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Cooling rate during PWHT

Cooling rate during PWHT

(OP)
Hi..

In a header assembly of diff thickness/size materials; which has to be heat treated inside a furnace, how to determine the cooling rate.?
How the code addresses deviations in cooling rate / heating rate? Any code references pl.

RE: Cooling rate during PWHT

It might help if you could describe your problem in a little more detail, including the applicable Code of Construction, and the purpose of the PWHT.

RE: Cooling rate during PWHT

Code of Construction would be nice:

ASME Sec. VIII Div. 1 - See UCS - 56 (d)
B31.3 - See 331.1.4
B31.1 - See 132.5

These paragraphs talk about a maximum rate of heating and cooling based on maximum thickness.....or 1/2 of max thickness for piping codes. Using max thickness will result in the cooling rate for thinner materials being slower than required, but that should not be a concern.....I say should not, as there are no thicknesses or materials mentioned.

RE: Cooling rate during PWHT

Section I would be my choice - LOL.

RE: Cooling rate during PWHT

(OP)
Sorry for the delayed response.
I have a header pipe with D606/~70mm thickness to which pipes of ~15mm & few nos of 5mm thick are welded.
All are P1 materials. How do i fix the soaking time? as my main pipe, dished end, stub etc are varying in thickness.
I have a lug of approx 40mm thick welded which is P5 material as well. How should i decide my PWHT cycle involving many materials / thicknesses ?

Secondly, while doing HT with 6 TCs, kindly clarify

During heating, if 1 thermocouple over shoots allowable rate of heating, whether it is allowed? What to do?
if 1 thermocouple drops temp by say 10deg in 10 min, & then regain heating whether it is allowed>
if 1 thermocouple overshoots allowable rate of heating for say 15 min, & then maintains very slow heating such that, hourly rate is maintained, is that allowed?

During soaking, if 1 thermocouple overshoots / drops the holding temperature , is it allowed? What to do in this case?

During cooling, if 1 thermocouple over shoots allowable rate of cooling, whether it is allowed? What to do?
if 1 thermocouple raised temp by say 10deg in 10 min, & then regain cooling whether it is allowed>
if 1 thermocouple overshoots allowable rate of cooling for say 15 min, & then maintains very slow cooling such that, hourly rate is maintained, is that allowed?

In big furnaces controlled manually, these kind of deviations bound to happen. Which code addresses these small issues? or where it is specified to ignore such deviations and that it doesnt affect quality of product?

RE: Cooling rate during PWHT

My suggestion, based on your questions above, first determine the code of construction and second, hire a reputable stress relieving company to perform the work.

RE: Cooling rate during PWHT

The cooling rate affects the type of crystal that is formed and also may generate thermal stresses in excess of yield stress( at temperature) and lead to residual stresses.Those thermal stresses vary by the square of the wall thickness. A fast cool leads to martensite formation ( required for P91 but not desired in the P1 material listed).

Another issue for furnaces is temperature unbalances in the furnace.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

RE: Cooling rate during PWHT

Metengr. But you can hire the best pwht company, they will give you good advise but the stamp holder bears the final responsaibility as he has to have and use his PWHT QC procedure.
Just commenting since you are the best on this subjects.

General Blr. CA,USA

RE: Cooling rate during PWHT

use ASME Section I
See PW-39

Holding Temp. (min) = 600~650C
Holding time = thickness of weld/25
Max. Heating Rate = 75C
Max. Cooling Rate = 95C
this is based on my experience

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