In article .com,
says...
I was not clear.

I wanted to use something like four inch on the bottom and
two inch on the top, something with a gap. And no sliding.


How strong does it need to be? Any reason you can't use a reducing
coupling and screw it together and avoid welding completely?

If this is common steel pipe and you're a decent welder, you don't need
any overlap. Cap the large pipe with a disc and fillet weld the smaller
to it. As long as the cross section of the welds is larger than that of
the pipe, and you follow good welding practice, the joint will be as
strong as the pipe.

Ned Simmons