M270M HPS345W Preheat — H16, Low HI, 20–40 mm: 250°F
Fracture-critical preheat requirement for M270M HPS345W / M270 HPS50W at 20–40 mm (3/4–1½ in) thickness with H16 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M HPS345W / M270 HPS50W
AASHTO M270M HPS345W (M270 HPS50W) is a high-performance weathering bridge steel with enhanced weldability through controlled chemistry — 0.11% max carbon, 0.006% max sulfur with calcium treatment for inclusion shape control. Developed under FHWA-funded research to eliminate the lamellar tearing and inconsistent toughness problems of earlier weathering steel bridge designs. The lower carbon equivalent compared to conventional Gr.345W reduces cracking sensitivity at flange splices. NFC preheat per Table 6.3 Group 1; FC per Tables 12.6/12.7.
Understanding the FC Preheat for M270M HPS345W / M270 HPS50W
High-performance weathering 345 MPa steel with enhanced weldability. Under D1.5 fracture-critical requirements (Clause 12), the combination of H16 hydrogen designation and this heat input band requires 250°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 HPS345W / M270 HPS50W Is Used
Preferred over conventional Gr.345W for new unpainted bridge construction. The HPS designation indicates FHWA-developed chemistry with 0.11% max carbon and controlled sulfur for enhanced weldability and lamellar tearing resistance. Flange splice CJP welds benefit from the lower carbon equivalent, reducing reject rates during cold-weather bridge fabrication. Material cost premium over standard Gr.345W is typically 15–25% per ton but eliminates weldability-related rework.
H16 Hydrogen Control for M270M HPS345W / M270 HPS50W
HPS345W (HPS50W) at H16 partially offsets the weldability advantage of HPS chemistry by allowing high hydrogen levels. The FC preheat requirement approaches that of conventional 345W at H8, reducing the cost advantage of the HPS specification. For this reason, most HPS345W fabrication targets H4 or H8 to capture the full preheat benefit.
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 HPS345W / M270 HPS50W at 20–40 mm (3/4–1½ in)
At 20–40 mm, HPS345W (HPS50W) appears in splice plates and web plates for new unpainted bridge construction. The calcium-treated, inclusion-shape-controlled chemistry virtually eliminates lamellar tearing at through-thickness-loaded connections — a critical advantage for corner joints at bearing stiffener-to-flange connections where Z-direction tensile stresses develop during welding contraction.
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 1.2–2.0 kJ/mm heat input, the 250°F (120°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 · 20–40 mm (3/4–1½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 200°F (100°C) |
| M270M Gr.345 / M270 Gr.50 | A | 200°F (100°C) |
| M270M Gr.345S / M270 Gr.50S | A | 200°F (100°C) |
| M270M Gr.345W / M270 Gr.50W | B | 250°F (120°C) |
M270M HPS345W / M270 HPS50W 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 HPS345W / M270 HPS50W welded with H16-designated consumables at 20–40 mm (3/4–1½ in) thickness and 1.2–2.0 kJ/mm heat input, the minimum preheat is 250°F (120°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 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.
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.