A709 HPS70WのGMAW用予熱 — 3/4" to 1-1/2"
A709 HPS70WをGMAWで板厚3/4" to 1-1/2"で溶接する場合の最低予熱およびパス間温度。AWS D1.1:2025 表5.11に基づく。
AWS D1.1:2025 表5.11に基づく — すべての値は条項に追跡可能。
低水素SMAW、SAW、GMAW、またはFCAWプロセス（高強度鋼）
GMAW (Gas Metal Arc Welding)
GMAW (MIG) feeds continuous solid wire with shielding gas — an inherently low-hydrogen process assigned to Category B in Table 5.11.
On high-strength plate, GMAW wire selection must meet the specified minimum tensile to satisfy weld metal matching. ER80S-D2 or ER100S-G wires serve higher-strength steels. The inherently low hydrogen input of solid wire makes GMAW particularly well-suited for crack-sensitive TMCP grades where minimizing diffusible hydrogen is the primary fabrication objective.
A709 HPS70W
ASTM A709 HPS70W is the highest-strength weathering bridge steel with 70 ksi minimum yield and 85-110 ksi tensile range, used in long-span bridges and heavily loaded members where weight reduction is critical. Produced as quenched-and-tempered plate in thicknesses up to 4", its high strength places it in Category C of Table 5.11 with correspondingly higher preheat requirements. Table 5.11 footnote (b) sets a maximum preheat/interpass of 400°F for thickness up to 1-1/2” and 450°F for thicker material — this upper limit is unusual in D1.1 and exists because the Q&T microstructure can be degraded by excessive heat. The chemistry includes copper (0.25-0.40%), nickel (0.65-1.00%), and chromium (0.40-0.65%) for atmospheric corrosion resistance, with 0.19% max carbon keeping the CE-IIW in the 0.46-0.52 range.
A709 HPS70WとGMAWでこの予熱が必要な理由
Highest-strength 70 ksi weathering bridge steel for long-span applications. The higher strength level of this steel places it in Category C of Table 5.11, which carries elevated preheat requirements compared to Category B grades. At 150°F minimum with GMAW, the preheat ensures the cooling rate stays slow enough to prevent hydrogen-induced cracking in this higher-hardenability material. Category C steels demand careful attention to interpass temperature control throughout the weld sequence.
A709 HPS70Wの代表的な適用例
Reserved for long-span bridge main girder flanges, cable-stayed bridge edge girders, arch rib plates, high-load interchange ramp girders, and situations where reducing member depth saves clearance or reduces dead load. HPS70W enables weight savings of 20-30% versus conventional Gr.50 designs, allowing shallower girder sections that reduce embankment costs on grade-separation bridges. Flange thickness often exceeds 2”, making preheat and interpass control critical at every CJP splice and web-to-flange joint. Note that Table 5.11 footnote (b) sets maximum preheat and interpass limits of 400°F for thicknesses up to 1-1/2" and 450°F above. This upper limit is unusual in D1.1 and requires monitoring both minimum and maximum interpass throughout the welding sequence. Fabricators must use dual-readout temperature monitoring to ensure the joint stays within the qualified band. Filler metals must match the 70 ksi minimum tensile while providing weathering-compatible chemistry for exposed joints.
3/4" to 1-1/2"で予熱が重要な理由
Preheat climbs at this range as thicker material slows heat dissipation, trapping hydrogen at crack-susceptible grain boundaries.
A709 HPS70WのカテゴリーC予熱
表5.11のカテゴリーCは、焼入性と残留応力の組み合わせにより高い予熱が必要な高強度鋼に適用されます。3/4" to 1-1/2"のA709 HPS70Wにおいて、150°Fの最低予熱は溶接冷却速度を遅くし、熱影響部での割れ感受性マルテンサイトの生成を防ぎます。拘束された継手の多パス溶接では、パス間温度をこの最低値以上に維持することが特に重要です。
GMAWと3/4" to 1-1/2"における他の鋼材
A709 HPS70WとGMAW
A709 HPS70W溶接ガイド
D1.1:2025参考データ。AWSとの提携なし。