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ISO 26304:2025 — Welding Consumables for Submerged Arc Welding of High Strength Steels
Classification of solid wire electrodes, tubular cored electrodes, and electrode-flux combinations
Introduction to ISO 26304:2025
ISO 26304:2025 is the fourth edition of the international standard that specifies classification requirements for solid wire electrodes, tubular cored electrodes, and electrode-flux combinations used in submerged arc welding (SAW) of high strength steels. This standard applies to steels with a minimum yield strength greater than 500 MPa or a minimum tensile strength greater than 570 MPa, covering both the as-welded and post-weld heat-treated conditions.
Global Market Reality: The standard recognizes two different classification approaches used worldwide — System A (based on yield strength and 47 J impact energy, originally from EN 14295) and System B (based on tensile strength and 27 J impact energy, mainly used around the Pacific Rim). Both systems are valid, and a single product may carry classification designations from either or both systems.
The 2025 edition introduces significant structural changes, reformatting the document in a single-column layout with System A and System B requirements presented in separate tables and clauses. Key updates include revised Table 3 values for System B to align with ISO 18275 and ISO 18276, updated chemical composition tables, and expanded designation examples.
Classification Systems and Requirements
System A — Yield Strength Based (47 J Impact Energy)
System A classifies consumables by yield strength with minimum 47 J impact energy. The designation includes up to seven parts: product/process symbol (S for solid wire, T for tubular cored), tensile properties symbol, impact properties symbol, flux type per ISO 14174, chemical composition symbol, post-weld heat treatment symbol, and an optional hydrogen content symbol.
System B — Tensile Strength Based (27 J Impact Energy)
System B classifies consumables by tensile strength with minimum 27 J impact energy. The designation includes five parts: product/process symbol (SU for solid wire, TU for tubular cored), strength and elongation symbol, impact properties symbol, chemical composition symbol, and an optional hydrogen content symbol. The letter “U” after the impact designator indicates that the deposit meets an optional 47 J requirement at the designated test temperature.
| System A Symbol | Min. Yield (MPa) | Tensile Strength (MPa) | Min. Elong. (%) | System B Symbol | Tensile Strength (MPa) |
|---|---|---|---|---|---|
| 55 | 550 | 640-820 | 18 | 59X | 590-790 |
| 62 | 620 | 700-890 | 18 | 62X | 620-820 |
| 69 | 690 | 770-940 | 17 | 69X | 690-890 |
| 79 | 790 | 880-1,080 | 16 | 76X | 760-960 |
| 89 | 890 | 940-1,180 | 15 | 83X | 830-1,030 |
Important Distinction: The two-run technique is only applicable to System B, as the 47 J impact energy requirement of System A is difficult to achieve with two-run welding. For System B two-run products, the symbol includes a “T” (e.g., 69TX) and is tested separately per Clause 6.2.
Chemical Composition Requirements
The standard includes detailed chemical composition tables for solid wire electrodes (Table 7) and tubular cored electrode-flux combinations (Table 8). These tables show the composition ranges for carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum, copper, and other alloying elements. The classification symbols use a logical system — for example, “SUN2M3” indicates a solid wire (S), with specific Ni-Mo alloy content. The standard also provides cross-references between System A and System B designations, with parentheses indicating near-matches across systems.
Engineering Insights for Welding Engineers
Matching Strength Properties
One critical insight from the standard is that the yield-to-tensile ratio of weld metal is generally higher than that of the parent material. Matching weld metal yield strength to parent metal yield strength does NOT necessarily ensure that tensile strengths match. This means engineers must carefully consider which property is critical for their application and select consumables accordingly — referencing either columns 3 or 6 of Table 3 as appropriate.
Selection Strategy: For applications requiring matching tensile strength (e.g., pressure vessels under ASME codes), select consumables by tensile strength rather than yield strength. For structural applications governed by yield strength limits, the yield-based System A provides a more direct match. For the highest integrity applications, the optional 47 J impact requirement in System B (denoted by “U” after the impact symbol) provides additional toughness assurance.
Flux-Electrode Interchangeability
A crucial engineering consideration is that combinations of electrodes and fluxes from different manufacturers may not be interchangeable even if they share the same classification. Two fluxes with the same ISO 14174 classification may produce different mechanical properties when paired with the same electrode. Similarly, two tubular cored wires of the same classification can produce different results with the same flux. Engineers should always qualify consumable combinations as a system rather than assuming interchangeability based on individual classifications.
Hydrogen Control
The standard includes optional symbols for diffusible hydrogen content determined in accordance with ISO 3690. Proper hydrogen control is critical for high-strength steel welding to prevent hydrogen-induced cracking (HIC). Annex A provides informative guidance on the possible risk of weld metal hydrogen cracking — a vital reference for welding engineers working with strength levels above 690 MPa where hydrogen sensitivity increases significantly.
Frequently Asked Questions
What is the difference between System A and System B in ISO 26304?
System A classifies consumables based on yield strength with a minimum average impact energy of 47 J, originally derived from European standard EN 14295. System B classifies based on tensile strength with a minimum average impact energy of 27 J, primarily used in Pacific Rim markets. System A uses 3 impact specimens, while System B uses 5 with a different evaluation method (discarding highest and lowest values).
Can the same welding consumable be classified under both systems?
Yes. By appropriately restricting the chemical composition to meet both sets of requirements, a single product can carry classifications from both System A and System B. The standard provides cross-reference tables showing near-matches between the two systems.
What does the two-run technique mean for System B?
The two-run technique is a welding procedure where the weld is completed in two passes — typically one root pass and one cover pass. It is only applicable to System B because achieving 47 J impact energy (System A requirement) is difficult with only two runs. System B two-run products use a “TX” suffix in their designation (e.g., 69TX).
How does ISO 26304:2025 handle hydrogen content?
Hydrogen content is addressed through an optional designation symbol (H followed by a number indicating maximum diffusible hydrogen in mL/100g of deposited metal, per ISO 3690). For high-strength steels above 690 MPa yield strength, the standard’s informative Annex A provides guidance on hydrogen cracking risk, which becomes increasingly critical as strength levels rise.
