OP was buying Maxway TW Made frames Batch Ordered for Importation By Merry Sales Imports ..SOMA is one of the brand names ..
Post heating Steels , in heat treatment is the Quenching while hot .. and that rate can be controlled..
part of the function of double butting is by having it thicker
where the Welding/ Brazing takes place , its still adequately strong without more than a controlled cool down rate ..
There were Pre heating strips along the seams to be welded in putting a Submarine together, in the Shipyard.
but this was Way thicker steel of Much Higher strength alloys .