M270M Gr.250 Preheat — H8, Mid HI, 20–40 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.

Understanding the FC Preheat for 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 150°F minimum preheat at 20–40 mm (3/4–1½ 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.

Where M270M Gr.250 / M270 Gr.36 Is Used

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.

H8 Hydrogen Control for 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.

Why Preheat Matters at 20–40 mm (3/4–1½ in)

Material from 20 to 40 mm (3/4 to 1-1/2 in) includes many girder web plates, splice plates, and bearing stiffener plates. Preheat increases to 20°C (70°F) for Group 1 and 50°C (125°F) for Group 2 under Table 6.3. The thicker section slows hydrogen diffusion, requiring higher preheat to maintain safe cooling rates.

M270M Gr.250 / M270 Gr.36 at 20–40 mm (3/4–1½ in)

At 20–40 mm, Gr.250 (36) appears in floor beam stiffener plates and diaphragm gussets where loads transfer between main girder and secondary members. Weld details are typically partial-penetration groove welds or large fillet welds at connection brackets. The thicker section slows hydrogen escape, making preheat compliance more important than at thin plate. Fabricators often tack-weld stiffener clips without preheat and then apply full preheat for final welding — D1.5 requires preheat for ALL passes including tacks.

Other Bridge Steels at H8 2.0–2.8 kJ/mm · 20–40 mm (3/4–1½ in)

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

Try Different Combinations

Use the D1.5 Bridge Preheat Calculator to look up any AASHTO M270 steel, hydrogen level, and heat input combination. Also see the D1.1 Preheat Calculator for structural steel.

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