One should consider the danger of welded in Arch Tubes which the National Board of Inspection Code allows (Part 3 - Repairs and Alterations S1.2.9.2).
The fear here (Australia) is because of relatively thin walled tubes (7swg) and relatively thin firebox plate thickness (3/8th"), The rapid and varying fluctuations in pressure and temperatures, plate and tube movement could cause a fracture at the plate or weld with the potential for disastrous results.
What has the experience been for Heritage Railways in the United States that use welded in Arch Tubes?
Attached are links that may interest, particularly the hot riveting of a tender.
http://www.powerhousemuseum.com/collect ... img=140281
http://www.youtube.com/watch?v=Am7e1ilnjgg
http://www.powerhousemuseum.com/collect ... otive_3265
My home preserved railway http://www.donriverrailway.com.au/