M270M Gr.345W Preheat — H8, Low HI, ≤ 20 mm: 125°F
Fracture-critical preheat requirement for M270M Gr.345W / M270 Gr.50W at ≤ 20 mm (3/4 in) thickness with H8 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 H8 hydrogen designation and this heat input band requires 125°F minimum preheat at ≤ 20 mm (3/4 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.
H8 Hydrogen Control for M270M Gr.345W / M270 Gr.50W
Weathering Gr.345W (50W) with H8 represents the standard practice for most unpainted bridge fabrication. Gas-shielded FCAW wires commonly achieve H8 designation, making this the natural hydrogen level for high-productivity shop welding on weathering grade plate girders.
Why Preheat Matters at ≤ 20 mm (3/4 in)
Material up to 20 mm (3/4 in) covers most cross-frame angles, stiffener clips, lateral bracing members, and light bridge plate. At this thickness, hydrogen diffusion is efficient and preheat requirements are the lowest in Table 6.3 — 10°C (50°F) for both groups. In FC service, this thickness tier also carries the lowest preheat in Tables 12.4–12.7, starting at 40°C (100°F) for H4 consumables.
M270M Gr.345W / M270 Gr.50W at ≤ 20 mm (3/4 in)
Gr.345W (50W) weathering steel at thin plate thickness appears in unpainted bridge bearing stiffeners and web attachment plates exposed to the atmosphere. Filler metal must match the weathering composition — E8018-W2 for SMAW or ER80S-G-W for GMAW. The patina development on thin plate is rapid (6–18 months) but requires proper drainage detailing to prevent pack rust between closely spaced plates.
Other Bridge Steels at H8 1.2–2.0 kJ/mm · ≤ 20 mm (3/4 in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 125°F (50°C) |
| M270M Gr.345 / M270 Gr.50 | A | 125°F (50°C) |
| M270M Gr.345S / M270 Gr.50S | A | 125°F (50°C) |
| M270M HPS345W / M270 HPS50W | B | 125°F (50°C) |
M270M Gr.345W / M270 Gr.50W at H8 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 H8-designated consumables at ≤ 20 mm (3/4 in) thickness and 1.2–2.0 kJ/mm heat input, the minimum preheat is 125°F (50°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 125°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.
Yes — D1.5 Table 6.3 requires a minimum of 10°C (50°F) even for the thinnest material in both groups. This is higher than D1.1’s Category B minimum of 0°C (32°F) at the same thickness, reflecting the more conservative approach for bridge structures where fracture consequences are severe.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.