التسخين المسبق M270M Gr.250 — H8، Mid HI، 40–60 mm: 200°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.

فهم التسخين المسبق FC لـ M270M Gr.250 / M270 Gr.36

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 200°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.

أين يُستخدم M270M Gr.250 / M270 Gr.36

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.

التحكم في الهيدروجين H8 لـ M270M Gr.250 / M270 Gr.36

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.

لماذا يهم التسخين المسبق عند 40–60 mm (1½–2½ in)

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.

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

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.

فولاذ جسور أخرى عند H8 2.0–2.8 kJ/mm · 40–60 mm (1½–2½ in)

M270M Gr.250 / M270 Gr.36 عند H8 2.0–2.8 kJ/mm

جرّب تركيبات مختلفة

استخدم حاسبة التسخين المسبق D1.5 للجسور للبحث عن أي فولاذ AASHTO M270 ومستوى هيدروجين وتركيبة مدخل حراري. انظر أيضاً حاسبة التسخين المسبق D1.1 للفولاذ الإنشائي.

أدلة ذات صلة