Class 300 malleable iron fittings: technical guide

Class 300 malleable iron fittings are NPT threaded connections made of ASTM A197 malleable iron with dimensions and ratings per ASME B16.3. At ambient temperature they withstand up to 2000 psi (137.9 bar) in 1/4 to 1 inch sizes, 1500 psi (103.4 bar) from 1-1/4 to 2 inch and 1000 psi (68.9 bar) from 2-1/2 to 3 inch, with a 300 psi (20.7 bar) saturated steam rating. TECTUL supplies 300 lb malleable iron threaded elbows, 45° elbows, tees, couplings and unions.

Verdict: for water, air or gas up to 300 psi and saturated steam up to 150 psig, Class 150 is sufficient; for steam between 150 and 300 psig, compressed air with pulsation and cycling, or liquids above 300 psi, use Class 300; above Class 300 ratings, or when the specification requires ASTM A105 forged steel, move to Class 3000 forged fittings per ASME B16.11. No malleable iron threaded fitting replaces a line calculation: the ratings in this guide are maximum non-shock pressures per ASME B16.3.

What malleable iron is and what ASTM A197 requires

Malleable iron is a white cast iron subjected to a long annealing treatment that transforms cementite into tempered carbon nodules. The result is a cast iron with real ductility, unlike gray cast iron, which is brittle. ASTM A197/A197M, the cupola malleable iron specification used for threaded fittings, requires a minimum tensile strength of 40,000 psi (275 MPa), a minimum yield strength of 30,000 psi (200 MPa) and 5% elongation in 50 mm.

That 5% elongation is the practical difference against a gray iron fitting: malleable iron tolerates thread tightening, assembly impacts and piping loads without sudden cracking. That is why ASME B16.3 is the dominant standard for small-diameter threaded fittings in industrial water, air, gas and low-pressure steam services.

Pressure and temperature: Class 150 versus Class 300 per ASME B16.3

Commercial catalogs use the acronyms WSP (working steam pressure) and WOG (water, oil, gas). In ASME B16.3 terms, Class 150 is rated 150 psi saturated steam and 300 psi for fluids at ambient temperature; Class 300 is rated 300 psi saturated steam and up to 2000 psi for fluids at ambient temperature depending on size. Ratings are independent of the contained fluid: they depend on the material temperature.

Class 150 ratings (all sizes):

Class 150 allows service up to 186 °C (366 °F), the saturated steam temperature at 150 psig. Class 300 ratings depend on size and reach 288 °C (550 °F):

Two notes from the standard that affect purchasing: intermediate ratings may be obtained by interpolation, and Class 300 street elbows (male by female) are not recommended above 600 psi because the male threaded end is the weak section.

The characteristic band or bead

Malleable iron fittings are recognized by the cast band or bead surrounding each threaded opening. That band is not decorative: it reinforces the section where the NPT thread thins the wall and where tightening stress concentrates. In Class 300 the body is visibly heavier than Class 150 of the same diameter and carries the figure 300 cast on the body together with the manufacturer's mark, per ASME B16.3 marking. If a used fitting does not show the 300 figure, it must be treated as Class 150 for pressure purposes.

Galvanized or black?

The same malleable fitting is offered in black finish (uncoated, with varnish or temporary protective oil) or hot-dip galvanized per ASTM A153/A153M, the specification for zinc coating by immersion on iron and steel hardware. Galvanizing protects against atmospheric corrosion and is the standard in water networks, outdoor compressed air and exposed installations. Black is used in lines that will be painted, in steam and condensate systems, and in fire protection networks where the specification calls for it. Zinc does not change the pressure rating: black Class 300 and galvanized Class 300 share the same pressure-temperature table.

Fitting types in the Class 300 family

• 90° elbow: perpendicular change of direction; the 300 lb malleable iron elbow is the most used fitting of the family.
• 45° elbow: the 300 lb 45° elbow softens the change of direction and reduces pressure drop versus a 90°.
• Tee: the 300 lb tee branches at 90°; straight and reducing versions exist.
• Coupling: the 300 lb coupling joins two threaded lengths in line; the reducing version (bell reducer) changes diameter.
• Union: the 300 lb union allows disassembling the line without rotating the pipe; it is governed by ASME B16.39, which covers malleable iron unions Classes 150, 250 and 300, usually with a brass seat.
• Reducers, plugs and caps: threaded bushings, plugs and locknuts are governed by ASME B16.14; caps close female ends.

NPT thread per ASME B1.20.1

All openings in this family carry NPT thread per ASME B1.20.1: a taper thread with 1/16 taper (3/4 inch per foot) that seals by metal-to-metal flank interference. In industrial practice the seal is completed with thread sealant or PTFE tape. NPT thread is not interchangeable with BSP thread: an imported malleable fitting with BSPT thread tightens on NPT pipe but does not seal reliably. Before buying, confirm that pipe and fitting use the same thread designation.

When to choose Class 300 over Class 150

With ASME B16.3 ratings, the decision thresholds are direct:

• Saturated steam: up to 150 psig, Class 150; between 150 and 300 psig, Class 300. Above 300 psig of steam, malleable iron is out and forged steel takes over.
• Temperature: above 186 °C (366 °F), Class 150 is out of rating; Class 300 reaches 288 °C (550 °F) at 300 psi.
• Water, oil or gas at ambient temperature: above 300 psi, Class 300 is mandatory; its ceiling runs from 1000 to 2000 psi depending on diameter.
• Compressed air: a 100 to 175 psig network fits Class 150 by rating, but Class 300 is specified when there is compressor pulsation, vibration and pressure cycling, because the heavier body provides mechanical margin; above 300 psig it is mandatory.
• Higher-demand lines: pump discharges with water hammer, hydrostatic test headers and services with frequent tightening and disassembly justify Class 300 even when the nominal pressure would fit Class 150.

Difference versus Class 3000 forged and Class 150 black iron

The Class 3000 forged fitting is made of forged steel (typically ASTM A105) under ASME B16.11, not cast iron: its rating correlates with Sch 160 pipe in the threaded version and withstands pressures far above any malleable iron fitting. The full comparison is in the Class 3000 forged fittings guide. So-called Class 150 black iron is the same ASTM A197 malleable iron in black finish and ASME B16.3 Class 150, that is, the lighter version of this same family; its dedicated guide is Class 150 black iron fittings.

Practical buying recommendation

First define the fluid, the operating pressure with its peaks and the temperature; with the table in this guide the class selects itself. Specify NPT diameter, class (300), finish (black or ASTM A153 galvanized) and fitting type, and verify on receipt the 300 figure cast on the body. To build the complete line, TECTUL quotes 300 lb malleable iron elbows, tees, couplings and unions delivered from Colombia. Guide updated July 22, 2026.