Code PWHT Requirements 1

[Sharik]

Probably a silly question but: why if a pressure vessel is built to ASME Section VIII, Div. 1, out of SA-516 70, 25.4 mm thick, MAWP 3500 kPag @ 300C, PWHT would not be required (by Code).

If a steam drum is built to ASME Section I, out of SA-516 70, 25.4 mm thick, MAWP 3500 kPag @ 300C, PWHT is required (by Code).

Why is there a difference in PWHT requirements for the same material and same design conditions?

 

[metengr]

Why is there a difference in PWHT requirements for the same material and same design conditions?

Very good question. The simple answer is that the Section I SC is the keeper of their own domain (Code book section) and considering that Section I has been around before Section VIII, past precedence is the rule regarding PWHT. There is a move to relax the PWHT exemption requirments, and it will happen and be more aligned with Section VIII.

 

[JoeTank]

metengr,
What is the technical basis of PWHT for CS materials? is it based entirely on residual stress due to weld shrinkage? Would there be any ASME VIII basis for a 2" thick shell of SA516-Grade 70 without PWHT is the design stresses were keep very low?

Joe Tank

 

[vesselfab]

if the vessel is asme section VIII and 2" thick, it WILL be PWHT. if standard P1

vessels over 1.25 thick (unless preheated per code rules up to 1.5") shall be PWHT if P1

any vessel over 1.5" WILL be PWHT if P1

 

[Mr168]

Residual stresses as you mentioned, reduction in hardness in the HAZ, improved notch toughness and ductility, and to some extent the diffusion of hydrogen can all factor into the PWHT of carbon steels.

 

[unclesyd]

I believe that PWHT is for stress relief or at least 90% is for the P1 materials.

A long time ago the what we now call PWHT was called stress relief.

 

[unclesyd]

I believe that PWHT for the P1 materials is/was for stress relief or at least 90% of it's benefit was.
A long time ago for P1 materials, we now call PWHT was called stress relieving. There was never any mention of any other reasons for the thermal treatment.

Having said that there was a memo from Dupont written in the late 40's where one of their metallurgist was recommending that piping for certain waste streams and process piping be PWHT to help prevent cracking in highly stressed areas. This was still just a stress relief but carried an additional benefit of preventing stress cracking.

 

[metengr]

For Section I stress relief was the reason, and I believe the reason was because of caustic SCC in boiler tubes/lower drums before water chemistry improved. The more conservative PWHT rules have remained in force. We are looking to relax the PWHT exemptions for PNo 1 material in PW-39 to align more with UCS-56.

 

[gengi]

Dear Sharik,

If I remember well also somewhere in the ASME VIII Div.1 is the PWHT required <8for any thickness,even lower than 1.1/2") if the vessel/equipment is STEAM GENERATOR...
may be UG......but I am not sure. please check.....

I would check by myself and then I will let you know.

Let me know if found.

rgrds

GENGI

 

[stanweld]

Some of the reasons why PWHT was required based on thickness rests with the higher carbon equivalents used in manufacture of thicker carbon steel plate in order to meet minimum tensile requirements. This coupled with the quench effect of the thicker materials could easily lead to a martensitic structure (also contribiting to higher welding residual stresses) in weld metal and HAZ, which could result in cold cracking.