Ok, if you are joining carbon steel to 9% Cr, my suggestion is to use Inco 82 or 182 filler metal depending on the welding process. I am going on the assumption here that the 9%Cr is a Grade 91 material (conservative approach). If it is Grade 9, the information below will still apply.
If you are joining the carbon steel to Grade 91, you need to exercise absolute caution in terms of preheat, interpass temperature and post weld heat treatment. I would butter the entire weld prep on the Grade 91 material, and PWHT. The intent is to apply a 1/4" thick layer of Inconel weld metal over the entire weld prep for the Grade 91 material and PWHT - this will avoid having to subject the carbon steel to the higher PWHT temperature required for Grade 91 material. A similar buttering operation could be used for carbon steel – it depends on the weld joint thickness. Over 1” thickness, I would butter and PWHT the carbon steel side of the weld prep.
The preheat for welding Grade 91 material should be 450 deg F with an interpass temperature at or below 800 deg F. Allow the buttered weld prep to cool to ambient temperature and post weld heat treat at 1380-1425 deg F for 30 minutes at temperature(this assumes a 1/4" thick butter layer). The narrow PWHT temperature band is critical and must be followed for Grade 91 material.
After PWHT of the Grade 91 material, weld to carbon steel using a 200 deg F preheat with Inconel weld rod.
