ASTM A420 / ASME SA420 is the standard specification by ASTM International covering wrought carbon steel and alloy steel pipe fittings for low temperature service. The specification defines four grades — WPL6 (–45°C), WPL9 (–75°C), WPL3 (–100°C), and WPL8 (–195°C) — with mandatory Charpy V-notch impact testing. Fittings are manufactured to ASME B16.9 (butt weld), ASME B16.11 (forged socket weld/threaded), MSS SP-79, and MSS SP-95. WPL6 is the industry standard for conventional low-temperature process piping and LNG facilities.

Zhengzhou Huitong (HT Pipe) supplies ASTM A420 fittings in all four grades with full Mill Test Certificates (MTC) per EN 10204 3.1 including Charpy V-notch impact test reports. 

1. Standard Scope & Designation

ASTM A420 covers wrought carbon and alloy steel pipe fittings with mandatory low-temperature Charpy V-notch impact requirements. The specification is dual-unit (inch-pound and SI) with both versions considered standard. Fittings are available in four grades covering temperatures from –45°C to –195°C.

2. Grades Covered by ASTM A420

3. Chemical Composition — ASTM A420 Table 1

Values are maximum unless shown as a range. Heat analysis per ASTM A420/A420M-22. W = wrought.

Note: The nickel content is the primary differentiator between grades. WPL6 has low nickel (≤0.40%), WPL9 has moderate nickel (1.60–2.24%), WPL3 has high nickel (3.2–3.8%), and WPL8 has very high nickel (8.4–9.6%). Higher nickel stabilizes the austenitic microstructure at cryogenic temperatures, providing superior impact toughness.

4. Mechanical Property Requirements — ASTM A420 Table 3

All values are minimum requirements per ASTM A420/A420M-22. Testing per ASTM A370.

5. Heat Treatment Requirements

6. Testing & Inspection Requirements

7. Fitting Dimensions — ASME B16.9 (Butt Weld) & ASME B16.11 (Forged)

Dimensions per applicable manufacturing standard. ASME B16.9 covers NPS 1/2 to NPS 48 butt-weld fittings (elbows, tees, reducers, caps). ASME B16.11 covers NPS 1/2 to NPS 4 socket weld and threaded forged fittings.

Note: ASME B16.9 dimensions apply to standard weight (Schedule 40) butt-weld fittings. For XS (Schedule 80) and lighter weights, refer to MSS SP-43 (lightwall) or ASME B16.9 directly. Butt-weld ends are beveled per ASME B16.25. Forging socket-weld ends per ASME B16.11.

8. Industries & Applications

9. ASTM A420 vs ASTM A350 — Low Temperature Fittings vs Flanges

10. Related Standards

11. How to Order — Required Information

12. Frequently Asked Questions

Q: What is the difference between ASTM A420 WPL6 and ASTM A333 Grade 6 pipe?

Both WPL6 (fittings) and A333 Gr.6 (pipe) are designed for low-temperature service to –45°C (–50°F) with similar chemical composition and Charpy impact requirements. They are the complementary specifications for low-temperature piping systems: A333 Gr.6 is the pipe material and A420 WPL6 is the fittings material. Both have maximum carbon of 0.30%, and both require Charpy V-notch impact testing at –45°C. The matching combination ensures consistent toughness throughout the entire piping system.

Q: Why does ASTM A420 require impact testing on weld metal?

Unlike standard fittings specifications, A420 mandates Charpy impact testing on weld metal as well as base metal. This is critical because the weld metal and heat-affected zone (HAZ) are often the weakest points in low-temperature service — they can have lower toughness than the base metal even when properly qualified. The weld metal impact test ensures the filler material and welding procedure provide adequate toughness at the design temperature. Impact requirements are specified in Tables 4 and 5 of A420.

Q: When should I specify WPL9 instead of WPL6?

Specify WPL9 when the design temperature is below –45°C and above –75°C, or when additional toughness margin is required. The higher nickel content (1.60–2.24% vs ≤0.40% in WPL6) provides superior impact toughness at cryogenic temperatures. WPL9 is standard for LNG import terminals, LNG tanker cooling lines, and deep refrigeration systems operating between –50°C and –75°C. WPL9 fittings are more expensive than WPL6 but may eliminate the need for thicker wall construction to achieve the same toughness.

Q: Can ASTM A420 fittings be used in ASME B31.1 or B31.3 piping codes?

Yes. ASTM A420 fittings are acceptable for use under ASME B31.1 (Power Piping), B31.3 (Process Piping), and B31.5 (Refrigeration Piping) codes. The applicable code determines the maximum allowable stress values, pressure-temperature ratings, impact test exemption temperature, and design margins. For B31.3, materials in Category D (low pressure) fluid service below –29°C require Charpy testing per A420. For Arctic and offshore applications, additional requirements may apply per the applicable jurisdiction.

Q: What is the marking requirement for ASTM A420 fittings?

ASTM A420 requires permanent marking on each fitting with: specification (A420), grade (WPL6, WPL9, WPL3, WPL8), heat number or lot identification, manufacturer identification, and schedule if special. If the impact test temperature is lower than the standard grade temperature, it must be marked (e.g., WPL6-60°F). If ultrasonic examination was used instead of radiographic examination, mark with "U". These markings are typically stamped or stenciled and must remain legible after installation.

Request a Quote — ASTM A420 Low Temperature Pipe Fittings

Zhengzhou Huitong (HT Pipe) supplies ASTM A420 / ASME SA420 pipe fittings in all four grades (WPL6, WPL9, WPL3, WPL8). Full EN 10204 3.1 CMTRs including Charpy V-notch impact test reports, heat treatment certificates, and RT/UT reports for LNG, cryogenic, and low-temperature process applications