M270M HPS345W Preheat — H8, Low HI, ≤ 20 mm: 125°F
Fracture-critical preheat requirement for M270M HPS345W / M270 HPS50W at ≤ 20 mm (3/4 in) thickness with H8 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 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 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 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 HPS345W / M270 HPS50W at ≤ 20 mm (3/4 in)
HPS345W (HPS50W) at thin thickness is specified for stiffener attachments and connection details where the improved weldability and lamellar tearing resistance justify the 15–25% material cost premium over conventional 345W. The 0.11% max carbon provides significantly lower carbon equivalent than conventional weathering steel, reducing preheat sensitivity and reject rates during production welding.
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 Gr.345W / M270 Gr.50W | B | 125°F (50°C) |
M270M HPS345W / M270 HPS50W 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 HPS345W / M270 HPS50W 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.