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Post weld Heat Treatment Section. I and VIII

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Vattappilly

Materials
Jun 27, 2005
7
I believe in ASME Section.I, PW-39 doesn't mention about thermocouples.

In Section.VIII div.1, I guess UCS-56 doesn't talk about it although UW-40 mentions about it in UW-40 (c).

The reason why I brought this up is because we send all section.VIII vessels outside for PWHT just because we do not have thermocouples here and that way we can't meet the requirements of Section.VIII. But if you read that paragraph, it says, "where more than one pressure vessel............."

Does it mean that if we just put one pressure vessel in the furnace, can we get away without putting thermocouples ?

I saw the foot note:10; can it be just applicable to cases where we want to fully load the furnace by putting more than one vessel or vessel components?

and again why is section.I not insisting on thermocouples while section.VIII makes it mandatory?
 
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Vattappilly;
No. Heat treatment temperature must be monitored, regardless. What paragraph UW-40 (c) is stating is if you have more than one component in a furnace charge, each component temperature shall be monitored using TC's - not just one component.

You must understand that the ASME B&PV code subcommittees handle their own work and agenda. In some cases, prior to revising their respective sections, the subcommittees will send proposed revisions for review and endorsement.

My guess is that Section VIII SG/SC was more conservative in their original approach for regulating temperature measurement because of the type of representation on the subgroup and subcommittee, and the various types of pressure vessel steels in comparison to the common carbon and low alloy steels used in Section I components.
 
I can't comment on Sec 1, however for Sec VIII vessels review UCS (d)(2).
 
I apologize, I should have said UCS 56, (d)(2). There, the use of a temperture monitoring device is called for. The word 'shall' indicates it's imperative. The only practical way to comply is to attach thermocouples at maximum intervals of 15 feet in each direction.
 
Thanks for the comments metengr and weldtek. I can't fully understand metengr's comments about the pressure vessel steels.

I mean both Table PW-39 as well as Table UCS-56 talk about more or less same materials, I mean same P numbers. So I don't get why they treat it conservatively in Section.VIII.
 
Ok. Let me re-phrase - If you read and compare between the various Code Sections (for example, I and VIII) there are process differences in how PWHT is to be performed - cooling rates and temperature measurement.

What I was trying to say was that Section VIII subgroup wanted to be more detailed in how PWHT is to be performed in comparison to Section I. Yes, the listed materials are the same. However, detail process steps could be handled differently, and Section VIII wanted to be more concise or specific in how heating, cooling and temperature measurement is to be handled.

I hope this helps to address your question.
 
shop inspection for PWHT and after PWHT

Postweld Heat Treatment

This operation can be defined as the heating of weldments immediately after welding in order to temper, stress relieve, or provide controlled cooling. It prevents the formation of hard or brittle structures prone to cracking in services. Postweld heat treatment is mandatory for certain alloys, combinations or alloys, and material thicknesses as outlined in the various codes and standards, such as ASME Section VIII Pressure Vessel Code. Consequently, welding procedures for such materials and/or thickness will contain the provisions for heat treatment.

Fabricators may have difficulty with prostweld heat treatment due to shop practices rather than the treatment itself. Cracking can occur in vessels that are held for heat treating until a full furnaces load is ready.

In general, at normal heat treating temperatures the time at temperature is specified as one hour per 1 in. (25 mm) of maximum thickness for vessels up to 2 in. (50 mm) thickness fabricators may suggest that since the component is being heated form both sides half-an-hour at temperature for each 1 in. (25) thickness is sufficient – but the one hour requirement should be maintained.

For vessels over 2 in (50 mm) in thickness, the usual holding time is two hour plus 15 minutes for each additional 1 in. (25 mm) over the 2 in. (50 mm) thickness. Again, some fabricators may want to cut this time by half since the component is being heated form all sides.

Holding time may be an accumulation of multiple heat-treat cycles. It dose not necessarily have to occur all at one time. Care must be taken to ensure that the accumulated time at the correct temperature equals that required to effect the desired treatment.

There is a maximum furnace temperature for introduction of the vessel. This varies with the alloy but is generally 800º F (426º C) for carbon steels. Heating rate vary with alloy as well but are usually in the 200 to 400º F (93 to 204º C) range. Some manufactures will try to heat as rapidly as possible in order to reduce the time required.

Cooling is normally performed in the furnace at a controlled rate unless the component si to be quenched. If quenching is performed then the component must be removed form the furnace and quenched as quickly as possible. Delays will cause such problems as carbide precipitation in austenitic alloy and reheat treatment will be necessary.
 
Both Codes address the strict monitoring of temp,
some Companies are getting away from the requirement of thermocouples because: they do not know the Code requirement and their AI does not enforce it; the PWHT subcontractor just use their procedure believing there is not need for thermocouples even not supplying the required chats and proper certs (I have seen it all)
But just wait when these shops get cought in an ASME review and then they will know the hard way.

gb

 
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