thePetroStreet

Dear all

Please share views on heat treat ment carried above tempering temperature for NACT plates? one of the case i have come across is stated below.


MOC: SA387 Gr11, Thickness 90mm, Delivery condition: Normalised+accelerated cooled+tempered (NACT)
Contract requirement: Cold formed D''end shall be normalised.
Since plate delivery condition is NACT, Mill certfies plate with 2NACT+3PWHT cycles on simulated coupons.
Fabricator will perform 2nd NACT (Fabricator assures that they do rapid quenching with water and tempering) after cold forming (since normalising is mandatory irrespective % strain)


I am of the view that so far no mill has disclosed how will they perform NACT (complete cycle and cooling rates), thou Simulation is done at mill, in my opinion it is impossible to replicate the same NACT process at fab shop, and can't represent the simulation done at mill.


Also it is not known how mill perform accelerated cooling (by direct water quenching or water sprinklers or forced draft....?)

Regards,

Inconel

Inconel,
What is the application of these plates? Boiler or some heat exchanger shell plates?

Ashfaq,

This is for pressure vessel shell plates.

Post forming heat treatment of NACT plates is normally advisable. I have gone through a special case of one heat exchanger, heads of which are in A387 Gr.11 Cl. 2 (annealed) material. Specimens have been made after forming operation & heat treatment and tested for intended mechanical properties.
My advise to you is to consult the mill to get the information regarding temperature range where embrittlement may occur.
And I agree that you would not be finding the NACT operation details from the mill. However, spare NACT plates can be made available for preparation of test specimens; and normalizing can be made on the same by the fabricator after forming operation.

Can you please write down your concerns / questions point wise? I am mixing what you exactly want to ask about.

@mechcolor,

Dear Friend,

So far no mill has disclosed how will they perform NACT (complete cycle and cooling rates).Do you have any info. regarding procedure of NACT operation from any steel manufacturer ?Please let me know is there any problem for doing PWHT above tempering temperature?

Regards,

Inconel

You would not be finding any information regarding accelerated cooling performed in the mill. What's your intention? Do you want to make a sample on which you may perform normalizing after forming of head and then to perform mechanical tests?
There is no problem in performing heat treatment (normalizing) above tempering temperature.
Post forming heat treatment (normalizing) is recommended after forming of heads if fiber elongation exceeds 5%. And after normalizing, mechanical tests shall also be made on specimens. Heat treatment of specimens shall be done following ASME UCS-85.

Inconel,
What I can add is that the mechanical properties of alloy steel are mainly determined by the tempering process after NACT treatment. And now if the material has to be welded or formed after subsequent stress relieving heat treatment is to be made, the temperature should not go beyond tempering temperature as it would adversely affect the properties of the material.
My advise to you is that the requirement of normalizing of NACT plates after dish head forming must be reviewed and the temperature during PWHT must not be taken beyond tempering temperature.