M270M Gr.345W Preheat — H16, Low HI, > 60 mm: 400°F
Fracture-critical preheat requirement for M270M Gr.345W / M270 Gr.50W at > 60 mm (> 2½ in) thickness with H16 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M Gr.345W / M270 Gr.50W
AASHTO M270M Gr.345W (M270 Gr.50W) is a weathering bridge steel with 345 MPa (50 ksi) yield that forms a protective oxide patina for unpainted bridge service. The copper-chromium-nickel alloying provides atmospheric corrosion resistance, eliminating lifetime repainting costs estimated at $15–25 per square foot per cycle. Weld filler must match the weathering composition (E8018-W2 or equivalent) for exposed joints. NFC preheat per Table 6.3 Group 1; FC per Tables 12.6/12.7 which carry higher preheat than the non-weathering grades.
Understanding the FC Preheat for M270M Gr.345W / M270 Gr.50W
Weathering 345 MPa (50 ksi) bridge steel for unpainted service. Under D1.5 fracture-critical requirements (Clause 12), the combination of H16 hydrogen designation and this heat input band requires 400°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.345W / M270 Gr.50W Is Used
Deployed in unpainted bridge plate girders across humid, coastal, and high-maintenance-cost environments. The weathering patina eliminates lifetime repainting cycles estimated at $15–25/sq ft per cycle. Weld filler must match the weathering composition (E8018-W2 or ER80S-G-W) for exposed joints to ensure the weld face develops the same protective oxide as the base metal. Conventional Gr.345W is being replaced by HPS345W in new designs due to superior weldability.
H16 Hydrogen Control for M270M Gr.345W / M270 Gr.50W
Weathering Gr.345W (50W) at H16 is used for field splices when self-shielded FCAW is the only practical process due to wind exposure on elevated bridge erection. The significantly higher preheat at H16 — often 40–60°F above H4 — makes weather-window scheduling critical for winter field splice operations.
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.345W / M270 Gr.50W at > 60 mm (> 2½ in)
Gr.345W (50W) above 65 mm is increasingly rare as HPS345W replaces it in new designs. The conventional weathering composition at this thickness has elevated lamellar tearing risk from sulfide inclusion elongation — the problem that drove development of the HPS grades with calcium-treated, inclusion-shape-controlled steel. Existing bridges with thick 345W flanges require careful Z-direction tension checks during rehabilitation welding.
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 > 60 mm (> 2½ in) with 1.2–2.0 kJ/mm heat input, the 400°F (200°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 1.2–2.0 kJ/mm · > 60 mm (> 2½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 350°F (180°C) |
| M270M Gr.345 / M270 Gr.50 | A | 350°F (180°C) |
| M270M Gr.345S / M270 Gr.50S | A | 350°F (180°C) |
| M270M HPS345W / M270 HPS50W | B | 400°F (200°C) |
M270M Gr.345W / M270 Gr.50W at H16 1.2–2.0 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.345W / M270 Gr.50W welded with H16-designated consumables at > 60 mm (> 2½ in) thickness and 1.2–2.0 kJ/mm heat input, the minimum preheat is 400°F (200°C) per D1.5 Table 12.6/12.7.
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 1.2–2.0 kJ/mm, the 400°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.