Preheat M270M Gr.250 — H4, Low HI, > 60 mm: 300°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.

Memahami Preheat FC untuk 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 H4 hydrogen designation and this heat input band requires 300°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.

Di Mana M270M Gr.250 / M270 Gr.36 Digunakan

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.

Kontrol Hidrogen H4 untuk M270M Gr.250 / M270 Gr.36

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.

Mengapa Preheat Penting pada > 60 mm (> 2½ in)

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.

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

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.

Bahan Habis Pakai Bersertifikat H4 untuk Pengelasan FC Jembatan

Penunjukan tambahan H4 menyertifikasi bahwa bahan habis pakai mengendapkan tidak lebih dari 4 mL hidrogen difusibel per 100g logam yang diendapkan. Untuk fraktur kritis M270M Gr.250 / M270 Gr.36 pada ketebalan > 60 mm (> 2½ in) dengan masukan panas 1.2–2.0 kJ/mm, bahan habis pakai H4 mencapai preheat terendah 300°F (150°C) di tabel FC.

Baja Jembatan Lain pada H4 1.2–2.0 kJ/mm · > 60 mm (> 2½ in)

M270M Gr.250 / M270 Gr.36 pada H4 1.2–2.0 kJ/mm

Coba Kombinasi Berbeda

Gunakan Kalkulator Preheat Jembatan D1.5 untuk mencari baja AASHTO M270, tingkat hidrogen, dan kombinasi masukan panas. Lihat juga Kalkulator Preheat D1.1 untuk baja struktural.

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