As a follow-up to Pablos comments, your AI will probably look for ultrasound thickness test of the suspect area, along with other points througout the pressure boundary. The data will be used to re-rate the vessel and will probably undergo an additional hydrotest to verify.
prex's interpretation is correct. Also, be advised that Sec. VIII Div.1 is intended for pressures below 3000 psi; beyond that ' deviations from and additions to the rules apply' meaning compliance with Div. 2 is required.
Why would you want to do this???
You have an entire vessel of stainless steel. What is your motivation for providing a carbon steel stiffener ring? To save money? How much will you save by choosing carbon steel over the same material, where expansion will not be a factor, where welding will not...