The pipe is assembled with other pipe, fittings, and flanges either by welding or threading. There are specific codes describing the permissibility of threading or welding. With welding, you also need to be aware of several options.

Flanges and fittings are either slip-on or weld-neck. Slip-on fittings slip onto the end of the pipe. The flange is then welded around the contact points on the inside and outside of the pipe and the flange.

Slip-on flanges are not considered as strong a joint as weld-neck or butt-welded connections. With butt-welded or weld-neck flanges, the two pieces—flange and pipe—are prepped and then welded together, with full penetration (a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces).

Socket welding describes when a slip-on fitting, usually used for small diameters, is inserted into the fitting until it bottoms out. The pipe is then pulled back from the bottom and welded to the fitting. Failure to pull the pipe back can cause weld failure due to stress.

Select the flange size that is appropriate to the steel pipe. The fit needs to be snug and not loose. The slip-on flange consists of a bored hole with a diameter just slightly larger than the steel pipe.