Mostly we are working with 304, 304L to 304, 304L or 316L. We are making vacuum chambers so the walls, flanges and passages are all SS. We often weld mild low carbon steel stiffeners and gussets on to the larger external areas for structure. Depending on size/spans, function, and number of penetration plate range from 1/2" to 1-1/2" finished. They are often Blanchard ground for finish and flatness prior to pre-machining. Once assembled into the weldment, we often do secondary machining and sometimes follow up secondary welding. To add more heat generated distortion we often welding cooling tubing/channels to carry water all over these chambers. The channels require continuous welds on both sides. Tubbing is only slightly less problematic as we still do 4" x 4" stitch weld to assure good conduction.
Usually we do structural GMAW stitch welds on the exterior and GTAW full length all around on the interior for vacuum seal. We then leak test the chambers with helium to requiring at least 2 x 10-8 atm-cc/sec.
A typical chamber will be a box 66" long, 60" wide and 12" tall. Construction will be to weld materials ground to 1.25" for top plates, 1.13" for end plates, .5" for side and bottom plates. Tops and bottoms will almost always have external carbons steel stiffeners. We almost always do post weld secondary machining on the end plates that requires flatnesses and parallelisms from end to end of .005".
