When welding to B31.3, your weld procedure qualification and qualification of welders/welding operators is in accordance with ASME Section IX. Section IX is not a fabrication code and therefore does not spell out where/when PWHT is required. It does, however, dictate variables, of which changes would require requalification of the welding procedure. In your case, Section IX variables for PWHT include QW-407.2 which is a supplementary essential variable (where impacts are required) and says "A change in the postweld heat treatment temperature and time ranges". It also goes on to say that the procedure qualification test shall be subjected to PWHT essentially equivalent to that encountered in the fabrication of production welds, including at least 80% of the aggregate times at temperature.
To clarify, this says that if your test weld were subjected to 1 hour PWHT at a given temperature, your WPS could show 1hr/.8 or 1.25 hours maximum at temperature. This means that your production weld can only see 1.25 hours at temperature. For most materials, B31.3 requires 1 hour minimum at temperature, therefore your previously PWHT'd welds could not be subjected to a second hour. If your situation meets this, you should only PWHT the new welds.
Hope this helps.
I tend to agree most closely with Fredspoppy response. The PQR supporting the WPS should say how long the original PWHT was (in many cases, knowledgable welding engineers specify 8 hours or more in case of repairs). For B31.3 and ASME IX purposes, as long as you don't exceed the PWHT time qualified on the PQR you should be okay rebaking the entire spool but if you're still uncertain then the safest bet is to just do localised on the welds that were cut-outs.
(in many cases, knowledgable welding engineers specify 8 hours or more in case of repairs)
I am yet to see such a "knowledgable" engineer.
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What material?
How long was the first cook?
Chances are, if it isn't an impact regime there will no problem with cooking it some more. Most likely ductility will be improved, but not always, and the tensiles (somewhat related to BHN) will hold to a certain min regardless of an extended PWHT time. In fact, if its a material with some precipitation hardening capability (say with Cu, Al, V/Nb) its tensile strength and hardness could go up with an extended PWHT.
Did you check it with a toothpick to see that it was fully cooked?
Al
I agree, I think its called "Larson - Miller parameter"
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The Larson Miller parameter (and the 50 or so others like it) is an equation used to take relatively low temp high stress rupture data and extrapolate to an estimated high temp low stress creep estimate. To put it crudely.
Let's put our common sense to work.
Two welds out of an unknown (for us) quantity of welds in a pipe spool that was heat treated were cut and remade.
Then, according to my common sense, only them should be heat treated by localized heat treatment, which can be carried out either by means of electric resistances or induction. Why should the entire spool piece sent back to the furnace? Cooperheat and Reliance are two companies that specialize in electric heat treatment of welds.
Do you Gentlemen agree with me?
Giovanni S. Crisi
Sao Paulo - Brazil
P.S I understand that induction is in little use nowadays and has lost its importance in favor of resistance heating.
Giovanni,
Metallurgical and mechanically, depending upon the alloy there is most often nothing wrong with re-heat treating. As for doing locals, that is certainly a good alternative but it isn't necessary. Most of what happens in heat treat happens quite early. 15 minutes to a half hour. The time is generally an issue more of thickness and homogenization than it is material changes.
If equipment for a local is available its a good alternative but I wouldn't call in a subvendor for a couple of welds or three. The decision to put it back in the furnace would be more economical, especially if you already have another batch fixin to go in.
But I still didn't notice the material we are talking about.
js,
what I tried to say (English being not my mother language sometimes I fail to explain what I mean), is that if only two welds of the whole spool need to be heat treated again, then there's no need to send the whole spool back to the furnace; it's sufficient to heat treat locally those two welds, either by resistance or induction, provided (and I agree with you) it's more convenient from an economical point of view.
Now, good engineering practice (and common sense) say that PWHT is the last thing to do in a weld, i.e., it's carried out after all inspection, X rays or UT and possible repairs have been made. It's useless to heat treat a weld and find out afterwards that it's got to be repaired because X rays have shown porosity.
Giovanni S. Crisi
Prof Crisi,
sorry, my x-ray & MT indicate cracks after the PWHT.
So I would suggest that NDT should be carried out before and after the PWHT.
Hydrotest / leak test should be the last testing.
Regards
Joey
1st. Well, of course, hydrotest must be the last test, that goes without saying.
2nd. I agree. X rays or UT testing should be performed before PWHT, so if they show any defect, it can be straighten out before PWHT; but they (XR or UT) should be performed also after PWHT, so if it (the PWHT) has developed any crack, it can be corrected before the hydrotest.
Giovanni S. Crisi
P.S. Here in Brazil, a "defect" is unacceptable and must be corrected, while "discontinuity" is acceptable and doesn't need to be corrected. Is that so in the USA?
Discontinuities aren't necessarily acceptable.
Same here. When a discontinuity exceeds an acceptable size it becomes a defect.
but by definition, it is still a discontinuity, right?
Here is how AWS A3.0 defines the two
discontinuity: An interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. a discontinuity is not necessarily a defect.
defect: A discontinuity or discontinuities that by nature or accumulated effect (for example crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. the term designates rejectability.
So to answer the Professor's original question....it is a little bit different here because he says discontinuities are acceptable. Discontinuities can be either acceptable or rejectable here.
That's not how I see it. discontinuities are acceptable. When they exceed an acceptable size then the discontinuity become a defect, not a rejectable discontinuity but a defect.
The first four words of the definition you gave above leads me to disagree. No biggie, though. We can agree to disagree.
Every weld ever made has discontinuities. The weld itself is a discontinuity.
The other thing is, froma pure logical point of view it oucld be argued, so to speak, that a defect is just a large discontinuity.
However, for the purposes of engineering and quality control as precise a definition as possible is required to establish criteria.
The industry standard definitions are the AWS documents.
In order of severity
discontinuity
indication
flaw
defect
The last one being considered a reject unless engineering determines that the failure of that component will not cause failure of the system in question, in which case, it is kicked back to the term flaw.
Prof Crisi,
When I was on the client side, I'm only interested to see the results of NDT after the PWHT. However, many contractors will go for NDT before PWHT.
I agree with your interpretation of defect and discontinuities. But will you agree with me that a discontinuity detected before PWHT has possibility to become a defect after PWHT? If that so, will you agree to repair the discontinuities before the PWHT?
If you're on the contractor side, then think of the consequences before you make a decision especially when you are doing those big pressure vessel and you have a very tight delivery schedule and delay penalty.
Regards
Joey
"But will you agree with me that a discontinuity detected before PWHT has possibility to become a defect after PWHT?"
It depends on the alloy and the defect. Cracks are the most problematic. If a preexisting crack undergoes the stresses and strains of the heat treat regime they can become detectable. If an the alloy is sensitive to PWHT cracking you can have post HT results different from those prior to HT.
Slag, fusion, porosity, penetration, etc. really makes no difference.
Some alloys by code (31.1) are required to be RT'd after PWHT. Most are not.
Joey, read my answer to your comment posted above and dated 4/16/2009 at 06:53. My answer is right below your comment. I think that it clears up my opinion.
I agree also with js55 that the big problems are cracks, or rather, microcracks that can become visible after PWHT. Porosity, slag, lack of penetration and fusion won't change their size because of heat treatment.
Giovanni S. Crisi
Prof Crisi,
I swear :) and I agree with js55. Crack is the main issue for NDT after the PWHT. Both of us are CRACK believer.
1. Just want to point out that some codes indeed require the RT after heat treatment. Like ASME code says that the final radiographs of all straight chromium ferritic welds including major repairs to these welds shall be made after postweld heat treatment has been performed. B31.3 says *For use under this Code, radiography shall be performed after heat treatment. And furthermore, it may not be easy to say to the client that their RT requirement after PWHT is an overkill inspection for material SA 516 Gr.60
2. Lack of fusion and lack of penetration are obvious indications that may need to be removed prior to PWHT. For those porosities or worm holes (having sharp tails), their sizes are important to know, for boundary cases, I will suggest to do the repair prior to PWHT. You would not want to wait for client rejection on final RT. Disagreement in radiograph interpretation is inevitable. Fyi, I've witnessed slapping incident before and shouting match in viewing room (sub-con QC vs owner QC). Similarly in school, there are those friendly professor lenient to their students and there also those terror professor who likes attention and recognition.
Regards
Joey
Where B31.3 says" For use under this Code, radiography shall be performed after heat treatment
regards
Robin,
It is only for P Nos 3,4 & 5 and it is Section 341.3 (a)
Regards,
Shane
page 135 of ASME B31.3-2006
Joey, I'm somewhere in between those types of professors you described.
Giovanni S. Crisi
By Joey 
Date 05-05-2009 05:41
Edited 05-05-2009 05:51
I want to be your student Sir!!
I'm interested to learn the Jiu-Jitsu for self defence while doing my duties as an inspector :)
Regards
Joey
B31.3 ; Table 331.1.1 Requirements for Heat Treatment ; you can also get some good information from ASME Section VIII Div 1 ; UW-40 (Procedure for
Postweld Heat Treatment).
It depends on the pipe size (dia x thk), it would be easy to use localize PWHT for your 2 welds rather than the entire spools. Large spools in furnace require proper supports or brace to avoid sagging. Choose which method is more effective and economical.
There are times that you have to repeat the PWHT not only once, twice,.. double baking in furnace are possible to happen not only due to repair / or modification after PWHT, but also due to trouble encountered during the time of PWHT operation (eg. power failure during soaking time, broken thermocouples, unable to reach the required soaking temp due to poor wrapping preparation, wrong heaters used, too fast heating / cooling rates, PWHT technicians caught cheating by Inspector etc,). Take note that there are many tricks in PWHT operations, some good inspectors are able to detect the trick made by technicians which may caused rejection of PWHT graph, hence, PWHT need to redo.
B31.3 ; Table 331.1.1 mentioned the min - max temp as well as the minimum holding time. It also stated the maximum BHN.
I believe that you have to redo the PWHT if you failed to achieve the heating requirement and BHN requirement.
Regards
Joey
Joey,
what's the maximum BHN allowed by Table 331.1.1? Back in my days of erector engineer it was 240. Is it still today?
Giovanni S. Crisi
Prof Crisi - In Table 331.1.1 - stated 225 for P3 & P4 and 241 for P5 & P6.
But at worksite, will see first the project specification to find the BHN requirement.
It depends on material and sometimes operating media.
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