M270M HPS345W Preheat — H8, Mid HI, 20–40 mm: 200°F

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 H8 hydrogen designation and this heat input band requires 200°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.

H8 Hydrogen Control for M270M HPS345W / M270 HPS50W

HPS345W (HPS50W) with H8 is common practice where the HPS weldability advantage partially compensates for the higher hydrogen level. The controlled chemistry means HPS345W at H8 often has lower total cracking susceptibility than conventional 345W at H4 — illustrating how base metal chemistry and hydrogen control interact.

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.

Other Bridge Steels at H8 2.0–2.8 kJ/mm · 20–40 mm (3/4–1½ in)

M270M HPS345W / M270 HPS50W at H8 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.

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