Bevel, Fillet Tolerances and PWHT gradient

[Sam Smith]

We are procuring an ASME Section VIII Div 1 designed heat exchanger from a vendor. Vendor has enlisted a few items in notes whose reference I'm unable to find. Does anyone know references of these?

Butt weld preparation tolerances: Bevel angle: ± 5 degree
Root gap: ± 1.5 mm
Face root: ± 1.5 mm

Fillet welding dimensions tolerances shall be -0 and +20%

Perform postweld heat treatment (for P No. 1 material) at 620 C ± 15 C for 1 hour, followed by slow cooling.
Gradient above 300 C, 150 C h maximum.

(In ASME Section VIII Div 1, PWHT holding temp is 595 C for P No 1 materials.)
Why has the vendor mentioned it as 620 C ± 15 C? Where do these values come from? Further, what do the values of 300 C and 150C h maximum mean (since both appear to be gradient) and where do they come from?

 

[david339933]

The tolerances on the weld bevel, root gap, root face come from the vendor's welding procedure (WPS). There is no reference in the Code to this as this is left up to good engineering judgement.
Fillet weld tolerances are stating that the weld can be oversized by 20% but not undersized at all.

A range is given for PWHT as it is hard to hit an exact number. The minimum is 595C. The Code stipulates that the rate of temperature increase can't be too quick after a certain temp(800F). Same as decrease in temperature. See UCS-56.

 

[EdStainless]

The cooling rate only needs to be controlled down to 300C. You can free cool or even fan cool below that.

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P.E. Metallurgy, consulting work welcomed

 

[david339933]

Cooling is the same as heating 800F (425C)

 

[Andrew ONeill]

I know for prevention of caustic stress corrosion cracking, the recommended heat treatment temperatures are higher than code required values. There is a specific NACE standard for caustic cracking that deals with this. It could be something similar in your case, or it could just be what the manufacturer does by default.

Andrew O'Neill
Specialist Mechanical Engineer
Rio Tinto
Australia

 

[Sam Smith]

Thanks for all the answers. Why is the temperature mentioned as above 300 C and not above 425 C (as per ASME Section VIII)? aAny reference for 300 C.

 

[r6155]

Please see ASME IX QW-184
(d) no concavity or convexity greater than 1/16 in. (1.5mm)
(e) no more than 1/8-in. (3-mm) difference between the fillet weld leg length.

Regards

 

[david339933]

I thought it was 600F in the past, but I think my memory is failing me as checking back as far as 2007 Ed. it was 800F. Could possibly be using piping code temp....B31.3 is 600F.

 

[r6155]

In ASME I Table PW 39-1 for P1 holding temp is 595C -705C
Regards