First thing to check out is UCS-66 and see what the impact testing exemption temperature is determined to be for the material.  You will have to check ALL components of the vessel in order to determine the vessel MDMT.

You may find that...
Impact testing is required (or nor required)
PWHT could be used as a option to lower the MDMT determined from UCS-66, but only if it's not otherwise a CODE requirement.
You can also lower the  pressure rating of the vessel as a means to lower the MDMT.

The Code has rules for all this stuff.  But, true to Code format, it's scattered all over the place.  And, the best info is often in the footnotes and tables.

Hope this gets you started in the right direction.

Steve Braune
Tank Industry Consultants
www.tankindustry.com

This could be considered an Alteration. Suggest you obtain a copy of the Data Report first. Look at NBIC Appendix 6. Contact the local jurisdiction. See ASME Div1 UG-20 as well as UCS-66.

You may need to consider WPS qualifaction beyond PWHT. This rerate may need to be performed by an "R" stamp holder.

Could be easy-could be cheaper to replace the vessel.

ARCH01,
You should specify the grade of your carbon steel material.

Don't forget to consider all welded attachments.  Also, there may be an opportunity to consider stress ratio, which can give you a temperature credit, in particular if the low temperature condition is at a lower pressure than the normal operating or design pressure (e.g. if your colder temperature is due to autorefrigeration due to some depressurizing event).

Cb4 and SteveBraune are right.  Read UCS-66 carefully.  You may not have to do anything.

Please follow up the below sequence by code.

Impact test requirement and mitigation of carbon steels in ASME Section VIII, Div.1 up to Ad.02

1. Base Metal;
(1) Basic requirement/exemption per UCS-66
(2) General : UG-20(f)
(3) Yield strength and thickness: Table UG-84.4 and UCS 66(d)
(4) Stress ratio (SR): UCS-66(b)(3) and Fig. UCS-66.1
(5) Heat treatment: Fig UCS-66 for Normalizing for base metal

2. Fabrication; UCS-66(h), UCS-68(c) for PWHT, Fig UCS-66 for Normalizing for vessel
 
3. WPQ and Production Impact Test; per UCS-67

If the vessel cannot meet all above, it shall be replaced.

Thomas Eun
thomaseun@shaw.ca

If permitted by the Jurisdiction, you could also use the procedures of API-579 and perform a fitness for service evaluation using the principles of fracture mechanics.

Steve Braune
Tank Industry Consultants
www.tankindustry.com