ASME B16.3

Covers — malleable iron threaded fittings:

• Sizes from NPS 1/8 to NPS 6.
• Pressure classes: Class 150 and Class 300.
• Fittings: Elbows, tees, crosses, 45° Y-branches, couplings, and reducers.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Heavy-duty forged steel threaded fittings (Class 2000+) → ASME B16.11.
• Gray cast iron fittings → ASME B16.4.

Malleable iron provides a critical balance between the cheap mass-producibility of casting and the ductility of steel:

• Material Treatment — Malleable iron is essentially cast iron that has been subjected to a prolonged heat-treatment (annealing) process to alter its microstructure, significantly improving its ductility and fracture toughness. This makes it far superior to brittle gray iron for threaded connections.
• Dimensional Control — B16.3 dictates the physical dimensions and band widths (the reinforced thick rim around the opening) to ensure the threads can handle immense tightening stresses from large pipe wrenches without cracking the fitting body.

Walk into any commercial building and look at the fire sprinkler piping, natural gas lines, or heating hot water systems—you are looking at ASME B16.3 malleable iron fittings. They are mass-produced, highly economical, and provide excellent performance for building and utility services where installing high-pressure forged steel (B16.11) would be a massive financial overkill.

Class 150 malleable iron fittings are the industry default for commercial low-pressure systems, offering a reliable, threadable connection that resists cracking during installation far better than traditional cast iron.