That depends on the materials and the thickness, if the tube is over 2mm wall and the steel a similar thickness, then the way I would tackle the job is with a stick welder and dissimilar metals rods, eutectic and easystrike do a range of rods where you can weld cast to steel I've done it a few times.
If you have a torch, pre warm the tube first to prvent cracking, if not then tap the tube as it cools after a weld with a small hammer.
The tube is unlikely to deform if its decent wall thickness, a circle is a strong shape, the steel box is more like to go on the blous.
THANKS finaly can you give me a spec on the rod i must use, the casting is +-6mm wall thiknes and the box is 2 mm thick.thanks once more
should the casting be red hot when i start welding some one told me to poet the welded asembly in sand to let it cool slowly