AASHTO/AWS D1.5:2025 · Table 12.6 · Fracture-Critical · H8

M270M HPS345W Preheat — H8, High HI, 20<t≤40 mm: 80°C

Q: What is the FC preheat for M270M HPS345W / M270 HPS50W with H8 at 20 < t ≤ 40 mm?

For fracture-critical M270M HPS345W / M270 HPS50W welded with H8-designated consumables at 20 < t ≤ 40 mm thickness and HI > 2.8 kJ/mm heat input, the minimum preheat is 80°C per D1.5 Table 12.6.

Q: What temperature applies to M270M HPS345W / M270 HPS50W at 20 < t ≤ 40 mm?

Table 12.6 requires a minimum preheat/interpass temperature of 80°C for this combination. Apply the value for the selected hydrogen designation and heat input.

Fracture-critical preheat requirement for M270M HPS345W / M270 HPS50W at 20 < t ≤ 40 mm thickness with H8 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.

Built on AWS D1.5:2025 — every value traced to the clause.

Fracture-Critical Minimum Preheat & Interpass

80°C

H8 hydrogen · HI > 2.8 kJ/mm heat input · 20 < t ≤ 40 mm thickness

AASHTO/AWS D1.5M/D1.5:2025 Table 12.6

H8 designation: consumable deposits ≤ 8 mL/100g diffusible hydrogen per AWS A4.3. Lower hydrogen = lower preheat requirement.

No °F value: D1.5 Table 12.7 (°F) lists M270 Gr. 50W and HPS 70W only; HPS 50W is not listed. This °C value is from Table 12.6, which names HPS 345W.

Reference tool. Verify against project-applicable edition and Engineer-approved WPS.

AASHTO M270 Standard Grades

M270M HPS345W / M270 HPS50W

AASHTO M270M HPS345W (M270 HPS50W) is a high-performance weathering bridge steel with controlled chemistry and 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 Table 12.6 (°C). Table 12.7 (°F) lists Gr.50W and HPS70W only, so D1.5 gives no °F FC value for HPS50W.

Why This Preheat

Understanding the FC Preheat for M270M HPS345W / M270 HPS50W

M270M HPS345W / M270 HPS50W. For H8 at 20 < t ≤ 40 mm, the selected fracture-critical cell requires 80°C minimum preheat. Check the cell again if the thickness, hydrogen designation, or heat input changes.

Bridge Applications

Where M270M HPS345W / M270 HPS50W Is Used

Preferred over conventional Gr.345W for new unpainted bridge construction. Flange splice CJP welds benefit from the lower carbon equivalent, reducing reject rates during cold-weather bridge fabrication.

H8 Fabrication Detail

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.

Thickness: 20 < t ≤ 40 mm

Why Preheat Matters at 20 < t ≤ 40 mm

The selected thickness interval is 20 < t ≤ 40 mm. Use its own fracture-critical table row; non-fracture-critical thickness bands do not select this result. Table 12.6 requires a minimum preheat/interpass temperature of 80°C for this combination. Apply the value for the selected hydrogen designation and heat input.

Fabrication Detail

M270M HPS345W / M270 HPS50W at 20 < t ≤ 40 mm

At 20 < t ≤ 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.

Interactive Calculator

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.

Learn More

Related Guides

FAQ

What is the FC preheat for M270M HPS345W / M270 HPS50W with H8 at 20 < t ≤ 40 mm?

For fracture-critical M270M HPS345W / M270 HPS50W welded with H8-designated consumables at 20 < t ≤ 40 mm thickness and HI > 2.8 kJ/mm heat input, the minimum preheat is 80°C per D1.5 Table 12.6.

What is the difference between FC and NFC preheat for M270M HPS345W / M270 HPS50W?

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.

How does heat input affect preheat for FC M270M HPS345W / M270 HPS50W?

Higher heat input means slower cooling rates, giving hydrogen more time to diffuse out of the weld zone. At HI > 2.8 kJ/mm, the 80°C 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.

What temperature applies to M270M HPS345W / M270 HPS50W at 20 < t ≤ 40 mm?

Table 12.6 requires a minimum preheat/interpass temperature of 80°C for this combination. Apply the value for the selected hydrogen designation and heat input.

D1.5:2025 reference data. Not affiliated with AWS or AASHTO.