M270M Gr.345 Preheat — H16, High HI, t>60 mm: 150°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 H16 at t > 2-1/2 in, the selected fracture-critical cell requires 300°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.

H16 Hydrogen Control for M270M Gr.345 / M270 Gr.50

Gr.345 (50) at H16 requires the highest FC preheat in the standard grade range. The elevated preheat compensates for the substantial hydrogen potential but adds significant production cost on thick girder flanges. Most fabricators avoid H16 for shop work, reserving it only for field processes where lower-hydrogen alternatives are not feasible.

Why Preheat Matters at t > 60 mm; t > 2-1/2 in

The selected thickness interval is t > 60 mm; 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 300°F (150°C) for this combination. Apply the value for the selected hydrogen designation and heat input.

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

Gr.345 (50) at t > 60 mm; t > 2-1/2 in represents the heaviest plate girder flanges on long-span bridges. At t > 60 mm; t > 2-1/2 in flange thickness, each CJP splice needs multiple weld passes with continuous interpass monitoring. Preheat soak-through needs heating mats or induction coils because torch heating alone cannot maintain uniform temperature through thick plate. Production sequencing must accommodate heating and cooling on heavy splices, making temperature management a major scheduling factor.

Preheat at H16 Designation

H16 consumables allow up to 16 mL of diffusible hydrogen per 100g — the highest level permitted for FC bridge welding. At t > 60 mm; t > 2-1/2 in with HI > 2.8 kJ/mm; HI > 70 kJ/in heat input, the 300°F (150°C) preheat compensates for the higher hydrogen potential. Recheck the applicable table cell before changing the hydrogen designation.

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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