AASHTO/AWS D1.5:2025 · Table 12.6/12.7 · Fracture-Critical · H4

M270M Gr.345W Preheat — H4, Mid HI, t>60 mm: 160°C

Q: What is the difference between FC and NFC preheat for M270M Gr.345W / M270 Gr.50W?

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.

Q: How does heat input affect preheat for FC M270M Gr.345W / M270 Gr.50W?

Higher heat input means slower cooling rates, giving hydrogen more time to diffuse out of the weld zone. At 50 < HI ≤ 70 kJ/in, the 325°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.

Fracture-critical preheat requirement for M270M Gr.345W / M270 Gr.50W at t > 60 mm; t > 2-1/2 in thickness with H4 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

325°F / 160°C

H4 hydrogen · 2.0 < HI ≤ 2.8 kJ/mm; 50 < HI ≤ 70 kJ/in heat input · t > 60 mm; t > 2-1/2 in thickness

AASHTO/AWS D1.5M/D1.5:2025 Table 12.6/12.7

H4 designation: consumable deposits ≤ 4 mL/100g diffusible hydrogen per AWS A4.3. Lower hydrogen = lower preheat requirement.

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

AASHTO M270 Standard Grades

M270M Gr.345W / M270 Gr.50W

AASHTO M270M Gr.345W (M270 Gr.50W) is a weathering bridge steel that forms a protective oxide patina for unpainted bridge service. Copper-chromium-nickel alloying provides atmospheric corrosion resistance. Weld filler must match the weathering composition (the specified filler metal 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.

Why This Preheat

Understanding the FC Preheat for M270M Gr.345W / M270 Gr.50W

M270M Gr.345W / M270 Gr.50W. For H4 at t > 2-1/2 in, the selected fracture-critical cell requires 325°F minimum preheat. Check the cell again if the thickness, hydrogen designation, or heat input changes.

Bridge Applications

Where M270M Gr.345W / M270 Gr.50W Is Used

Deployed in unpainted bridge plate girders across humid, coastal, and high-maintenance-cost environments. Weld filler must match the weathering composition (a specified weathering-match filler metal) 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.

H4 Fabrication Detail

H4 Hydrogen Control for M270M Gr.345W / M270 Gr.50W

Weathering Gr.345W (50W) with H4 benefits from reduced preheat on field splice joints where maintaining temperature in wind-exposed bridge erection conditions is challenging. The specified filler metal low-hydrogen electrode commonly used for weathering steel field work typically carries H4 or H8 designation.

Thickness: t > 60 mm; t > 2-1/2 in

Why Preheat Matters at t > 60 mm; t > 2-1/2 in

The selected thickness interval is t > 60 mm; t > 2-1/2 in. Use its own fracture-critical table row; non-fracture-critical thickness bands do not select this result. Table 12.6/12.7 requires a minimum preheat/interpass temperature of 325°F (160°C) for this combination. Apply the value for the selected hydrogen designation and heat input.

Fabrication Detail

M270M Gr.345W / M270 Gr.50W at t > 60 mm; t > 2-1/2 in

Gr.345W (50W) at t > 60 mm; t > 2-1/2 in is increasingly rare as HPS345W replaces it in new designs. The conventional weathering composition at this thickness has elevated lamellar tearing risk from sulfide inclusion elongation — the problem that drove development of the HPS grades with calcium-treated, inclusion-shape-controlled steel. Existing bridges with thick 345W flanges require careful Z-direction tension checks during rehabilitation welding.

H4 Hydrogen Control

H4-Certified Consumables for FC Bridge Welding

The H4 supplementary designator certifies that the consumable deposits no more than 4 mL of diffusible hydrogen per 100g of deposited weld metal. For fracture-critical M270M Gr.345W / M270 Gr.50W at t > 60 mm; t > 2-1/2 in thickness with 2.0 < HI ≤ 2.8 kJ/mm; 50 < HI ≤ 70 kJ/in heat input, H4 consumables achieve the lowest preheat of 325°F (160°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.

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 Gr.345W / M270 Gr.50W with H4 at t > 60 mm; t > 2-1/2 in?

For fracture-critical M270M Gr.345W / M270 Gr.50W welded with H4-designated consumables at t > 60 mm; t > 2-1/2 in thickness and 2.0 < HI ≤ 2.8 kJ/mm; 50 < HI ≤ 70 kJ/in heat input, the minimum preheat is 325°F (160°C) per D1.5 Table 12.6/12.7.

What is the difference between FC and NFC preheat for M270M Gr.345W / M270 Gr.50W?

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 Gr.345W / M270 Gr.50W?

Higher heat input means slower cooling rates, giving hydrogen more time to diffuse out of the weld zone. At 50 < HI ≤ 70 kJ/in, the 325°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.

What temperature applies to M270M Gr.345W / M270 Gr.50W at t > 60 mm; t > 2-1/2 in?

Table 12.6/12.7 requires a minimum preheat/interpass temperature of 325°F (160°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.