Threaded flanges are joined to pipes by screwing the pipe onto the flange, without seam welds (in certain cases, though, small welds are applied to increase the strength of the connection).
Threaded flanges are available in sizes up to 4 inches and multiple pressure ratings, however, they are used, mostly, small size piping in low pressure and low-temperature applications, like water and air utility services.
Threaded flanges are also a mandatory requirement in explosive areas, such as gas stations and plants, as the execution of welded connections in such environments would be dangerous.