M270M Gr.345 Preheat — H16, Mid HI, 20–40 mm: 175°F
Fracture-critical preheat requirement for M270M Gr.345 / M270 Gr.50 at 20–40 mm (3/4–1½ in) thickness with H16 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 H16 hydrogen designation and this heat input band requires 175°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.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.
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 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.345 / M270 Gr.50 at 20–40 mm (3/4–1½ in)
At 20–40 mm, Gr.345 (50) serves splice plates, web plates, and bearing sole plates in standard highway bridge girders. CJP butt welds at flange splices are the most critical joint at this thickness — full-penetration ultrasonically-inspected joints that must pass D1.5 acceptance criteria. Preheat compliance is production-critical: 70°F minimum means heated fabrication bays in northern climates during winter bridge contracts.
Higher 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 20–40 mm (3/4–1½ in) with 2.0–2.8 kJ/mm heat input, the 175°F (80°C) preheat compensates for the higher hydrogen potential. Switching to H8 or H4 consumables would reduce the required preheat for this joint.
Other Bridge Steels at H16 2.0–2.8 kJ/mm · 20–40 mm (3/4–1½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 175°F (80°C) |
| M270M Gr.345W / M270 Gr.50W | B | 225°F (110°C) |
| M270M HPS345W / M270 HPS50W | B | 225°F (110°C) |
| M270M HPS485W / M270 HPS70W | B | 225°F (110°C) |
M270M Gr.345 / M270 Gr.50 at H16 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.
Related Guides
For fracture-critical M270M Gr.345 / M270 Gr.50 welded with H16-designated consumables at 20–40 mm (3/4–1½ in) thickness and 2.0–2.8 kJ/mm heat input, the minimum preheat is 175°F (80°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 2.0–2.8 kJ/mm, the 175°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.
Group 2 steels (HPS485W, HPS690W) have higher hardenability from their increased alloy content, forming harder microstructures on cooling. The 50°C (125°F) minimum versus Group 1’s 20°C (70°F) compensates for the greater cracking susceptibility of these higher-strength grades.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.