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Malleable and Cast Iron Threaded Fittings

Threaded iron fittings come in malleable iron (ASME B16.3, Class 150 and 300) and gray cast iron (ASME B16.4, Class 125 and 250). This sheet explains how they differ, how to read the markings and where each is used.

Ref PV-0015ASME B16.3ASME B16.4

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Malleable iron fittings are cast and then heat treated, which makes them tougher and less brittle than gray iron. They are the common threaded fitting for gas, air, oil and general piping, in black or galvanized finish. ASME B16.3 covers them in Class 150 and Class 300. Gray cast iron fittings are heavier and more brittle. ASME B16.4 covers them in Class 125 and Class 250, and they are used mainly in steam, low-pressure and some heating systems. Both use tapered pipe threads (NPT). Cast iron threaded drainage fittings are a different product under ASME B16.12.

How to identify

Read the cast marks on the fitting body. Most show the maker's mark, the size and the class number (such as 150 or 300). Malleable fittings usually have a raised bead (band) at each opening and may be marked "M" or "MI." Class 300 malleable fittings are visibly heavier than Class 150. Gray iron fittings are bulkier and may be marked "CI." See the fitting and flange marking chart.

Malleable vs cast iron

Item	Malleable iron	Gray cast iron
Standard	ASME B16.3	ASME B16.4
Classes	150 and 300	125 and 250
Toughness	Ductile; handles shock better	Brittle; can crack from impact or water hammer
Finish	Black or galvanized	Black, sometimes galvanized
Common uses	Gas, air, oil, water, general piping	Steam and heating, low-pressure lines
Ends	NPT, usually beaded	NPT

Pressure and temperature ratings for each class are in the tables of B16.3 and B16.4.

Key points

The class number is a designation, not a working pressure. Ratings change with temperature and service. Use the standard's tables.
Many fuel gas codes require malleable iron fittings for threaded steel gas pipe. Gray iron fittings are not allowed for gas in many codes.
Do not mix galvanized fittings into a black iron gas system or the reverse, unless the code allows.
Use a thread sealant rated for the fluid and do not over-tighten. Cracked fittings often come from too much torque on brittle iron.
Check NPT size and thread engagement. See the NPT thread chart.

Common mistakes

Treating "150" as a 150 psi rating.
Using a gray iron fitting on a gas line.
Replacing a Class 300 fitting with a Class 150 fitting.
Using drainage fittings (B16.12) on pressure lines.

Governing standards: ASME B16.3 (malleable iron threaded fittings, Classes 150 and 300); ASME B16.4 (gray iron threaded fittings, Classes 125 and 250); ASME B1.20.1 (NPT threads).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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