M270M Gr.345 Preheat — H4, Mid HI, 40<t≤60 mm: 80°C

M270M Gr.345 / M270 Gr.50

AASHTO M270M Gr.345 (M270 Gr.50) is the standard bridge plate and shape grade, corresponding to ASTM A709 Gr.50. It is the baseline strength for modern highway bridge design per AASHTO LRFD. Used for plate girder flanges, webs, floor beams, cross-frames, and splice plates. Flanges on large plate girders are heavy-section fabrication details, making preheat compliance at upper thickness tiers a significant production consideration. NFC preheat per Table 6.3 Group 1; FC per Tables 12.4/12.5.

Understanding the FC Preheat for M270M Gr.345 / M270 Gr.50

M270M Gr.345 / M270 Gr.50. For H4 at 1-1/2 < t ≤ 2-1/2 in, the selected fracture-critical cell requires 175°F minimum preheat. Check the cell again if the thickness, hydrogen designation, or heat input changes.

Where M270M Gr.345 / M270 Gr.50 Is Used

Standard for highway bridge plate girder flanges, box girder webs, cross-frame angles, bearing sole plates, and splice plates in bolted-welded connections. Gr.345 (50) is the baseline strength for modern AASHTO LRFD bridge design. Flange butt splices, web-to-flange continuous fillet welds, and bearing stiffener clips are the dominant weld types in girder fabrication. On girder fabrication lines with long continuous seams, confirm the hydrogen designator on the consumable package and plan the temperature-measurement sequence in the weld traveller.

H4 Hydrogen Control for M270M Gr.345 / M270 Gr.50

Gr.345 (50) with H4 consumables achieves the lowest FC preheat requirements available for the workhorse bridge grade.

Why Preheat Matters at 40 < t ≤ 60 mm; 1-1/2 < t ≤ 2-1/2 in

The selected thickness interval is 40 < t ≤ 60 mm; 1-1/2 < t ≤ 2-1/2 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 175°F (80°C) for this combination. Apply the value for the selected hydrogen designation and heat input.

M270M Gr.345 / M270 Gr.50 at 40 < t ≤ 60 mm; 1-1/2 < t ≤ 2-1/2 in

Gr.345 (50) at 40 < t ≤ 60 mm; 1-1/2 < t ≤ 2-1/2 in includes the main flange plates on medium-span highway bridges. Interpass temperature must stay above preheat minimum between passes and below the Table 6.4 maximum — this dual constraint makes temperature monitoring continuous during flange splice welding. Magnetic particle examination of each completed splice adds to the production schedule.

H4-Certified Consumables for FC Bridge Welding

The H4 supplementary designator certifies that the consumable deposits no more than 4 mL of diffusible hydrogen per 100g of deposited weld metal. For fracture-critical M270M Gr.345 / M270 Gr.50 at 40 < t ≤ 60 mm; 1-1/2 < t ≤ 2-1/2 in thickness with 2.0 < HI ≤ 2.8 kJ/mm; 50 < HI ≤ 70 kJ/in heat input, H4 consumables achieve the lowest preheat of 175°F (80°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.

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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