What D1.1 Requires for 異種金属 Joints
D1.1:2025 §5.7.2 addresses joints between base metals that are not the same 仕様書 or strength level. The governing rule is straightforward: the minimum preheat for the joint shall be the highest of the minimum preheats required for either base metal. Both metals must be listed in Table 5.6 as approved base metals for 事前認定 use.
This means you look up each steel independently in Table 5.11 using the same process, hydrogen カテゴリー, and governing 板厚, then apply the higher of the two results. There is no blending, averaging, or interpolation between the two values. The more conservative requirement governs without exception.
Dissimilar joints are common in 構造用鋼 製作. Connecting a lighter secondary member (A36) to a heavier primary framing member (A992 or A572 Gr.65) is a 規格 detail. The preheat requirement for that joint is determined by whichever steel demands the higher 温度 at the applicable thickness and process category.
Filler Metal Selection for Mixed Joints
D1.1:2025 provides two compliant paths for filler metal selection in dissimilar joints:
| Option | Filler Metal Strength | Condition |
|---|---|---|
| Option A | Matches the higher-strength base metal | No additional condition |
| Option B | Matches the lower-strength base metal | Must produce a low-hydrogen deposit |
Option B is widely used in practice. Welding A36 to A572 Gr.65 with E7018 (which matches A36 at 70 ksi minimum tensile) is compliant because E7018 is a low-hydrogen 電極. The joint will develop adequate strength for the A36 member, and the low-hydrogen requirement controls the cracking risk in the HAZ of the higher-strength A572.
Filler metal must also appear in Table 5.7 (prequalified filler metals). The hydrogen designator on the filler metal label directly affects which Table 5.11 preheat category applies — E7018-H8, for example, qualifies for Category D rather than Category B, which may allow reduced preheat on the higher-strength steel.
Preheat for Mixed-Category Joints
The practical application of §5.7.2 is best illustrated with examples. Consider two common dissimilar pairings in 鉄骨製作:
Example 1: A36 to A572 Gr.65 (Category B, SMAW with E7018, 1 in plate)
A36 at 1 in thick, Category B: 50°F minimum preheat. A572 Gr.65 at 1 in thick, Category B: 150°F minimum preheat. Governing preheat for the dissimilar joint: 150°F. Many fabricators make the mistake of using the A36 requirement (50°F) for this joint. Per §5.7.2, that is a 基準 violation.
Example 2: A36 to A992 (Category B, SMAW with E7018, 3/4 in or less)
A36 at 3/4 in or less, Category B: no preheat required. A992 at 3/4 in or less, Category B: no preheat required. Both steels fall in Group I and have the same Table 5.11 要求事項 at light thicknesses. The dissimilar joint requires no preheat for this combination. The rule still applies — it just yields the same 結果 for both metals.
Thickness note: The governing thickness for preheat is the thicker of the two base metals at the joint. Even if the thinner member would require no preheat on its own, the joint preheat is set by the higher of (a) the thicker member's requirement and (b) the higher-strength member's requirement.
Common Dissimilar Combinations in Structural Welding
Several dissimilar pairings appear frequently in building and bridge fabrication under D1.1:
| Base Metal 1 | Base Metal 2 | Group Pairing | Governing Steel for Preheat |
|---|---|---|---|
| A36 (36 ksi yield) | A992 (50 ksi yield) | I + I | Same group — same preheat |
| A36 (36 ksi yield) | A572 Gr.50 (50 ksi yield) | I + II | A572 Gr.50 at higher thicknesses |
| A572 Gr.50 (50 ksi yield) | A572 Gr.65 (65 ksi yield) | II + III | A572 Gr.65 governs |
| A572 Gr.50 (50 ksi yield) | A913 Gr.65 (65 ksi yield) | II + III | A913 Gr.65 governs |
| A992 (50 ksi yield) | A514 (100 ksi yield) | I + V | A514 governs — 400°F max interpass also applies |
The A992-to-A514 pairing illustrates an important point: when one of the dissimilar metals is a quenched and tempered steel like A514, the 最大 パス間温度 limit (400°F for thickness up to 1-1/2 in) also applies to the entire joint. Both the minimum preheat and the maximum interpass constraint come from the A514 side of the joint.
In Practice: Writing the WPS for a Dissimilar Joint
When completing a 事前認定WPS for a dissimilar joint, both base metal specifications must appear on the WPS form. Table 5.5 lists 母材群 as an 重要変数 — a WPS written for Group I to Group I base metals does not cover Group I to Group III. If the project requires multiple dissimilar combinations, each unique pairing needs its own WPS line or a separate WPS document.
The preheat field on the WPS records the governing minimum — the highest Table 5.11 value for either base metal at the applicable process, hydrogen category, and thickness. The 溶加材分類 and hydrogen designator must also be recorded, since these drive both the Table 5.11 category and the Option A/B filler metal 適合 path.
For non-prequalified dissimilar joints — for example, when one of the base metals is not in Table 5.6 — a 溶接施工法確認試験記録 (PQR) is required under 条項 6. The PQR test assembly should represent the actual dissimilar combination, not a like-metal substitute. The PQR guide covers the 資格試験 requirements in detail.
CWI Exam Tip
CWI Part C question pattern: A question may describe a dissimilar joint and ask which preheat applies. The answer is always the higher of the two Table 5.11 values — not an average, not the lower. A second common pattern asks about filler metal: the correct answer is that both options are allowed, but matching the lower-strength metal requires a low-hydrogen deposit. A frequent distractor is "the filler must always match the higher strength metal" — that is not what D1.1 §5.7.2 says.