Malleable iron threaded fittings NPT, classes 150 and 300 (black and galvanized)

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NPT-threaded fittings (ASME B1.20.1) in ASTM A197 malleable iron, manufactured to ASME B16.3: class 150 (300 psi up to 66 °C; 150 psig on saturated steam, 186 °C limit) and class 300 (2,000 psi cold in 1/4″–1″, decreasing with size). Black or hot-dip galvanized finish per ASTM A153. This sheet covers the 34 catalog references of the family, 1/4″ to 6″.

SKU VT-58528 (galvanized 90° elbow) · family of 34 NPT-threaded malleable iron references · price from $1,700 COP + VAT per unit depending on figure and size (Jul 2026, confirmed on quotation)

1. Technical specifications

Class 150 dimensions — 90° elbows, tees, crosses and 45° elbows (ASME B16.3-2021, Table 7.1-1)

Class 300 dimensions — 90° elbows, tees and 45° elbows (ASME B16.3-2021, Table 7.1-5)

The dimensional scope of class 300 in ASME B16.3-2021 is NPS 1/4″ to 3″. Class 300 sizes above 3″ appearing in the catalog are supplied to manufacturer standard and confirmed on quotation. Unions are dimensioned per ASME B16.39 and plugs, bushings and locknuts per ASME B16.14 (standards referenced by B16.3); their dimensions are confirmed on quotation per figure.

Materials

Body	ASTM A197/A197M cupola malleable iron: tensile strength ≥ 275 MPa (40 ksi), yield strength ≥ 200 MPa (30 ksi), elongation ≥ 5% in 50 mm. The chemical composition is left to the manufacturer provided it achieves those mechanical properties (as A197 itself defines).
Galvanized coating	Hot-dip per ASTM A153/A153M, Class A (castings): average zinc mass ≥ 610 g/m² (2.00 oz/ft²) and ≥ 550 g/m² (1.80 oz/ft²) for any individual specimen. ASME B16.3-2021, para. 12.1, requires a minimum thickness of 0.0864 mm (86 µm) applied before threading; it allows as an alternative electrodeposited zinc per ASTM B633 Type I, service condition 4, thickness ≥ 0.025 mm.
Black finish	No galvanic coating; same A197 body, same dimensions and same pressure rating (see section 3).
Threads	Taper NPT per ASME B1.20.1. Exception in para. 8.1 of B16.3-2021: wrought couplings and caps in NPS 1/8″, 1/4″ and 3/8″ may be supplied with straight threads (and per para. 6.2, in class 150 those sizes may be made from steel rod or bar with yield ≥ 207 MPa / 30 ksi).

General specifications

Product	Family of malleable iron threaded fittings in 3 catalog subfamilies: galvanized class 150 (17 figures), black iron class 150 (10 figures) and malleable iron class 300 (7 figures) — 34 references in total.
Figures	90° elbow, 45° elbow, street elbow, reducing elbow, tee, reducing tee, cross, lateral (Y-branch), coupling, union, cap, plug, countersunk plug, bushing, locknut and hose adapter, depending on the subfamily.
Size range	1/4″ to 6″ in class 150 (per figure); 1/4″ to 3″ in class 300 within the B16.3 scope.
Working pressure	By class, temperature and (in class 300) size group — full table in section 2.
Applicable standards	ASME B16.3-2021 (dimensions, tolerances and P-T ratings) · ASTM A197/A197M-00 (R2019) (material) · ASTM A153/A153M-23 (galvanizing) · ASME B1.20.1-2013 (R2018) (NPT threads) · ASME B16.39 (unions) · ASME B16.14 (plugs, bushings and locknuts).

2. Working pressure by class, temperature and size (ASME B16.3-2021, Table 3-1)

For malleable iron threaded fittings the allowable pressure is not computed with a formula: ASME B16.3-2021 assigns it directly in Table 3-1, by class, temperature and — in class 300 only — by size group. This is the complete table:

¹ Note (1) of Table 3-1: the 150 psi value is permissible up to a service temperature of 186 °C (366 °F), which is the temperature of saturated steam at 150 psig (10.3 bar). Psi values follow the °F temperature series of Table 3-1 of ASME B16.3-2021; the °C and bar figures in this table are rounded conversions (the standard also publishes its own SI series with slightly different °C temperature steps). Ratings are maximum non-shock pressures, independent of the contained fluid; the standard allows interpolation for intermediate temperatures.

The two reading rules

Rule 1 — class 150 is the 150 psig saturated steam class, not "always 300 psi". The class number names its steam service: a class 150 fitting carries saturated steam up to 150 psig, whose temperature is 186 °C (366 °F) — that is its real temperature boundary. The 300 psi (20.7 bar) quoted in catalogs applies only cold, up to 66 °C (150 °F), and drops progressively: at 149 °C it is already 185 psi. Likewise, class 300 carries saturated steam up to 300 psig, with a 288 °C (550 °F) limit.

Rule 2 — in class 300 the rating depends on size; in class 150 it does not. Class 300 is the only fitting type in the TECTUL catalog whose rating falls as size grows: cold, 2,000 psi up to 1″, 1,500 psi from 1-1/4″ to 2″ and 1,000 psi from 2-1/2″ to 3″. A 3″ class 300 tee takes half the pressure of a 1″ one. Class 150 keeps a single rating for all sizes. Additionally, ASME B16.3-2021 (para. 3) does not recommend class 300 street elbows above 600 psi.

3. Black vs galvanized iron: same body, same rating

The black iron and the galvanized class 150 fitting are the same structural product: ASTM A197 malleable iron body, dimensions from ASME B16.3-2021, Table 7.1-1, and the same rating from Table 3-1 (300 psi at ≤66 °C). Table 3-1 does not distinguish finishes: zinc neither adds nor subtracts pressure. The only normative difference is the galvanized version's additional zinc layer: ASTM A153 Class A hot-dip, average mass ≥ 610 g/m² (equivalent to the 86 µm minimum thickness required by B16.3, para. 12.1), applied before threading.

Body and dimensions	Identical: ASTM A197 + ASME B16.3 (same dimensional tables)
Pressure rating	Identical: Table 3-1 of B16.3-2021 by class and temperature
Coating	Black: none · Galvanized: A153 Class A zinc, ≥ 610 g/m² average (≥ 86 µm)
Protection against water and weather	Black: exposed surface rusts (requires paint or non-aqueous service) · Galvanized: zinc protects as a barrier and sacrificially

Why in Colombian practice gas runs on black iron and water on galvanized. The split is about compatibility and cost, not strength: in gas and steam lines the fluid is not aqueous and the fitting bore suffers no water corrosion, so the zinc layer adds nothing and is omitted (black iron, lower cost); in potable water, moist air and outdoor lines, the galvanized zinc protects the iron from aqueous corrosion, so the typical installation pairs galvanized A53 pipe with fittings of the same finish, keeping coating continuity along the whole line. The final finish choice belongs to the project specification or the local gas utility.

4. ASME B16.3 (malleable) vs ASME B16.4 (gray iron): not interchangeable

The malleable iron fitting (ASME B16.3, classes 150 and 300, ASTM A197 material) and the gray or cast iron fitting (ASME B16.4, classes 125 and 250, ASTM A126 material) are different products that the market confuses because both are sold threaded and look alike. The differences that matter when specifying:

Standard and classes	Malleable: ASME B16.3, classes 150 and 300 · Gray: ASME B16.4, classes 125 and 250
Material	Malleable: ASTM A197 — iron malleabilized by heat treatment, with a guaranteed minimum elongation of 5% in 50 mm and yield ≥ 200 MPa · Gray: ASTM A126 — gray cast iron, no elongation requirement (brittle behavior)
Mechanical behavior	Malleable iron tolerates deformation before fracture (thread make-up, installation impacts, vibration); gray iron fails by fracture with no prior deformation
Dimensions	Different dimensional tables in each standard: center-to-end and thicknesses do not match between B16.3 and B16.4
Field identification	A class 300 fitting is cast-marked "300" and "MI" (malleable iron, para. 5.2 of B16.3); B16.4 fittings do not carry the "MI" mark

Practical specification rule: if the class number on the drawing or requisition is 125 or 250, the part is B16.4 gray iron; if it is 150 or 300, it is B16.3 malleable iron. Substituting one for the other changes material, rating and dimensions at once. All 34 references in this sheet are B16.3 malleable iron.

5. Marking and receiving inspection

ASME B16.3-2021 defines different markings per class — and that asymmetry is itself a receiving-inspection datum:

Class 150 (para. 5.1)	Only requires the manufacturer's name or trademark. The absence of the numeral "150" on the body is normal and standard-compliant — it is not a sign of a generic product.
Class 300 (para. 5.2)	Requires four marks: (a) manufacturer's name or trademark, (b) the numeral "300", (c) the letters "MI" identifying malleable iron, (d) the size; additional markings per MSS SP-25 are allowed.

Additional acceptance criteria set by the standard itself:

• Metal thickness: at no point of the casting may the thickness be less than 90% of the tabulated value (para. 7.3). For a 1/2″ class 150 elbow (G = 2.67 mm), the absolute minimum at any point is 2.40 mm.
• Design proof for reducing fittings: the design is validated by a hydrostatic test at no less than 5 times the rating of the largest end, held at least 1 minute with no cracking or leakage (para. 7.2.3). It is a manufacturer design-qualification test, not a per-lot test.
• Threads: NPT per ASME B1.20.1, with internal countersink ≥ half a pitch at ~45° and external chamfer at 30° to 45° (para. 8.3); maximum thread misalignment between openings is 0.5% (5.0 mm/m, para. 8.5).
• Class 300 street elbows: not recommended above 600 psi (para. 3) — the only case where a figure carries its own restriction on top of its class rating.

The manufacturer's certificate (A197 requires the manufacturer to be prepared to certify the material, para. 6.1 of B16.3) is supplied upon request when quoting.

6. Frequently asked questions

How much pressure does a 1/2″ galvanized elbow take?

300 psi (20.7 bar) up to 66 °C, per Table 3-1 of ASME B16.3-2021 for class 150 fittings — the rating drops with temperature: 265 psi at 93 °C, 225 psi at 121 °C, 185 psi at 149 °C and 150 psi at 177 °C, with a 186 °C service limit. In class 150 size does not change the rating: a 2″ elbow takes the same as a 1/2″ one.

Does "class 150" mean the fitting takes 150 psi?

No: 150 psig is its saturated steam rating, whose temperature is 186 °C (366 °F); cold (≤66 °C) the same fitting takes 300 psi. The class number names the defining steam service, not the absolute maximum pressure — the full table by temperature is in section 2.

Does the galvanized fitting take less pressure than the black one?

No: both share the rating (Table 3-1), the ASTM A197 body and the B16.3 dimensions; Table 3-1 does not distinguish finishes. Galvanizing only adds the ASTM A153 Class A zinc layer (≥ 610 g/m² average) to protect against aqueous corrosion — it changes neither the pressure nor the assembly dimensions.

Why does a 3″ class 300 tee take half the pressure of a 1″ one?

Because ASME B16.3 assigns the class 300 rating by size group: 2,000 psi up to 1″, 1,500 psi from 1-1/4″ to 2″ and 1,000 psi from 2-1/2″ to 3″ (cold). It is the only fitting type in the catalog whose rating depends on size; class 150 keeps 300 psi across all sizes.

How do I identify a class 300 fitting in the field?

It carries the numeral "300", the letters "MI" and the size cast on the body, plus the manufacturer's mark (para. 5.2 of B16.3-2021). Class 150 fittings carry only the manufacturer's mark — the missing "150" is standard-compliant, not a defect.

7. Application notes

8. Technical notice and limitation of liability