Quick Answer: ASTM A234 = carbon and alloy steel butt weld fittings for moderate and high-temperature service (WPB is the most common grade). ASTM A420 = carbon and alloy steel butt weld fittings for low-temperature service (WPL6 is the primary grade) with mandatory Charpy impact testing at -46°C and fine grain practice requirements.
Selecting the right butt weld fitting standard for your temperature environment is a safety-critical decision. This comparison by HT PIPE details the differences between ASTM A234 (WPB) and ASTM A420 (WPL6) — two of the most frequently specified carbon steel fitting grades.
1. Grade Comparison: WPB (A234) vs WPL6 (A420)
| Attribute |
ASTM A234 WPB |
ASTM A420 WPL6 |
| Standard |
ASTM A234 — Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperatures |
ASTM A420 — Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low-Temperature Service |
| Grade Designation |
WPB (Wrought Pipe Grade B) |
WPL6 (Wrought Pipe Low Temperature Grade 6) |
| Service Temperature |
Moderate to high temperature (-29°C to 425°C+) |
Low temperature (-46°C to ~345°C) |
| Impact Testing |
Not required by base spec |
Mandatory — Charpy V-notch at -46°C |
| Fine Grain Practice |
Not required |
Required |
| Primary Use |
Process piping, steam, boilers, general service |
LNG, cryogenic, cold climate, low-temp piping |
| Governing Codes |
ASME B31.1, B31.3 (moderate/high temp) |
ASME B31.3 (low-temp service), B31.8 (cold climate gas) |
Key Insight: The fundamental difference is temperature service orientation. A234 WPB is designed to perform at elevated temperatures without losing strength. A420 WPL6 is designed to maintain toughness at sub-zero temperatures. They address opposite ends of the temperature spectrum and are not interchangeable.
2. Chemical Composition Comparison
| Element |
A234 WPB (max / range) |
A420 WPL6 (max / range) |
| Carbon (C) |
≤ 0.30 |
≤ 0.30 |
| Manganese (Mn) |
0.29 – 1.06 |
0.60 – 1.35 |
| Phosphorus (P) |
≤ 0.050 |
≤ 0.040 |
| Sulfur (S) |
≤ 0.050 |
≤ 0.040 |
| Silicon (Si) |
≥ 0.10 |
≥ 0.10 |
| Chromium (Cr) |
≤ 0.40 (residual) |
≤ 0.40 (residual) |
| Copper (Cu) |
≤ 0.40 (residual) |
≤ 0.40 (residual) |
| Nickel (Ni) |
≤ 0.40 (residual) |
≤ 0.40 (residual) |
| Molybdenum (Mo) |
≤ 0.15 (residual) |
≤ 0.15 (residual) |
| Vanadium (V) |
≤ 0.08 (residual) |
≤ 0.08 (residual) |
Notable Differences: A420 WPL6 has a higher minimum manganese range (0.60–1.35 vs 0.29–1.06) and lower max phosphorus and sulfur (0.040 vs 0.050). The higher Mn content improves toughness at low temperatures, while lower P/S reduces brittleness. These chemical controls are essential for the Charpy impact performance required at -46°C.
3. Mechanical Properties & Impact Testing
| Property |
A234 WPB |
A420 WPL6 |
| Tensile Strength |
415 – 585 MPa (60 – 85 ksi) |
415 – 655 MPa (60 – 95 ksi) |
| Yield Strength (min) |
240 MPa (35 ksi) |
240 MPa (35 ksi) |
| Charpy Impact Test |
Not required by standard |
Mandatory at -46°C (-50°F) |
| Impact Energy (avg of 3 specimens) |
N/A |
≥ 18 J (13 ft-lbf) |
| Impact Energy (single specimen min) |
N/A |
≥ 14 J (10 ft-lbf) |
| Fine Grain Practice |
Not required |
Required (fine austenitic grain size per A20) |
| Heat Treatment |
As required (hot-finished, annealed, normalized, etc.) |
Normalized or normalized & tempered; quench & temper per agreement |
The Impact Test is the Deciding Factor: A420 WPL6's mandatory Charpy impact test at -46°C with minimum average energy of 18J and minimum single value of 14J is the critical distinction. This proves the fitting will not fail by brittle fracture in cold service. HT PIPE performs and documents all impact tests per ASTM E23, with full results included in each MTC.
4. Temperature Range Comparison
| Temperature Range |
A234 WPB |
A420 WPL6 |
Notes |
| Upper Limit |
425°C (800°F) and above per B31.3 |
~345°C (650°F) |
WPB excels at elevated temperatures; WPL6 optimized for low-temp |
| Lower Limit (unrestricted) |
-29°C (-20°F) per B31.3 without impact test |
-46°C (-50°F) with mandatory impact test |
WPL6 is qualified below -29°C; WPB is not |
| Sub-zero (below -29°C) |
Not recommended — no impact toughness guarantee |
Qualified — impact tested at -46°C |
Critical safety difference |
| -46°C to -101°C |
Not applicable |
WPL3 (tested at -101°C) available under A420 |
A420 has grades for even lower temperatures |
| -196°C (cryogenic) |
Not applicable |
WPL8 (tested at -196°C) available under A420 |
A420 covers deep cryogenic service |
HT PIPE manufactures fittings across the full temperature spectrum: A234 WPB for hot service and A420 WPL6/WPL3/WPL8 for progressively colder environments. Choosing the wrong grade for a temperature zone can result in catastrophic brittle fracture — always specify based on the minimum design metal temperature (MDMT).
5. Application Recommendations
| Application |
Recommended Grade |
Reason |
| Steam piping (high-pressure, high-temp) |
A234 WPB |
Designed for elevated temperature; per B31.1 / B31.3 |
| Boiler connections & fittings |
A234 WPB |
High-temp carbon steel; seamless-equivalent performance |
| Process piping (refinery, petrochemical, moderate temp) |
A234 WPB |
Standard carbon steel fitting for -29°C to 425°C service |
| Cold-climate gas distribution (winter temps below -29°C) |
A420 WPL6 |
Mandatory impact testing ensures toughness in cold weather |
| LNG processing & storage |
A420 WPL6 or WPL3 |
Low-temp service per B31.3; impact tested at -46°C / -101°C |
| Cryogenic piping (liquid nitrogen, liquid oxygen) |
A420 WPL8 |
Impact tested at -196°C for deep cryogenic service |
| Refrigeration system piping |
A420 WPL6 |
Low-temperature operation requires impact-qualified fittings |
| Offshore cold environment piping |
A420 WPL6 |
Sub-zero ambient + operating temperatures; mandatory toughness |
6. Selection Guide
How to choose between A234 WPB and A420 WPL6?
- Is the minimum design metal temperature (MDMT) below -29°C? → A420 WPL6 (mandatory impact testing ensures toughness)
- Is the operating temperature above 345°C (high-temp service)? → A234 WPB (designed for elevated temperature strength)
- Is MDMT between -29°C and 345°C (moderate range)? → A234 WPB is acceptable per B31.3; but WPL6 can also be used if impact assurance is desired
- Does the project code require impact testing regardless of temperature? → A420 WPL6 (impact-tested by default)
- Is it an LNG or cryogenic application? → A420 WPL6 (at -46°C), WPL3 (at -101°C), or WPL8 (at -196°C)
- Is it a standard steam/process piping project? → A234 WPB (most economical and widely available)
Decision Flowchart
- Determine the Minimum Design Metal Temperature (MDMT) for your system.
- MDMT below -29°C? → Use A420 WPL6 (or WPL3/WPL8 for colder).
- MDMT between -29°C and ~345°C? → A234 WPB is acceptable. WPL6 may be used for extra assurance.
- Operating temperature above 345°C? → A234 WPB (or alloy grades WP5/WP11/WP22 for even higher).
- Code or client specification mandates impact testing? → A420 WPL6 regardless of temperature.
Need help determining your MDMT or selecting the correct grade? HT PIPE's engineering team can review your project specifications and recommend the right standard and grade for your conditions.
7. HT PIPE Supply Note
HT PIPE manufactures and exports butt weld fittings per both ASTM A234 and ASTM A420:
- A234 WPB: The most commonly supplied carbon steel BW fitting grade — elbows, tees, reducers, caps, 1/2"–48" NPS, Sch 10–XXS
- A234 alloy grades: WP5, WP9, WP11, WP22, WP91 for high-temperature alloy steel service
- A420 WPL6: Low-temperature carbon steel BW fittings with mandatory Charpy impact testing at -46°C
- A420 WPL3 & WPL8: For ultra-low and cryogenic service (-101°C and -196°C respectively)
- All products with full MTC per EN 10204 3.1, including impact test results for A420 grades
- Third-party inspection available (SGS, BV, TUV, Lloyd's)
- Competitive pricing and reliable delivery to over 50 countries worldwide
8. Frequently Asked Questions
What is ASTM A234 WPB?
ASTM A234 WPB is the most common grade of wrought carbon steel butt weld fittings for moderate and high-temperature service. WPB stands for Wrought Pipe Grade B. It covers elbows, tees, reducers, and caps with a minimum tensile strength of 415 MPa and minimum yield strength of 240 MPa. WPB fittings are widely used in process piping, steam systems, and boiler applications per ASME B31.1 and B31.3.
What is ASTM A420 WPL6?
ASTM A420 WPL6 is the standard grade for wrought carbon steel butt weld fittings intended for low-temperature service. WPL6 stands for Wrought Pipe Low Temperature Grade 6. It requires mandatory Charpy V-notch impact testing at -46°C (-50°F) with minimum average energy of 18J and minimum single value of 14J. WPL6 also requires fine grain practice and is used for LNG, cryogenic, and cold-climate piping applications.
A234 vs A420 for low temperature?
ASTM A420 is the correct standard for low-temperature service. A420 WPL6 requires mandatory Charpy impact testing at -46°C and fine grain practice, which are not required by A234. A234 WPB does not guarantee adequate toughness at sub-zero temperatures and should not be used where design temperatures fall below -29°C. For low-temperature and cryogenic applications, always specify A420 WPL6 (or WPL3/WPL8 for even lower temperatures).
What temperature can A420 WPL6 be used at?
ASTM A420 WPL6 fittings are qualified for use at temperatures as low as -46°C (-50°F), with the Charpy impact test performed at this temperature. The standard also includes WPL3 (tested at -101°C) and WPL8 (tested at -196°C) for even more severe cryogenic service. For the upper end, WPL6 is generally used up to approximately 345°C (650°F) for carbon steel low-temperature service.
Is WPB the same as WPL6?
No, WPB and WPL6 are different grades under different ASTM standards. WPB is under ASTM A234 (moderate/high-temperature service) and WPL6 is under ASTM A420 (low-temperature service). While they share similar minimum tensile and yield strength values, WPL6 has mandatory Charpy impact testing at -46°C, fine grain practice requirements, and tighter manganese control — all designed to ensure toughness at low temperatures that WPB does not guarantee.
9. Request a Quote
Need WPB or WPL6 Butt Weld Fittings? Get a Quote from HT PIPE
Whether your project needs A234 WPB for high-temperature service or A420 WPL6 for low-temperature/cryogenic applications — HT PIPE delivers fully certified fittings with complete test documentation.
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