Welding 1/4" NPT black iron pipe tee fittings

That's interesting. How could I tell? Maybe try to smash one with a sledgehammer? I vaguely recall that I could try to grind one on a grinder and look at sparks.

I did weld a black pipe nipple into a hole in a steel plate (think bulkhead fitting) yesterday.

Roy is right, they are cast iron and are not weldable.

unless you have fittings that are designed for fire sprinkler systems they are steel and you can weld them.Sprinkler fitting are malleable iron.Regular screw fittings are weldable.

As others here (and your lawyer) say, 'it depends'.

You do need to identify what materials you are working with. You do not say what air pressure you are designing for and also do not identify the type or quality of T fitting.

If you are talking about common hardware store threaded T fittings then they are probably a poor or dubious quality cast. With experience you can identify different types of cast using a grinder and a spark test but the easiest way is to test cut a fitting with a torch, if the fitting is cast it will not CUT (burn) nicely but will just melt.

If you are talking about threaded fittings then the real question is - Why do you want to weld them? Of course while hardware store threaded fittings will (probably/maybe?) work you should be aware that they are designed for water pressure and NOT for air pressure containment. Fittings designed for air (and energy) containment will be stronger and better quality material than water fittings. These pressure fittings are available from industrial plumbing suppliers and are probably no more expensive than the consumer hardware junk. These better quality fittings are available as threaded or socket or beveled, for welding (with a socket or butt weld). Socket fittings are easier to fit up and (fillet) weld. In this small size you will need to be very skilled to do a pressure quality butt weld with SMAW, TIG or OA is easier but still requires proper technique and skill level (and rod). A proper butt weld root pass cannot be learned on Usenet.

Another option, (depending on the type of manifold you need) is to use a larger (and thicker) steel pipe to make a header and to fit and weld the smaller pipes onto it. You could also weld on 'threadelettes' which are weld on fittings designed for this and also available from any good industrial plumbing supplier.

An industrial plumbing supplier can show you the different types of fittings and provide much guidance and advice.

The bottom line is that pressure (energy) containment is potentially very dangerous and you need guidance that is more dependable than Usenet.

Make sure they are STEEL fittings and not cast iron.

Use 309L Stainless Steel filler to offset the poor quality steel.

Around 120 amps should be fine for the 1/8" wall thickness.

Clean the weld areas with sand paper or a flap wheel.

Thanks Ernie. I decided to just get a brass hex rod and drill and tap it.

why do you wish to weld then instead of screwing them together?Properly done screw joint will hold any amount of air pressure you would safely put on the pipe.

One other option that you might discover is that they are leaded steels.

You'll often find that in threaded rod.

The manufacturer of these materials takes advantage of the machinability of the free machining steels to the detriment of people who want to weld them. So, I'd be wary of any threaded commercial product.