I'm welding an exhaust system.  The welds are 1) tube to tube miter cuts, 2) tube to heat shield, 3) sensors and a flange to the tube, and 4) tube to the muffler and catalytic converter.  The tube, heat shields, converter, and muffler are ASTM A439 material.  The sensors and flange are 304 material.  I've been welding the 439 to 439 joints using ER409Cb filler material, and ER309L for the 439 to 304 joints.

I'm wondering if this is my best choice? Also, I'm wondering if I could use one filler material?

AWS D1.6, Annex F, F2 (2) explains, "weld metal equal to or exceeding the base metal strength."  So. I should pick the weakest metal to select the Filler metal?

What do you think?

Thanks,

Keith