Practical limits - preheat temps ACHE

[RandEng007]

While playing around with preheat calculations for the repair on an existing ACHE (Sour service), it seems that we need a preheat of about 120 and in some case 150degC, to make the required hardness levels. (Depending on heat input, cooling rate etc) While interesting to play with these calculations, I do wonder what is the general experience out there in terms of the maximum preheat that can practically be achieved for a ACHE (fully welded header boxes) This expample is nothing special. P1 material, box with PWHT, but tubes in as-welded condition, 1" tubes on a traingular pitch. (The reason for worrying is that it just seems that the finned tubes will work against you constantly drawing away heat from the weld)