TUBE AND PIPE

The great solution: Use connectors

More like adapters. Tube doesn't match with pipe.

Pipe vs Tube

It is difficult to get the Tubes and pipes with the same inside diameters.

In the best recourse, in case you have to do it, and no codes are involved, You must get the pipe with just a bit less diameter and thene bevel it to match the Inside diameter of the tube. Then join and weld.

Ensure material compatibility.

As mentioned- ensure no codes involved. None of the codes I know will permit this.

Also this will be just a bit difficut (since if you see the dimensions, pipes have just a bit more ID than the DN (or NPS) except for very high schedules. Where as the Tubes have to have the ID less than the size.

So the choices- Adaptor (like a reducer) at the joint. Or a imperial pipe+ metric tube.

visit sites of tube fitting manufacturers like Swagelok. They have tube to pipe fittings.