Ön Isıtma M270M Gr.250 — H8, Low HI, > 60 mm: 325°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 için FC Ön Isıtmayı Anlama

Basic 250 MPa (36 ksi) bridge steel for secondary members. Under D1.5 fracture-critical requirements (Clause 12), the combination of H8 hydrogen designation and this heat input band requires 325°F minimum preheat at > 60 mm (> 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 Nerede Kullanılır

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 için H8 Hidrojen Kontrolü

H8 designation (8 mL/100g max) on Gr.250 (36) provides a balanced approach between preheat economy and consumable availability. Most structural SMAW electrodes and FCAW wires carry H8 classification as the standard low-hydrogen tier. For secondary bridge members, H8 is typically the default specification unless the Engineer specifically mandates H4.

> 60 mm (> 2½ in)'de Ön Isıtma Neden Önemlidir

Material over 65 mm (2-1/2 in) includes the heaviest bridge girder flanges and box-section walls. Table 6.3 requires 110°C (225°F) for both groups at this thickness. Extended preheat soak time is necessary to achieve uniform through-thickness temperature. FC preheat for the heaviest sections reaches 180–200°C (350–400°F) at the H16 hydrogen level.

> 60 mm (> 2½ in)'de M270M Gr.250 / M270 Gr.36

Gr.250 (36) above 65 mm is rare — typically only in large bearing assemblies or rehabilitation projects repairing older bridges where original plate was overspecified. Preheat soak-through at 225°F requires flame heating on both sides for 15–20 minutes on plate this thick. The cost of preheat compliance at this thickness often triggers a design review to consider switching to Gr.345 which achieves the same capacity with thinner, lighter sections.

H8 1.2–2.0 kJ/mm · > 60 mm (> 2½ in)'de Diğer Köprü Çelikleri

H8 1.2–2.0 kJ/mm'de M270M Gr.250 / M270 Gr.36

Farklı Kombinasyonları Dene

Herhangi bir AASHTO M270 çelik, hidrojen seviyesi ve ısı girdisi kombinasyonunu aramak için D1.5 Köprü Ön Isıtma Hesap Makinesini kullanın. Yapısal çelik için D1.1 Ön Isıtma Hesap Makinesine de bakın.

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