Can I build smoker by cast iron

It is possible, but cast iron is very heavy making it very expensive to ship.

It would hold the heat, so this may work for you.

Keep us informed....

Well a few things come to mind right off the bat, like mentioned it is very heavy. Welding cast iron requires special welding rod ( a high nichoal content). The cooling process needs to be control for a quality/durable weld. I don't believe you would see much thermal difference between the same thickness of cold or hot rolled steel as cast iron. Trying to "cast" a smoker, the design of the casting would take someone who knows all the in's and out's to be able to get the smoker out of the mold and have no porosity that would case a later crack. $$$$ Not that it couldn't be done, it just doesn't show any real big advantage. Also like ceramic, if you ran it hot in cold climates it just might crack if it cooled to quick.
Off the top of my head.

i wouldn't do it..

Ever hear of cast iron wood stoves? Used to work for granny, should work for you. Would hold heat better than uninsulated thin steel. Heavy yes- thermal mass = good. Made in sections and bolted together would work.
This opinion is worth what you paid for it :heh:

that why they called it cast iron , it have been casted, wouldn't be practical trying to weld one together, if you could find the plates they would have to be bolted together. Also would be easy too crack while shipping

the reason i wouldn't do it is because this pit is going to be outside.. at least most pits are.. and the weather will be unforgiving to a cast iron pit.. if it heats or cools to fast.. will most likely crack..

Why not I can build one from a Cardboard box. Mind you I would not buy a CI Pit but you can darn sure make one