ASTM A350/A350M is the standard specification by ASTM International covering carbon and low-alloy steel forged or ring-rolled flanges, fittings, valves, and parts intended primarily for low-temperature service and requiring mandatory notch toughness (Charpy V-Notch) testing. The latest edition is A350/A350M-24 (2024). The companion specification ASME SA-350 adopts identical requirements for ASME Boiler and Pressure Vessel Code and B31.3 Process Piping compliance.
Zhengzhou Huitong (HT PIPE) supplies ASTM A350 / SA-350 compliant forged flanges and fittings across all standard grades (LF1, LF2, LF3, LF5, LF6, LF9, LF787) with full Charpy V-Notch impact test reports, Mill Test Certificates (MTC) per EN 10204 3.1, and complete traceability per ASTM A788/A961.
1. Standard Scope & Designation
ASTM A350/A350M covers carbon and low-alloy steel forgings requiring notch toughness testing for piping components used in low-temperature service. The steel must be completely killed and fine-grained. Products are made to specified dimensions or to dimensional standards such as ASME and API Specifications. The material must be suitable for welding to compatible pipe per ASTM A53, A106, A333, API 5L, and related standards.
| Attribute |
Inch-Pound (A350) |
SI / Metric (A350M) |
| Issuing Body |
ASTM International |
ASTM International |
| Unit System |
Inch-Pound (ksi, ft·lbf) |
SI (MPa, Joules) |
| Grade Designation |
LF1, LF2, LF3, LF5, LF6, LF9, LF787 |
LF1, LF2, LF3, LF5, LF6, LF9, LF787 |
| Dimensional Standards |
ASME B16.5, B16.9, B16.11, B16.47 |
ASME B16.5, B16.9, B16.11, B16.47 |
| Latest Edition |
A350/A350M-24 (2024) |
A350/A350M-24 (2024) |
| Governing General Req. |
ASTM A961 / A961M |
ASTM A961 / A961M |
Scope Statement (ASTM A350-24): "This specification covers several grades of carbon and low-alloy steel forged or ring-rolled flanges, forged fittings and valves intended primarily for low-temperature service and for use in pressure vessels at the applicable pressure-temperature ratings."
Defining Characteristic — Mandatory Impact Testing: Unlike ASTM A105 (no impact testing required), ASTM A350 requires mandatory Charpy V-Notch (CVN) impact testing per Table 3 at designated low temperatures. This makes A350 the preferred specification for all cryogenic, LNG, and sub-ambient temperature piping applications where notch toughness is critical for safety.
2. Grades Covered by ASTM A350
ASTM A350 covers 7 grades (LF1 through LF787) with multiple classes within several grades. Grades range from basic carbon steel (LF1) for moderately low temperatures to nickel-alloyed grades (LF3 with 3.3–3.7% Ni) for cryogenic service at −150°F (−101°C). LF787 is a unique precipitation-hardening steel with Cr, Mo, Cu, and niobium.
| Grade |
Classes |
Key Alloying Element |
Lowest Service Temp. |
Typical Application |
| LF1 |
Class 1 |
Basic carbon steel, Mn 0.60–1.35% |
−20°F (−29°C) |
Moderate low-temp, general industrial |
| LF2 |
Class 1, 2, 3 |
Carbon steel, Ni ≤0.40%, controlled residuals |
−50°F (−46°C) |
Most widely used; natural gas, LNG, cryogenic |
| LF3 |
Class 1, 2 |
Nickel steel, Ni 3.30–3.70%, C ≤0.20% |
−150°F (−101°C) ⭐ |
Cryogenic / LNG; deepest low-temp grade |
| LF5 |
Class 1, 2 |
Nickel steel, Ni 1.00–2.00% |
−75°F (−59°C) |
Petrochemical, low-temp processing |
| LF6 |
Class 1, 2, 3 |
HSLA, Mn 1.15–1.50%, N 0.01–0.030%, V 0.04–0.11% |
−60°F (−51°C) |
Enhanced toughness, offshore platforms |
| LF9 |
Class 2 |
Ni 1.60–2.24%, Cu 0.75–1.25% |
−100°F (−73°C) |
Deep cryogenic, offshore gas processing |
| LF787 |
Class 1, 2, 3 |
Cr 0.60–0.90%, Mo 0.15–0.25%, Cu 1.00–1.30%, Nb ≥0.02% |
−100°F (−73°C) (Class 3) |
Precipitation-hardening; maximum strength |
Grade LF2 is the most widely used ASTM A350 grade for low-temperature flange and fitting applications. It is the direct equivalent of ASTM A352 LCB (casting). LF2 Class 1 provides service to −50°F (−46°C) with mandatory impact testing at that temperature, making it the standard choice for natural gas, LNG, and cryogenic piping systems worldwide.
3. Chemical Composition — Heat Analysis (ASTM A350 Table 1)
Values are maximum unless shown as a range. For vacuum carbon-deoxidized steel per Supplementary Requirement S3/S4, silicon shall be 0.12% maximum. Residual elements must be reported. Carbon equivalent (CE) formula: CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.
| El. |
LF1 |
LF2 |
LF3 |
LF5 |
LF6 |
LF9 |
LF787 |
| C% |
0.30 |
0.30 |
0.20 |
0.30 |
0.22 |
0.20 |
0.07 |
| Mn% |
0.60–1.35 |
0.60–1.35 |
0.90 |
0.60–1.35 |
1.15–1.50 |
0.40–1.06 |
0.40–0.70 |
| P% |
0.035 |
0.035 |
0.035 |
0.035 |
0.025 |
0.035 |
0.025 |
| S% |
0.040 |
0.040 |
0.040 |
0.040 |
0.025 |
0.040 |
0.025 |
| Si% |
0.15–0.30 |
0.15–0.30 |
0.20–0.35 |
0.20–0.35 |
0.15–0.30 |
— |
0.40 |
| Ni% |
0.40 |
0.40 |
3.30–3.70 |
1.00–2.00 |
0.40 |
1.60–2.24 |
0.70–1.00 |
| Cr% |
0.30 |
0.30 |
0.30 |
0.30 |
0.30 |
0.30 |
0.60–0.90 |
| Mo% |
0.12 |
0.12 |
0.12 |
0.12 |
0.12 |
0.12 |
0.15–0.25 |
| Cu% |
0.40 |
0.40 |
0.40 |
0.40 |
0.40 |
0.75–1.25 |
1.00–1.30 |
| Nb% |
0.02 |
0.02 |
0.02 |
0.02 |
0.02 |
0.02 |
0.02 min |
| V% |
0.08 |
0.08 |
0.03 |
0.03 |
0.04–0.11 |
0.03 |
0.03 |
| N% |
— |
— |
— |
— |
0.01–0.030 |
— |
— |
Key Footnotes:
• Sum of Cu + Ni + Cr + V + Mo ≤ 1.00% on heat analysis (for LF1, LF2, LF6)
• Sum of Cr + Mo ≤ 0.32% on heat analysis
• Niobium (columbium) limit may be increased to 0.05% heat / 0.06% product by agreement
• LF787 requires Nb ≥ 0.02% minimum — a unique precipitation-hardening requirement
Note: Vacuum carbon-deoxidized steel (Supplementary Requirement S3/S4) requires Si ≤ 0.12% maximum. All elements must be reported on heat analysis. Carbon equivalent (CE) should be calculated and reported for weldability assessment.
4. Mechanical Property Requirements (ASTM A350 Table 2)
All values are minimum requirements after required heat treatment. Yield strength determined by either 0.2% offset method or 0.5% extension under load method. Testing per ASTM A370. Reduction of area applies to round specimens only.
| Grade / Class |
Tensile Strength
ksi [MPa] |
Yield Strength
min ksi [MPa] |
Elong. %
(round) |
Elong. %
(strip ≥5/16") |
RA %
min |
| LF1 Class 1 |
60–85 [415–585] |
30 [205] |
25 |
28 |
38 |
| LF2 Class 1 & 2 |
70–95 [485–655] |
36 [250] |
22 |
30 |
30 |
| LF2 Class 3 |
65–85 [450–585] |
55 [380] |
20 |
28 |
38 |
LF3 Class 1 & 2
LF5 Class 2 |
70–95 [485–655] |
37.5 [260] |
22 |
30 |
35 |
| LF5 Class 1 |
60–85 [415–585] |
30 [205] |
25 |
28 |
38 |
| LF6 Class 1 |
75–100 [515–690] |
60 [415] |
20 |
28 |
40 |
| LF6 Class 2 & 3 |
66–91 [455–630] |
46 [315] |
25 |
28 |
40 |
| LF9 Class 2 |
75–95 [515–655] |
65 [450] |
20 |
28 |
45 |
| LF787 Class 1 |
63–88 [435–605] |
46 [315] |
25 |
28 |
45 |
| LF787 Class 2 & 3 |
75–95 [515–655] |
65 [450] |
20 |
28 |
45 |
Note: Strip specimen elongation for thickness < 5/16" is calculated by equation: 48t + 13 (or 48t + 15 for higher-elongation grades). Hardness testing per Section 7.3 of A350. Maximum hardness limits apply per heat treatment condition.
5. Charpy V-Notch Impact Test Requirements (ASTM A350 Table 3 & Table 4)
Impact testing is mandatory for all ASTM A350 forgings — this is the defining requirement that separates A350 from A105. The test verifies notch toughness at the specified low temperature, ensuring material suitability for cryogenic and sub-ambient service. Tests are performed per ASTM E23 on standard 10×10 mm specimens (subsize rules per A370 apply for thin sections).
5.1 Standard Impact Test Temperature (Table 4)
| Grade / Class |
Test Temp (°F) |
Test Temp (°C) |
Service Category |
| LF1 Class 1 |
−20°F |
−29°C |
Moderate low-temp |
| LF2 Class 1 |
−50°F |
−46°C |
Standard low-temp |
| LF2 Class 2 |
0°F |
−18°C |
Near-ambient low-temp |
| LF3 Class 1 & 2 |
−150°F |
−101°C |
Cryogenic / LNG ⭐ |
| LF5 Class 1 & 2 |
−75°F |
−59°C |
Deep low-temp |
| LF6 Class 1 |
−60°F |
−51°C |
Low-temp offshore |
| LF6 Class 2 & 3 |
0°F |
−18°C |
Near-ambient low-temp |
| LF9 Class 2 |
−100°F |
−73°C |
Deep cryogenic |
| LF787 Class 2 |
−75°F |
−59°C |
Precipitation-hardened |
| LF787 Class 3 |
−100°F |
−73°C |
Max cryogenic PH |
5.2 Minimum Impact Energy Requirements (Table 3)
| Grade / Class |
Min. Avg. (3 spec.)
ft·lbf [J] |
Min. Single Spec.
ft·lbf [J] |
| LF1 Class 1, LF9 Class 2 |
13 [18] |
10 [14] |
| LF2 Class 1 |
15 [20] |
12 [16] |
| LF2 Class 2 |
20 [27] |
15 [20] |
| LF3 Class 1 & 2 |
15 [20] |
12 [16] |
| LF5 Class 1 & 2 |
15 [20] |
12 [16] |
| LF6 Class 1 |
15 [20] |
12 [16] |
| LF6 Class 2 & 3 |
20 [27] |
15 [20] |
| LF787 Class 2 & 3 |
15 [20] |
12 [16] |
Grade LF3 — The Cryogenic Champion: ASTM A350 LF3 with its 3.30–3.70% nickel content provides the deepest low-temperature capability (tested at −150°F/−101°C). This makes LF3 the preferred grade for LNG storage terminals, LNG carrier marine systems, and deep cryogenic process units. Its low carbon (≤0.20%) and controlled alloying optimize impact toughness at extreme cold.
Subsize Specimens: When material thickness is insufficient for standard 10×10 mm specimens, subsize specimens (7.5×10, 5×10, 3.3×10, 2.5×10 mm) may be used per ASTM A370. Impact values for subsize specimens are typically adjusted by correlation factors provided in the standard.
6. Heat Treatment Requirements
All ASTM A350 forgings must be heat treated after hot working and before delivery. The steel must be completely killed and fine-grained. Permitted heat treatments are Normalized (N), Normalized + Tempered (N+T), or Quenched + Tempered (Q+T). Grade LF787 requires precipitation heat treatment. After hot working, forgings shall be allowed to cool substantially below the transformation range before heat treatment.
| Heat Treatment |
Applicable Grades |
Min. Tempering Temp. |
Typical Application |
| Normalized (N) |
LF1, LF2, LF3, LF5 |
1100°F (590°C) |
Standard low-temp forgings; optimal toughness |
| Normalized + Tempered (N+T) |
LF1, LF2, LF3, LF5, LF6, LF787 |
1100°F (590°C) |
Improved toughness; stress-relieved forgings |
| Quenched + Tempered (Q+T) |
LF2 Class 3, LF6 Class 2/3, LF787 |
1100°F (590°C) |
Higher strength classes; quenched forgings |
| Normalized + Precipitation HT (N+PH) |
LF787 |
— |
Precipitation-hardened; Class 1 only |
| Quenched + Precipitation HT (Q+PH) |
LF787 Class 3 |
— |
Max strength; Q+T + precipitation age |
Hold Time Requirement: For Q+T forgings, the minimum hold time at the tempering temperature is 30 minutes per inch (30 min/25 mm) of maximum thickness, with a minimum hold time of 30 minutes regardless of section size.
Melting Practice: Steel shall be produced by open hearth, basic oxygen, electric furnace, or vacuum-induction melting (VIM). Steel must be completely deoxidized and fine-grained. Vacuum carbon deoxidation (per Supplementary Requirement S3/S4) requires Si ≤ 0.12% and provides superior cleanliness for critical cryogenic service.
7. Testing & Inspection Requirements
| Test / Inspection |
Requirement |
Reference Standard |
| Chemical Analysis (Heat) |
One per heat; all elements including residuals per Table 1 |
ASTM A961 |
| Tensile Test (Room Temp.) |
One per lot; yield, tensile, elongation, RA per Table 2 |
ASTM A370 |
| Charpy V-Notch Impact Test |
MANDATORY; 3 specimens per lot; avg + single spec. min per Table 3 |
ASTM E23 |
| Hardness Test |
Per lot; hardness per Section 7.3 and HT condition |
ASTM E10 / E18 |
| Hydrostatic Test |
Per flange/fitting size; test pressure per B16.5 / B16.47 |
ASME B16.5 / B16.47 |
| Ultrasonic Testing (UT) |
Supplementary S1; for critical service forgings |
ASTM A388 |
| Magnetic Particle (MT) |
Supplementary S2; surface inspection |
ASTM E709 |
| Liquid Penetrant (PT) |
Supplementary; surface defect detection |
ASTM E165 |
| PMI (Positive Material ID) |
Mandatory for alloy grades; verify chemistry at delivery |
ASTM E572 / E1471 |
| Drop Weight Test |
Supplementary S5; nil-ductility test for thick sections |
ASTM E208 |
8. Flange Types & Compatible A350 Grades
ASTM A350 forgings are manufactured to dimensional standards ASME B16.5 (NPS ½–24) and ASME B16.47 (NPS 26–60). LF2 is the most common grade for standard low-temperature flanges (up to −50°F/−46°C). For deeper cryogenic service, use LF3 (−150°F/−101°C) or LF9 (−100°F/−73°C).
| Flange Type |
Pressure Ratings |
Common A350 Grades |
| Weld Neck Flange (WN) |
150# – 2500# |
LF1, LF2, LF3, LF5, LF6 |
| Slip-On Flange (SO) |
150# – 600# |
LF1, LF2 |
| Socket Weld Flange (SW) |
150# – 900# |
LF1, LF2, LF3 |
| Threaded Flange (TH) |
150# – 600# |
LF1, LF2 |
| Blind Flange (BL) |
150# – 2500# |
LF1, LF2, LF3, LF5, LF6, LF9, LF787 |
| Lap Joint Flange (LJ) |
150# – 2500# |
LF1, LF2, LF3 |
| Ring Joint Flange (RTJ) |
150# – 2500# |
LF2, LF3, LF5, LF9, LF787 |
| Long Weld Neck Flange |
150# – 900# |
LF2, LF3 |
Flange Face Types: Raised Face (RF), Flat Face (FF), Ring Joint (RTJ), Tongue & Groove (T&G), Male & Female (M&F). Dimensional requirements per ASME B16.5 (NPS ½–24) and ASME B16.47 Series A & B (NPS 26–60). API 605 for large diameter flanges.
9. Related Standards & Companion Specifications
| Standard |
Scope |
Relationship to A350 |
| ASTM A788 / A788M |
General requirements for steel forgings |
Primary governing general spec |
| ASTM A961 / A961M |
Common requirements for flanges, fittings, valves, parts |
Supplementary governing spec |
| ASME B16.5 |
Steel pipe flanges and flanged fittings (NPS ½–24) |
Mandatory dimensional standard |
| ASME B16.47 |
Large diameter steel flanges (NPS 26–60) |
Large size dimensional standard |
| ASME B16.34 |
Valves — Flanged, Threaded, and Welding End |
Valve dimensional & rating standard |
| ASTM A352 / A352M |
Steel castings, low-temp service (LCB, LC3) |
Casting equivalent of A350 |
| ASTM A420 / A420M |
Wrought carbon & alloy fittings for low-temp service |
Low-temp pipe fittings standard |
| ASTM A333 / A333M |
Seamless and welded steel pipe for low-temp service |
Compatible pipe material |
| API 5L |
Specification for line pipe |
Compatible line pipe |
| ASTM A370 |
Mechanical testing of steel products |
Required test method |
| ASTM E23 |
Notch bar impact testing of metallic materials |
Impact test method |
10. A350 vs A105 vs A694 — Flange Material Specifications Compared
| Criteria |
ASTM A350 (LF2) |
ASTM A105 |
ASTM A694 (F52) |
| Grade Designation |
LF1, LF2, LF3... (by temp) |
A105 (single grade) |
F42–F70 (by yield) |
| Impact Testing |
MANDATORY CVN at low temp |
Not required |
Not required (optional) |
| Min. Yield (ksi) |
36 (LF2) |
36 |
52 (F52) / 60 (F60) |
| Min. Tensile (ksi) |
70 (LF2) |
70 |
66 (F52) / 75 (F60) |
| Steel Deoxidation |
Completely killed & fine-grained |
Killed steel preferred |
Completely deoxidized |
| Primary Application |
Low-temp / cryogenic (−150°F to 0°F) |
Ambient / moderate temp |
High-pressure transmission |
| Max C% |
0.30 (LF2) / 0.20 (LF3) |
0.35 |
0.30 |
| Forging Required |
Yes — forged or ring-rolled |
Yes — forged only |
Yes — no bar/plate machining |
Grade Selection: Choose A350 LF2 for all low-temperature service (≤−29°C/−20°F) where impact toughness is required by code (ASME B31.3, B31.8, etc.). Choose A105 for ambient-temperature general-purpose piping. Choose A694 F60/F65 for high-pressure transmission pipelines where maximum yield strength is the priority over cryogenic capability.
11. How to Order — Required Information
| No. |
Item |
Requirement / Example |
| 1 |
Specification |
ASTM A350/A350M or ASME SA-350 |
| 2 |
Grade & Class |
e.g., LF2 Class 1 (most common), LF3 Class 2, LF9 Class 2 |
| 3 |
Product Type |
Weld Neck Flange, Slip-On Flange, Blind Flange, Threaded Flange, Socket Weld Flange, Lap Joint Flange |
| 4 |
NPS & Pressure Rating |
e.g., NPS 6 – 300# WN RF Flange; NPS 16 – 150# RTJ Blind |
| 5 |
Face Type & Finish |
Raised Face (RF), Flat Face (FF), Ring Joint (RTJ) |
| 6 |
Dimensional Standard |
ASME B16.5, B16.47, API 605 |
| 7 |
Heat Treatment |
Normalized (N) or Normalized + Tempered (N+T) — Q+T if specified |
| 8 |
Impact Test Temp & Energy |
Confirm test temperature per grade class; e.g., LF2 Cl.1 @ −50°F (−46°C), 15 ft·lbf avg |
| 9 |
Supplementary Requirements |
S1 (UT), S2 (MT), S3 (Vacuum deoxidation), S5 (Drop weight), etc. |
| 10 |
Certification |
EN 10204 3.1 Certified Mill Test Report with impact test results |
Download ASTM A350 / A350M Standard (PDF)
The official ASTM A350/A350M-24 (2024) PDF is available from our HT Pipe Download Center. The document includes full mechanical property Table 2, chemical composition Table 1, mandatory Charpy V-Notch impact test Table 3, impact temperature Table 4, heat treatment requirements, and supplementary testing options.
Go to Download Center | Request MTC / Quotation
12. Frequently Asked Questions
Q: What is ASTM A350 LF2 used for?
ASTM A350 LF2 is used for forged flanges, fittings, valves, and parts in low-temperature and cryogenic service where Charpy V-Notch impact toughness is required. Typical applications include natural gas pipelines and city gate stations, LNG storage and regasification terminals, offshore platform piping in cold environments, petrochemical process units at sub-ambient temperatures, and air separation (ASU) plants. LF2 Class 1 is tested at −50°F (−46°C) and is the most widely specified A350 grade globally.
Q: What is the impact test requirement for ASTM A350 LF2 Class 1?
ASTM A350 LF2 Class 1 requires Charpy V-Notch impact testing at −50°F (−46°C). The minimum average absorbed energy for 3 specimens is 15 ft·lbf (20 Joules); no single specimen may be less than 12 ft·lbf (16 Joules). Testing is performed per ASTM E23 on standard 10×10 mm specimens (or approved subsize per ASTM A370). This mandatory impact testing is the key differentiator between A350 and A105.
Q: What is the difference between ASTM A350 LF2 and ASTM A352 LCB?
ASTM A350 LF2 is the forging specification; ASTM A352 LCB is the casting equivalent. Both are carbon steel low-temperature service materials with similar chemistry and mechanical property requirements (yield ≥36 ksi). The main difference is manufacturing process: LF2 forgings are produced by hot working (hammering or pressing), while LCB castings are poured into molds. Forgings generally have better structural integrity and toughness than castings of similar composition.
Q: What is the lowest temperature service grade in ASTM A350?
Grade LF3 with Class 1 and Class 2 is the deepest low-temperature grade in ASTM A350, rated for service down to −150°F (−101°C). LF3 achieves this through 3.30–3.70% nickel content, which stabilizes the microstructure and maintains ductility at cryogenic temperatures. For comparison: LF2 (−50°F/−46°C), LF5 (−75°F/−59°C), LF9 (−100°F/−73°C), and LF787 Class 3 (−100°F/−73°C) all have higher temperature limits than LF3.
Q: Can ASTM A350 LF2 be used at ambient temperature?
Yes, ASTM A350 LF2 can be used at ambient and above-ambient temperatures. In fact, LF2 at room temperature exceeds the mechanical property requirements of A105 (both have min 36 ksi yield, 70 ksi tensile). However, the impact testing cost makes A105 more economical for non-low-temperature applications. LF2 is specified when the piping system may experience low temperatures (shutdown, maintenance, cold product) or when project specifications require A350-level quality assurance regardless of operating temperature.
Q: What heat treatment is required for ASTM A350 LF3 forgings?
ASTM A350 LF3 forgings are typically delivered in the Normalized (N) or Normalized + Tempered (N+T) condition. Normalizing refines the grain structure and optimizes the impact toughness required for cryogenic service at −150°F (−101°C). Quench and Temper (Q+T) is generally not recommended for LF3 as the 3.5% nickel steel responds well to normalizing. LF3 may not be ordered in the as-forged condition. The minimum tempering temperature of 1100°F (590°C) applies when N+T or Q+T is selected.
Request a Quote — ASTM A350 Carbon & Low-Alloy Steel Flanges & Forgings
Zhengzhou Huitong (HT Pipe) supplies ASTM A350 / SA-350 compliant forged flanges and fittings in all standard grades (LF1, LF2, LF3, LF5, LF6, LF9, LF787) with full Charpy V-Notch impact test reports, EN 10204 3.1 CMTRs, and competitive lead times from our manufacturing facility in China.
