M270M Gr.345 Preheat — H4, High HI, > 60 mm: 250°F
Fracture-critical preheat requirement for M270M Gr.345 / M270 Gr.50 at > 60 mm (> 2½ in) thickness with H4 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M Gr.345 / M270 Gr.50
AASHTO M270M Gr.345 (M270 Gr.50) is the standard bridge plate and shape grade with 345 MPa (50 ksi) minimum yield, 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 routinely reach 50–75 mm (2–3 in) thick, 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
Standard 345 MPa (50 ksi) bridge plate for girders and floor beams. Under D1.5 fracture-critical requirements (Clause 12), the combination of H4 hydrogen designation and this heat input band requires 250°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.
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. Girder flanges typically range from 20 to 75 mm (3/4 to 3 in) thick with widths from 300 to 750 mm (12 to 30 in), requiring extended preheat soak times on thicker flange splices.
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. On girder fabrication lines producing 50–100 meters of flange-to-web fillet weld per shift, the reduced preheat from H4 translates directly to faster welding speed and lower fuel costs for preheat maintenance.
Why Preheat Matters at > 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.345 / M270 Gr.50 at > 60 mm (> 2½ in)
Gr.345 (50) above 65 mm represents the heaviest plate girder flanges on long-span bridges. At 75–100 mm flange thickness, each CJP splice requires 80+ weld passes with continuous interpass monitoring. Preheat soak-through needs heating mats or induction coils because torch heating alone cannot maintain uniform temperature through 3+ inches of plate. Production rates drop to 1–2 splices per shift at this thickness, making preheat efficiency a major scheduling factor.
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 > 60 mm (> 2½ in) thickness with > 2.8 kJ/mm heat input, H4 consumables achieve the lowest preheat of 250°F (120°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.
Other Bridge Steels at H4 > 2.8 kJ/mm · > 60 mm (> 2½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 250°F (120°C) |
| M270M Gr.345W / M270 Gr.50W | B | 300°F (150°C) |
| M270M HPS345W / M270 HPS50W | B | 300°F (150°C) |
| M270M HPS485W / M270 HPS70W | B | 300°F (150°C) |
M270M Gr.345 / M270 Gr.50 at H4 > 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.
Related Guides
For fracture-critical M270M Gr.345 / M270 Gr.50 welded with H4-designated consumables at > 60 mm (> 2½ in) thickness and > 2.8 kJ/mm heat input, the minimum preheat is 250°F (120°C) per D1.5 Table 12.4/12.5.
Non-fracture-critical (Table 6.3) preheat is a simple thickness-based lookup. Fracture-critical (Tables 12.4–12.8) adds hydrogen level and heat input as variables, typically requiring higher preheat. For FC members, the hydrogen designator on the consumable classification directly determines the minimum preheat.
Higher heat input means slower cooling rates, giving hydrogen more time to diffuse out of the weld zone. At > 2.8 kJ/mm, the 250°F preheat balances the hydrogen level and cooling rate. Moving to a higher heat input band would typically reduce the required preheat for the same hydrogen level and thickness.
Table 6.3 converges at 110°C (225°F) for both groups at this thickness because the dominant factor becomes hydrogen diffusion distance through the thick section rather than the steel’s hardenability. Even Group 1 steels need substantial preheat at 65+ mm to keep cooling rates slow enough for safe hydrogen escape.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.