Heat treatment above and beyond 1

[Jman69]

Hey all, Ive looking through ASME Section 8 Div. 1 for an answer to my question and have not found anything.
My question is: Can a vessel be heat treated too much?

For instance, im reviewing a vessel package where the vessel wall is .75" in thickness. Not required to be heat treated by code, but for some reason the customer is requiring it. Ok. Well they ended up haveing to upsize one of the nozzles after this vessel was already heat treated. Rather than doing a local HT on the afected weld, they heat treated the intire vessel again. It was heat treated for an hour on the first round and another hour on the second round. Again, Can material be heat treated too much? Im sure there must me a limit but I do not see an absolute in the code book. Any guidence would be a help.

Thanks, Jmann

 

[racookpe1978]

What alloy vessel, attachments, pipe nozzles, pads, etc.?
What temperature for how long?

 

[Jman69]

It is made of carbon steel, three nozzles with repads. This vessel as I call it is actually a sphere. Two hemi heads welded together with three nozzles.

 

[racookpe1978]

What MATERIAL and what specification metal?
Again, what times at what temperatures was the vessel heated to?

Heat treatment is a spec ific and very important control function of the EXACT MATERIAL BEING treated to very specific times, temperatures, heat-up and cool-down rates!

 

[Jman69]

SA-516-70
1st) 1 hr at 1150 heat up and cool down per ASME
2nd) 1 hr at 1150 heat up and cool down per ASME

 

[metengr]

For same pressure vessel materials, repeated PWHT cycles can alter notch toughness properties of the weld metal. This would have a direct affect on if the vessel was qualified with impact testing.

If the vessel was not qualified with impact testing, I would have a weld coupon done and run through a first and second PWHT cycle to verify mechanical properties, and ensure correct information on the data report.

 

[Jman69]

I appreciate your answer, which I’m sure it does affect the material. My question is, where is it in code that states, if it does, that it has violated ASME code? That is all I have to stand on. If it does not violate code, than there goes my argument.

 

[metengr]

Jman69;
The code provides minimum rules for material properties, design, fabrication and construction, which cannot be altered. If one selects a fabrication step which is not needed, one needs to be prepared to perform supplemental testing to ensure no harm to the material.

Again, a fabrication step was performed which is essentially a nonconformance which can be easily addressed by the advice I gave above. If the weld coupon which goes through 2 simulated PWHT reveals no significant change you are fine.

 

[Jman69]

Im sorry. I dont want to keep dragging this out. One more question. If the weld procedure used was qualified with an equal or more amount of heat treatment time, would that accomidate the testing you are speaking of?

 

[metengr]

Yes. But here is something that was in the back of my head and finally found it....
See Section VIII, Div 1 UCS-68 Design


(c) If postweld heat treating of a pressure-retaining
weld is performed when it is not otherwise a requirement
of this Division, a 30°F (17°C) reduction in impact testing
exemption temperature may be given to the minimum
permissible temperature from Figure UCS-66 for P‐No. 1
materials. The resulting exemption temperature may be
colder than −55°F (−48°C) when the PWHT exemption
in (b) is applicable.

So, for P-No 1 materials the code states that you can actually take credit for a reduction in MDMT by PWHT. So, precedence has been set. In your case it was done two times and I would still expose the weld coupon for 2 PWHT cycles because if the WPS used in production was not qualified for PWHT you have a nonconformance issue. This way there is no argument.

 

[Jman69]

Thanks as always metengr. You too racookpe1978.

 

[Bmechz]

Dear Jman69,

You run a PQR with 2 cycle PWHT and prove that the mechanical properties are within the range..

Above mentioned clause UCS 68 is for exemption of impact testing, not related to the mechanical properties of materials after PWHT..

Bmechz

 

[Duwe6]

Please note that the customer may not be 'excessive' in requiring PWHT. Some services demand PWHT, such as strong amines and liquid ammonia. Being a service-related item, it is not addressed in Sect VIII or IX.

 

[marty007]

Jman69,

With CS materials, long-term heat treatment will "soften" the material. If the heat treatment is performed for a very long time, it is possible that the tensile strength of the material could drop below the code allowable.

In digging through the code however, I have never found any specific paragraphs that require testing for this. Good practice would say you should find some scrap and do a simulated test then check for properties, but it's not required. All the code states is a minimum PWHT time.

Cheers,

 

[SnTMan]

Jman69, per Sec VIII, Div 1. mechanical properties of P-1, Group 1 & 2 materials are more-or-less considered to be unaffected by PWHT. See UCS-85, which exempts these materials from "simulated PWHT", which insures that as-fabricated materials mechancal properties meet specifications.

Regards,

Mike