ASME BPVC.II.C-2025 · SFA-5.32M/SFA-5.32 · AWS A5.32M/A5.32:2021 · ISO 14175:2008 MOD · Pages 1001–1023
Ever had an ASME-coded weld rejected for porosity or oxidation — and your technique was perfect? Nine times out of ten the culprit is the shielding gas: wrong classification, wrong purity, or a gas that doesn’t meet code. This complete technical guide walks through every clause, table, and designation rule in SFA-5.32M/SFA-5.32 — the ASME Boiler & Pressure Vessel Code specification for welding shielding gases (Section II Part C, 2025 Edition).
📋 In This Article
- What Is SFA-5.32?
- Scope & Covered Processes (Clause 1)
- Properties of Gases — Table 1
- Classification System — Table 2 (Clause 5.1)
- Designation System — Clause 5.2
- Optional P Designator — Table 3 (Clause 6)
- Mixture Tolerances — Table 4 (Clause 7)
- Purity & Dew Point — Table 5 (Clause 8)
- Dew Point Conversion — Table 6
- Gas Selection Guide — Annex A6 & A7
- Cylinder Marking — Clause 11
- Certification & Testing — Clauses 9, 10, 13
- Safety — Annex A8
- Quick Reference & Compliance Checklist
1. What Is SFA-5.32?
SFA-5.32M/SFA-5.32, published in ASME BPVC Section II Part C, is the mandatory standard for shielding gases used in ASME-coded welding. Full title: “Welding Consumables — Gases and Gas Mixtures for Fusion Welding and Allied Processes.” It is identical with AWS Specification A5.32M/A5.32:2021, which is a modification of ISO 14175:2008. Per the title page: “In case of dispute, the original AWS text applies.” The 2025 edition is ASME BPVC.II.C-2025, covering pages 1001–1023.
Any WPS written to ASME Section IX must reference shielding gases conforming to SFA-5.32. Gas not listed under a valid SFA-5.32 classification cannot be used on ASME-coded pressure vessels, boilers, or pressure piping without engineering justification.
2. Scope and Covered Processes (Clause 1)
Per Clause 1.1, SFA-5.32 covers gases and gas mixtures used in: GTAW, GMAW, FCAW, PAW, PAC, PAG, LBW, LBC, ABW, and EGW. Application modes include manual, semi-automatic, mechanised, and automatic weld methods.
1. Fuel gases (acetylene, natural gas, propane) are NOT covered by SFA-5.32.
2. Resonator gases used in gas lasers are NOT covered.
3. Purity values are specified as delivered by the supplier — NOT at the point of use. This is the code’s language on cylinder-to-arc variation.
Per Clause 1.2, the classification system is independent of US Customary and SI units. Classification is based solely on chemical composition — not trade names or cylinder sizes.
3. Properties of Gases — Table 1
Per Clause 4, the relevant physical and chemical properties are given in Table 1. All density values are at 0°C and 0.101 MPa [32°F and 1 Atmosphere] — Table 1 footnote (a).
| Type of Gas | Symbol | Density kg/m³ [lb/ft³] | Relative Density to Air | Boiling Point °C [°F] | Reactivity During Welding |
|---|---|---|---|---|---|
| Argon | Ar | 1.784 [0.1114] | 1.380 | −185.9 [−302.6] | Inert |
| Helium | He | 0.178 [0.0111] | 0.138 | −268.9 [−452.0] | Inert |
| Carbon Dioxide | CO₂ | 1.977 [0.1234] | 1.529 | −78.5 [−109.3] ᵇ | Oxidizing |
| Oxygen | O₂ | 1.429 [0.0892] | 1.105 | −183.0 [−297.4] | Oxidizing |
| Nitrogen | N₂ | 1.251 [0.0781] | 0.968 | −195.8 [−320.4] | Low reactive ᶜ |
| Hydrogen | H₂ | 0.090 [0.0056] | 0.070 | −252.8 [−423.0] | Reducing |
| ᵃ All values at 0°C and 0.101 MPa (1.013 bar) [32°F and 1 Atmosphere (14.7 psia)]. ᵇ CO₂ value is a sublimation temperature (solid-to-gas transition). ᶜ “The behaviour of nitrogen varies with different materials and applications. Possible influences must be considered by the user.” — Table 1 Note c. | |||||
Argon (1.380) and CO₂ (1.529) are significantly heavier than air and will accumulate at the bottom of pressure vessels, pits, and confined spaces, creating an asphyxiation hazard. See Section 13 — Safety.
4. Classification System — Table 2 (Clause 5.1)
Per Clause 5.1.1: “Gases and gas mixtures shall be classified by the number of this standard, followed by the symbol for the gas in accordance with Table 2. The classification is based on the reactivity of the gas or gas mixture.” A gas classified under one classification shall not be classified under any other (Clause 5.1.1).
Per Clause 5.1.2, seven main groups are defined:
Full Table 2 — Classification of Gases
| Main Group | Sub-group | CO₂ % | O₂ % | Ar [Balance] ᵃ | He % | H₂ % | N₂ % |
|---|---|---|---|---|---|---|---|
| I | 1 | — | — | 100% | — | — | — |
| 2 | — | — | — | 100% | — | — | |
| 3 | — | — | Balance ᵃ | 0.5–95% | — | — | |
| M1 | 1 | 0.5–5% | — | Balance ᵃ | — | 0.5–5% | — |
| 2 | 0.5–5% | — | Balance ᵃ | — | — | — | |
| 3 | — | 0.5–3% | Balance ᵃ | — | — | — | |
| 4 | 0.5–5% | 0.5–3% | Balance ᵃ | — | — | — | |
| M2 | 0 | 5–15% | — | Balance ᵃ | — | — | — |
| 1 | 15–25% | — | Balance ᵃ | — | — | — | |
| 2 | — | 3–10% | Balance ᵃ | — | — | — | |
| 3 | 0.5–5% | 3–10% | Balance ᵃ | — | — | — | |
| 4 | 5–15% | 0.5–3% | Balance ᵃ | — | — | — | |
| 5 | 5–15% | 3–10% | Balance ᵃ | — | — | — | |
| 6 | 15–25% | 0.5–3% | Balance ᵃ | — | — | — | |
| 7 | 15–25% | 3–10% | Balance ᵃ | — | — | — | |
| M3 | 1 | 25–50% | — | Balance ᵃ | — | — | — |
| 2 | — | 10–15% | Balance ᵃ | — | — | — | |
| 3 | 25–50% | 2–10% | Balance ᵃ | — | — | — | |
| 4 | 5–25% | 10–15% | Balance ᵃ | — | — | — | |
| 5 | 25–50% | 10–15% | Balance ᵃ | — | — | — | |
| C | 1 | 100% | — | — | — | — | — |
| 2 | Balance | 0.5–30% | — | — | — | — | |
| R | 1 | — | — | Balance ᵃ | — | 0.5–15% | — |
| 2 | — | — | Balance ᵃ | — | 15–50% | — | |
| N | 1 | — | — | — | — | — | 100% |
| 2 | — | — | Balance ᵃ | — | — | 0.5–5% | |
| 3 | — | — | Balance ᵃ | — | — | 5–50% | |
| 4 | — | — | Balance ᵃ | — | 0.5–10% | 0.5–5% | |
| 5 | — | — | — | — | 0.5–50% | Balance | |
| O | 1 | — | 100% | — | — | — | — |
| G | — | Gas mixtures containing components not listed, or mixtures outside the composition ranges listed. Note ᵇ: two G-classified gases may NOT be interchangeable. | |||||
| ᵃ For classification purposes, argon may be substituted partially or completely by helium (Table 2, Note a). ᵇ Two gas mixtures with the same G-classification may not be interchangeable (Table 2, Note b). | |||||||
Key Group Restrictions from Annex A7
“Reactive and oxidizing gases such as carbon dioxide (CO₂) and oxygen (O₂) can have detrimental effects on base metals such as aluminum, nickel, titanium, zirconium, and tungsten. For this reason, carbon dioxide or oxygen cannot be used as the shielding gas for gas tungsten arc welding.”
This prohibition covers ALL base metals in GTAW — not just reactive metals.
Per Clause A7.2.2: “AWS A5.32 M21 – ArC-20+ mixtures containing more than 20% carbon dioxide will not support spray transfer, but rather globular type transfer.”
“Shielding gases containing nitrogen are not recommended for welding carbon steel.” Also: nitrogen root shielding of stainless steel welds may reduce ferrite levels — critical where ferrite content control is specified. Small additions up to 3% N₂ in Ar are used for duplex stainless GMA/GTA welding.
“Using GMAW, 100% helium will only produce globular transfer. The argon percentage must be at least 20% when mixed with helium to produce and maintain a stable spray transfer.”
5. Designation System — Clause 5.2
The designation extends classification by adding chemical component symbols and actual volume percentages. Per Clause 5.2.1, six component symbols: Ar (argon), C (CO₂), H (H₂), N (N₂), O (O₂), He (helium). Base gas comes first, followed by other components in decreasing order of percent, values separated by a forward slash (/).
All 7 Official Designation Examples — Clause 5.2.2
| Ex. | Gas Composition | Classification | Full Designation | Note |
|---|---|---|---|---|
| 1 | 100% Argon | AWS A5.32 – I1 | AWS A5.32 – I1 | Single-component: no symbol or 100 required |
| 2 | 6% CO₂ + 4% O₂ in Argon | AWS A5.32 – M25 | AWS A5.32 – M25 – ArCO – 6/4 | Most common ternary mix pattern |
| 3 | 30% Helium in Argon | AWS A5.32 – I3 | AWS A5.32 – I3 – ArHe – 30 | Binary inert blend |
| 4 | 5% Hydrogen in Argon | AWS A5.32 – R1 | AWS A5.32 – R1 – ArH – 5 | Reducing; H₂ safety rules apply |
| 5 | 90% He + 7.5% Ar + 2.5% CO₂ (He base) | AWS A5.32 – M12 | AWS A5.32 – M12 – HeArC – 7.5/2.5 | He replaces Ar as base (Table 2 Note a) |
| 6 | 0.05% O₂ in Argon (below Table 2 0.5% minimum) | AWS A5.32 – G | AWS A5.32 – G – ArO – 0.05 | Outside Table 2 range → G |
| 7 | 0.05% Xenon (Xe) in Argon — Xe not in Table 2 | AWS A5.32 – G | AWS A5.32 – G – Ar+Xe – 0.05 | Unlisted component: + prefix required |
6. Optional P Designator — Table 3 (Clause 6)
Per Clause 6, the letter P placed at the end of a designation (separated by a dash) indicates the gas meets the stricter maximum moisture requirements of Table 3. This is an optional higher-purity tier for critical applications such as reactive metal GTAW and precision stainless welding.
| Classification | Maximum Moisture (ppmv) | Approx. Dew Point (°C) via Table 6 |
|---|---|---|
| I1, I2, I3 | 10.5 | ≤ −60°C [−76°F] — strictest |
| M11, M12, M13, M14 | 32 | ≤ −44°C approx. |
| M20, M21, M22, M23, M24, M25, M26, M27 | 32 | ≤ −44°C approx. |
| M31, M32, M33, M34, M35 | 32 | ≤ −44°C approx. |
| C (all sub-groups) | 32 | ≤ −44°C approx. |
| R, N, O groups | Not in Table 3 | P designator NOT available |
7. Mixture Tolerances — Table 4 (Clause 7)
Per Clause 7, mixture tolerances apply to volumetric percentages per Table 4. These define the allowable deviation from nominal and are the code basis for incoming gas CoA acceptance criteria.
| Nominal Concentration % | Allowable Tolerance | Tolerance Type | Code Example (Clause 7) |
|---|---|---|---|
| Greater than 5% | ± 10% of the nominal value | RELATIVE | 25% CO₂ nominal → ±2.5% → range: 22.5% to 27.5% |
| 1% to 5% | ± 0.5% absolute | ABSOLUTE | 2.5% O₂ nominal → ±0.5% → range: 2.0% to 3.0% |
| Less than 1% | Not specified | N/A | No tolerance limit defined in the code |
Above 5%: tolerance is relative — ±10% of nominal. Below 5%: tolerance is absolute — ±0.5 percentage points. A 50% CO₂ mix has ±5% tolerance; a 3% O₂ mix has only ±0.5%.
8. Purity and Dew Point Requirements — Table 5 (Clause 8)
Per Clause 8: “The maximum dew point and purity of gases and gas mixtures shall meet the requirements of Table 5.” These are mandatory minimums — all values are as delivered by the supplier, not at point of use.
| Main Group | Gas Type | Min. Purity % by Volume | Max. Dew Point °C [°F @ 14.7 psia] | Max. Moisture ppmv |
|---|---|---|---|---|
| I | Inert | 99.99 | −50 [−58] | 40 |
| M1 ᵃ | Gas mix | 99.9 | −50 [−58] | 40 |
| M2 ᵃ | Gas mix | 99.9 | −44 [−48] | 80 |
| M3 ᵃ | Gas mix | 99.9 | −40 [−40] | 120 |
| C ᵃ | Carbon dioxide | 99.8 | −40 [−40] | 120 |
| R | Reducing | 99.95 | −50 [−58] | 40 |
| N | Nitrogen | 99.9 | −50 [−58] | 40 |
| O | Oxygen | 99.5 | −50 [−58] | 40 |
| ᵃ Nitrogen: 1000 ppm maximum as impurity in M1, M2, M3, and C groups (see Clause A7.1.3). General note: for certain applications a higher purity and/or lower dew point may be recommended. | ||||
9. Dew Point Conversion — Table 6 (Clause 8)
Per Clause 8, moisture can be expressed as ppmv or as dew point in °C at 0.101 MPa [°F at 14.7 psia]. Table 6 (at 1 atmosphere: 21°C @ 760 mm Hg / 70°F @ 14.7 psia) is used to convert between the two. The table contains 57 rows from −90.0°C (0.10 ppmv) to −20.0°C (1020 ppmv).
| Dew Point °C | Dew Point °F | Moisture ppmv | Code Relevance |
|---|---|---|---|
| −60.0 | −76 | 10.5 | Table 3 — P designator limit for I1, I2, I3 groups |
| −50.0 | −58 | 39.0 ≈ 40 | Table 5 — Max moisture for I, M1, R, N, O groups |
| −44.4 | −48 | 76 ≈ 80 | Table 5 — Max moisture for M2 group |
| −40.0 | −40 | 128 ≈ 120 | Table 5 — Max moisture for M3 and C groups |
| −35.0 | −31 | 222 | Below code minimum — non-conforming for all groups |
| −20.0 | −4 | 1020 | Far below code minimum — not acceptable |
1. Identify your gas group from Table 2.
2. Find the Table 5 maximum moisture ppmv or dew point °C for that group.
3. If your CoA reports dew point in °C, use Table 6 to convert to ppmv and compare.
4. If WPS specifies P designator, also check against Table 3.
Example: CoA shows dew point −52°C for I1 (Argon). Table 6: −52°C ≈ 34 ppmv. Table 5 limit for Group I: ≤40 ppmv → PASS Table 5. Table 3 P-designator for I-group: ≤10.5 ppmv → FAILS Table 3 → cannot use –P suffix.
10. Gas Selection Guide — Annex A6 and A7
Annex A is informative (not mandatory) but represents the code committee’s guidance. Clause A6 lists 15 gas selection factors: base metal type and thickness, arc characteristics, metal transfer, travel speed, depth and width of fusion, cost, mechanical properties, root opening, base material cleanliness, spatter, arc cleaning action, gas purity, joint configuration, welding position, and fume generation.
Gas Selection Matrix
| Base Metal | Process / Transfer | Classification | Typical Designation | Annex Ref. |
|---|---|---|---|---|
| Carbon Steel | GMAW Short circuit / Globular | M21 or M31 | AWS A5.32 – M21 – ArC-25 | A7.2.2.3 |
| Carbon Steel | GMAW Spray transfer | M20 or M12 | AWS A5.32 – M20 – ArC-8 | A7.2.2.1 |
| Carbon Steel | GMAW 100% CO₂ | C1 | AWS A5.32 – C1 | A7.1.2 |
| Carbon Steel | FCAW all positions | M21 or M31 | AWS A5.32 – M21 – ArC-25 | A7.2.2.3 |
| Stainless Steel | GTAW — all positions | I1 | AWS A5.32 – I1 | A7.1.1 / Clause A6 |
| Stainless Steel | GMAW Spray transfer | M13 | AWS A5.32 – M13 – ArO-2 | A7.2.1.2 |
| Stainless Steel | GMAW Short circuit (TriMix) | M12 | AWS A5.32 – M12 – HeArC – 7.5/2.5 | A7.3.2.3 |
| Aluminium | GTAW / GMAW | I1 or I3 | AWS A5.32 – I1 or I3 – ArHe-30 | A7.1.1 / A7.2.3 |
| Nickel Alloys | GTAW / PAW | I1 or R1 | AWS A5.32 – R1 – ArH-5 | A7.1.1 / A7.2.5 |
| Duplex Stainless | Back purging — root pass | N1 or N2 | AWS A5.32 – N2 – ArN-3 | A7.1.3 |
| Titanium / Zirconium | GTAW — reactive metals | I1 (P grade) | AWS A5.32 – I1 – P | A7.1.1 |
| High-strength steel | GMAW Pulsed spray | M12 (ArHeC) | AWS A5.32 – M12 – HeArC – 20/5 | A7.3.2.1 |
| ⛔ PROHIBITION (Clause A6): CO₂ and O₂ CANNOT be used as shielding gas for GTAW on ANY base metal — ASME BPVC.II.C-2025. | ||||
11. Cylinder Marking — Clause 11
Per Clause 11, every cylinder must be clearly marked with at minimum: (1) name of manufacturer or supplier; (2) trade name; (3) full designation per Clause 5.2; (4) health and safety warnings per local/national regulations.
12. Certification and Testing — Clauses 9, 10, 13
Clause 9 — Testing: Samples shall be drawn from the individual cylinder, vessel, or gas outlet source. Testing may be conducted by the manufacturer or supplier. Results must fulfil Tables 2, 4, and 5. Any special testing requirements shall be stated in the purchase order (Clause A3). Liquid premixed gases are limited to argon/oxygen mixes (Clause 9.1).
Clause 10 — Retesting: If any test fails, it shall be repeated twice. Both retests must pass. If either fails, the gas does not conform.
Clause 13 — Certification: Per Clause 13: “By affixing the AWS specification and classification designations on the packaging enclosing the product, the supplier (manufacturer) certifies that the product meets all of the requirements of the specification.”
Per Clause A4: Certification “is not to be construed to mean that tests of any kind were necessarily conducted on samples of the specific product shipped.” It is based on testing of representative product. For critical ASME work, state specific testing requirements explicitly in the purchase order.
Clause 14 — Cylinder Residual Gases: All containers shall be evacuated or residual gases analysed per CGA P-15 before refilling, with exemptions for containers equipped with backflow prevention devices (unless gas type changes or maintenance occurs).
Clause 15 — Packaging: Per DOT regulations (49 CFR 173.301). Cylinder sizes agreed between purchaser and supplier.
13. Safety — Annex A8
“Argon, carbon dioxide, helium, and nitrogen can displace oxygen in a worker’s breathing zone which can result in asphyxiation, and possibly death, when released in poorly vented, confined work areas. Argon and carbon dioxide cause a special concern since they are heavier than air and may concentrate in low areas such as in the bottom of pressure vessels, tanks, pits, and ships. Unless adequate ventilation and breathing air are supplied, care must be taken with any of these gases when they are released in enclosed areas or confined spaces. A safety watch should be provided and in attendance anytime a worker is using any of these gases in a vessel.”
“Hydrogen is a highly flammable gas. A mixture of hydrogen with oxygen or air in a confined area will explode when brought in contact with a flame or other source of ignition. Concentrations of hydrogen between 4% and 75% by volume in air are relatively easy to ignite by a low-energy spark and may cause an explosion. Smoking, open flames, unapproved electrical equipment, and other ignition sources must not be permitted in hydrogen areas. Store containers outdoors or in other well-ventilated areas.”
Escaping hydrogen cannot be detected by sight, smell, or taste. Consult NFPA 50-A (gaseous hydrogen) and 50-B (liquefied hydrogen).
For ventilation requirements, refer to Clause A5 and ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes. Additional guidance: AWS F3.2, Ventilation Guide for Welding Fume.
14. Quick Reference and Compliance Checklist
| Group | Gas Type | Min. Purity % | Max. Dew Pt °C [°F] | Max. Moisture ppmv | P-Designator Limit | Key Application |
|---|---|---|---|---|---|---|
| I | Inert (Ar, He, Ar/He) | 99.99 | −50 [−58] | 40 | ≤10.5 ppmv | GTAW all metals; GMAW non-ferrous |
| M1 | Mildly oxidizing | 99.9 | −50 [−58] | 40 | ≤32 ppmv | Stainless spray transfer GMAW |
| M2 | Moderately oxidizing | 99.9 | −44 [−48] | 80 | ≤32 ppmv | Carbon steel GMAW — most common |
| M3 | Highly oxidizing | 99.9 | −40 [−40] | 120 | ≤32 ppmv | Heavy section; high penetration FCAW |
| C | CO₂ based | 99.8 | −40 [−40] | 120 | ≤32 ppmv | GMAW/FCAW carbon steel; NOT spray |
| R | Reducing (H₂ mix) | 99.95 | −50 [−58] | 40 | Not in Table 3 | GTAW austenitic SS & Ni alloys; PAC |
| N | Nitrogen | 99.9 | −50 [−58] | 40 | Not in Table 3 | Back purging; duplex SS additions |
| O | Oxygen | 99.5 | −50 [−58] | 40 | Not in Table 3 | Minor additive; PAC plasma gas only |
✅ 10-Point ASME Code Compliance Checklist for Shielding Gas
- Cylinder label shows the full AWS A5.32 designation per Clause 11 (supplier, trade name, designation, safety info).
- Classification on label matches the classification specified in the WPS.
- Certificate of Analysis (CoA) confirms purity meets Table 5 minimum for the gas group.
- CoA dew point or ppmv value is within Table 5 maximum limits — cross-checked using Table 6 if needed.
- If WPS specifies the P designator, CoA confirms moisture meets Table 3 limits (10.5 or 32 ppmv depending on group).
- Mixture percentages fall within Table 4 tolerances (±10% of nominal for >5%; ±0.5% absolute for 1–5%).
- Gas is welding-grade — food-grade CO₂ and instrument-grade gases do not automatically conform to SFA-5.32.
- No CO₂ or O₂ used as primary shielding gas for GTAW — explicit prohibition per Clause A6.
- G-classified gases are treated as unique — no two G-classified gases are interchangeable per Table 2 Note b.
- Any special testing requirements stated in the purchase order per Clause A3; CoA retained for ASME documentation.
References
- ASME BPVC.II.C-2025 — Section II Part C, Specifications for Welding Rods, Electrodes, and Filler Metals. ASME, New York, 2025. SFA-5.32M/SFA-5.32, pp. 1001–1023.
- AWS A5.32M/A5.32:2021 — Welding Consumables — Gases and Gas Mixtures for Fusion Welding and Allied Processes. American Welding Society, Miami.
- ISO 14175:2008 — Welding consumables — Gases and gas mixtures for fusion welding and allied processes. ISO, Geneva.
- ANSI Z49.1 — Safety in Welding, Cutting, and Allied Processes.
- CGA P-15 — Filling of Industrial and Medical Nonflammable Compressed Gas Cylinders.
- 49 CFR 173.301 — US DOT requirements for shipment of compressed gases.
- ASTM E29 — Standard Practice for Using Significant Digits in Test Data.
- AWS F3.2 — Ventilation Guide for Welding Fume.