M270M Gr.250 Preheat — H8, High HI, 20<t≤40 mm: 50°C

M270M Gr.250 / M270 Gr.36

AASHTO M270M Gr.250 (M270 Gr.36) is the basic structural bridge steel grade. 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

M270M Gr.250 / M270 Gr.36. For H8 at 3/4 < t ≤ 1-1/2 in, the selected fracture-critical cell requires 125°F minimum preheat. Check the cell again if the thickness, hydrogen designation, or heat input changes.

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 determine CVN testing conditions; retain the zone assignment and toughness-test certificate with mill records, and check the contract’s fracture-critical member designation.

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 < t ≤ 40 mm; 3/4 < t ≤ 1-1/2 in

The selected thickness interval is 20 < t ≤ 40 mm; 3/4 < t ≤ 1-1/2 in. Use its own fracture-critical table row; non-fracture-critical thickness bands do not select this result. Table 12.4/12.5 requires a minimum preheat/interpass temperature of 125°F (50°C) for this combination. Apply the value for the selected hydrogen designation and heat input.

M270M Gr.250 / M270 Gr.36 at 20 < t ≤ 40 mm; 3/4 < t ≤ 1-1/2 in

At 20 < t ≤ 40 mm; 3/4 < t ≤ 1-1/2 in, 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.

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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