the weld, you only heat the weldment up to 250-300 degrees C, for PWHT you heat it up to 550-740 depending on alloy.

I agree with the third party inspector, pre, post and PWHT should all be listed on the WPS used for a given joint.
I doubt very much - regardless of code - that you have to qualify a new procedure in case of post heating (hydrogen bakeout)

Let's say your WPS has a thickness range 5-30 mm......
In order to make it a bit easy on yourself you can write the following, note this is just an example;

5-10 mm 1 hour post heat
10-20 mm 2 hours post heat
20-30 mm 3 hours post heat

You mention thickness of "about over 25 mm" are you sure this particular thickness don't require PWHT?

Parent - - By tigerlee Date 10-24-2009 08:44
Thank you for your reply.

but I want to know where is mandatory (essential variable) requirement of post heating on AWS D1.1.
Generally post heating is apply to welding line and adjacent area.
and it had been done by fabricator's experience as per AWS D1.1(2004) C8.5.5.

My question's key point is "post heating is essential variable"? or for only recommanded item?
it need defined and then we can follow up continue.

please advise one more.

Thank you

Parent - - By 3.2 Inspector (***) Date 10-24-2009 09:19 Edited 10-24-2009 09:23
I should have kept my mouth shut as I am not aware of the AWS D1.1 requirements.
My common sense tells me that if you do the qualification test without hydrogen bake out, you can write your WPS with or without it.
However if you do your test with a hydrogen bake out, it must be on the WPS.

Instead I will ask you this, why is the hydrogen bake out needed?
What is your welding process?
How is the ambient conditions during welding?
How do you store your filler materials?
Do you apply preheat?

Parent - By tigerlee Date 10-26-2009 07:27 Edited 10-26-2009 07:34
I reply as below

why is the hydrogen bake out needed?
==>It is my experience. not required contract spec.
What is your welding process?
How is the ambient conditions during welding?
==>20 ℃ degree
How do you store your filler materials?
==>Inventory room and daily issue to shop
Do you apply preheat?
==>Yes, I do according to WPS
Parent - - By js55 (*****) Date 10-23-2009 15:26
I'll take a stab at this. And lets see where this one goes.
AWS has, unless someone finds something I am not aware of, been remarkably silent on this. You will not find any clarification of PWHT as opposed to bake out like you do in ASME. Therefore, I do not think you have much of an argument to oppose even though IMO your TPI is being a s#&$head.
In my opinion, and from an ASME Section IX perspective, bakeout is not PWHT thus, IX clarified them seperately in their definitions, even though the language of IX and D1.1 are almost EXACTLY the same as pertains to PWHT.
I think this is justified when you consider what PWHT is intended to do and what a bake out is intended to do, though this discussion could get esoteric real fast.
If bake outs are PWHT then what about heat straightening per 5.26.2? Are they required to be in the WPS? And those temps will certainly be higher than a bake out. Isn't this a "heat treament after welding"? What does 'after' mean? I they aren't, then how is it bake out is? If they are then why aren't they required on the WPS?
Sometimes common sense gets lost in an ego trip.
Parent - - By 803056 (*****) Date 10-23-2009 16:28
My personal opinions on the subjects are;

If the post weld heat treatment is required, it belongs in the WPS. Whether it needs to be qualified is a subject that should be addressed by the applicable code. AWS D1.1 is intended for structural applications where both the carbon content and the carbon equivalencies of the steels are limited. The post weld heat treatments most often encountered are for the purpose of stress relief, but even for that purpose, stress relief is not a frequent operation for structural applications. Even the hydrogen bake-out is not a normal production operation in structural applications.

The same cannot be said of weldments that are used as pressure containment. ASME construction codes require post weld heat treatment for many applications and alloys. They are more definitive and place requirements on what, when, and how a material is to be heat treated. Section IX provides fairly explicit definitions on different types of post weld heat treatment when it comes to qualifying the welding procedure. 

AWS B2.1 can apply when AWS D1.1 isn't applicable and it is closely aligned with ASME when it comes to essential and nonessential welding variables, including post weld heat treating operations. Welding procedures that include PWHT have to be qualified using the same HT procedures as will be used in production. As such, B2.1 requires the WPS to be qualified with the appropriate PWHT conditions as follows: no PWHT, PWHT below the lower transformation temperature (which in my opinion would include hydrogen bake-out as well as stress relief at temperatures below 1330 degrees F), PWHT within the transformation temperature range (slightly above 1330 degrees F), PWHT above the transformation temperature, etc. There are additional requirements, but I think the general approach is very similar to Section IX.

As for flame straightening, it isn't a welding process, but if it is a controlled process, which it should be, there should be a procedure for it as well. Many of the states have specific requirements and limitations on how flame straightening is performed when the weldment is intended for a bridge.

Best regards – Al
Parent - - By js55 (*****) Date 10-23-2009 17:11
Since B2.1 refers to A3.0 I would have to concur that bake out has to be considered a PWHT process by official definition.
This is not so under ASME Section IX where the definitions are segregated.
Also, in regards to heat straightening, I would agree it is not a welding process. But neither is PWHT. And if you consider PWHT to be a bake out then neither is bake out.
Interesting dichotomy.
AWS combines PWHT and bake out by default and therefore bake out is NOT part of the welding process though it has to be on the WPS, ASME seperates them (See also QW-406.2, though there is no specificity as to how it must be addressed as a non essential) and therefore it can be considered part of the welding process.
Parent - - By 803056 (*****) Date 10-23-2009 17:54 Edited 10-23-2009 17:56
The point I was trying to make was that flame straightening is completely separate from the welding operation and it doesn't have to be qualified by any of the welding standards I've worked with.

Hydrogen bake-out on the otherhand is part of the welding operation and is typically performed after welding is completed (not always true, the Navy requires heat soaks on HY grades of steels for the purpose of hydrogen evolution at various stages of the welding operation). Since it is part of the welding operation, it should be included in the WPS and it should be included in the qualification of the welding procedure. 

This is where the welding consultant or the in-house welding engineer earns his keep. Even if the applicable welding standard doesn't list hydrogen bake-out as an essential variable, the WE should recognize when it is essental to the successful of the welding operation. The WE wanabe only takes into consideration those variables the welding standard mandates and is often surprised when success eludes him.

I hope you are doing well and the stars are aligned in your favor.

Best regards - Al
Parent - - By js55 (*****) Date 10-23-2009 18:35
Doing well, but I don't know about the stars.
I just never seem to be able to learn fast enough to have a command of what is required.
Its all just too big.
I'll be in St Louis for Code Week if your around and then in Houston (Westheimer) for 9 days starting on the 11th. If you're gonna be around let me know. Dinner is on me. Er, uh, my boss actually. But your always welcome here if you're passin through.
Hope all is well with you.
Parent - By 803056 (*****) Date 10-24-2009 04:29
I have a gig in Orlando the week after next and then it's off to FabTech. No trips planned for the Houston area until next year.

Thanks for the offer though. It's the thought that counts!

Best regards - Al
Parent - By ssbn727 (*****) Date 10-23-2009 18:41
I forgot how long it would be before the USN required to delay NDE on HY 80, 100 & 120 CJP completed deposits ,but I think it was 48 for some and 72 hours for the thicker sections in order to make sure that any hydrogen would be baked out, and there wouldn't be any possibility of delayed cracking that would occur if the NDE was performed earlier and within the periods I mentioned above.

Nowadays I read that they're using HY 130 and in some locations, HY 140 Q&T steels which probably have a different set of procedures when compared to the earlier versions of HY steels. I have to agree with both interpretations which make things from a code perspective clear as mud or better yet, some Black liquor sludge!!! :) :) :)

Parent - - By jarcher (**) Date 10-24-2009 00:40
I'd just like to throw a thought in the mix. Hydrogen bake out takes place so far below the lower critical temperature and any carbon steel tempering temperature that the effect on microstructure is virtually nil. Yes, there is some release of residual stress, around 20-25% if memory serves, but the only other effect is the one purposed, the removal of a contaminant from the weld area in less time than would occur at ambient temperature. OTOH, there are sound metallurgical reasons to describe and more tightly control PWSR and PWHT as both effect changes in mechanical properties. The point being that possessing a PQR where the base material and filler were successfully qualified without a bake out should allow the option of with or without, like the current option of using backing if the WPS was without backing. Just my two cents. Of course that doesn't help the orignal poster's situation until it is spelled out in D1.1.
Parent - By RonG (****) Date 10-24-2009 02:01
I confess I did not read all the post real carefully and I do not think I read any exactly like I want to say it so here goes. If I am being redundant, my apologies. :-)

For most intent and purpose PWHT is a Thermal Stress Relief with the single purpose to relieve residual stress from weld shrinkage.

Thermal Stress relief (PWHT) is a Time & Temperature Phenomenon. Meaning the higher the temp the less time needed to accomplish Stress Relief.

The same can be accomplished at a lower temp at a greater time interval.

Hydrogen Bake out would be just that, to prevent "Cold Cracking" but not necessarily a Stress Relief.
Up Topic Welding Industry / Technical Discussions / What is different PWHT and post heating?

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