M270M Gr.250予熱 — H4、High HI、40–60 mm: 150°F

M270M Gr.250 / M270 Gr.36

AASHTO M270M Gr.250 (M270 Gr.36) is the metric/US customary designation for the basic structural bridge steel with 250 MPa (36 ksi) minimum yield. It is the direct bridge equivalent of ASTM A709 Gr.36, procured under AASHTO M270 with mandatory Charpy V-notch testing per temperature zone. Used for secondary bridge members — lateral bracing, diaphragm plates, floor beam stiffeners, and bearing components where Gr.345 strength is not needed. Non-fracture-critical preheat follows Table 6.3 Group 1; fracture-critical follows Tables 12.4/12.5 with hydrogen and heat input as additional variables.

M270M Gr.250 / M270 Gr.36のFC予熱を理解する

Basic 250 MPa (36 ksi) bridge steel for secondary members. Under D1.5 fracture-critical requirements (Clause 12), the combination of H4 hydrogen designation and this heat input band requires 150°F minimum preheat at 40–60 mm (1½–2½ in). Lower hydrogen levels (H4 < H8 < H16) allow lower preheat because less hydrogen enters the weld deposit. Similarly, higher heat input reduces preheat requirements because slower cooling rates give hydrogen more time to diffuse out.

M270M Gr.250 / M270 Gr.36の使用箇所

Specified for secondary bridge members under AASHTO LRFD — lateral bracing angles, diaphragm plates, floor beam stiffeners, bearing seat components, and railing posts. Gr.250 (36) requires CVN testing per AASHTO temperature zone, distinguishing it from plain ASTM A36 by guaranteeing fracture resistance at the bridge design service temperature. Transverse stiffener fillet welds and floor beam web-to-flange joints are high-frequency fabrication details. Temperature zones (1 through 3) determine CVN test temperature: Zone 1 at 21°C (70°F) for moderate climates, Zone 2 at 4°C (40°F) for cold, Zone 3 at -12°C (10°F) for severe cold.

M270M Gr.250 / M270 Gr.36におけるH4水素管理

For Gr.250 (36), the H4 restriction to 4 mL/100g diffusible hydrogen provides the widest margin against cold cracking in secondary bridge members. The low carbon equivalent of Gr.250 already makes it one of the least crack-sensitive bridge steels, so the practical benefit of H4 over H8 is primarily preheat reduction — often 10–20°C (20–40°F) lower, which speeds production on high-volume stiffener and bracing fabrication.

40–60 mm (1½–2½ in)で予熱が重要な理由

Material from 40 to 65 mm (1-1/2 to 2-1/2 in) covers heavy girder flanges, thick splice plates, and main member plate. This is the critical thickness range for bridge fabrication — preheat reaches 65°C (150°F) for Group 1 and 80°C (175°F) for Group 2. FC preheat at this thickness can exceed 200°C (400°F) depending on hydrogen level and heat input.

40–60 mm (1½–2½ in)でのM270M Gr.250 / M270 Gr.36

Gr.250 (36) at 40–65 mm is uncommon in modern designs because Gr.345 handles the same loads at lighter weight. When specified, it appears in bearing seats and massive foundation assemblies where material cost is secondary to simplicity. At this thickness the HAZ cooling rate is significantly slower, and the 150°F preheat reflects the need to keep cooling rates below the critical rate for hydrogen-induced cracking in the coarse-grained HAZ.

橋梁FC溶接用H4認定溶接材料

H4補足記号は、溶接材料が溶着金属100gあたり4mL以下の拡散性水素しか溶着しないことを証明します。> 2.8 kJ/mm入熱での40–60 mm (1½–2½ in)板厚における破壊臨界M270M Gr.250 / M270 Gr.36において、H4溶接材料はFCテーブルで最低予熱150°F (70°C)を達成します。予熱低減が優先される場合、これが推奨される水素レベルです。

H4 > 2.8 kJ/mm · 40–60 mm (1½–2½ in)における他の橋梁鋼材

H4 > 2.8 kJ/mmでのM270M Gr.250 / M270 Gr.36

別の組み合わせを試す

D1.5橋梁予熱計算機を使用して、AASHTO M270鋼材の水素レベルと入熱の任意の組み合わせを検索できます。構造用鋼にはD1.1予熱計算機もご覧ください。

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