M270M Gr.345S Preheat — H4, Low HI, 20–40 mm: 150°F
Fracture-critical preheat requirement for M270M Gr.345S / M270 Gr.50S at 20–40 mm (3/4–1½ in) thickness with H4 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M Gr.345S / M270 Gr.50S
AASHTO M270M Gr.345S (M270 Gr.50S) is the structural shape variant of Gr.345, intended for rolled W-shapes and channels used in bridge framing. The "S" suffix indicates the shape-specific chemistry and mechanical property requirements. Used in cross-frame angles, diaphragm members, and built-up sections where rolled shapes are specified rather than plate. Preheat requirements match Gr.345 — Table 6.3 Group 1 for NFC, Tables 12.4/12.5 for FC applications.
Understanding the FC Preheat for M270M Gr.345S / M270 Gr.50S
Shape-grade 345 MPa (50 ksi) steel for rolled bridge members. Under D1.5 fracture-critical requirements (Clause 12), the combination of H4 hydrogen designation and this heat input band requires 150°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 Gr.345S / M270 Gr.50S Is Used
Used as rolled W-shapes and channels in bridge cross-frames, diaphragm members, and built-up sections. The "S" shape designation indicates procurement under AASHTO M270 with shape-specific chemistry and mandatory CVN testing. Common applications include WT sections for cross-frame members, MC channels for diaphragms, and W-shapes for floor beams on shorter spans. Weld details are primarily fillet welds at gusset connections and clip angles.
H4 Hydrogen Control for M270M Gr.345S / M270 Gr.50S
Shape-grade Gr.345S (50S) benefits from H4 at gusset-to-shape connections where access for torch preheat is limited by flanges and stiffeners. The lower preheat achievable with H4 reduces the thermal management complexity in tight spaces typical of cross-frame subassemblies.
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 Gr.345S / M270 Gr.50S at 20–40 mm (3/4–1½ in)
At 20–40 mm flange thickness, Gr.345S (50S) W-shapes serve as floor beams on shorter spans and as bracing members in steel box girders. Connection details include moment connections to girder webs requiring CJP flange welds with backing bars. Preheat is applied to the thicker element at the connection — when a 25 mm shape connects to a 50 mm plate, preheat must meet the requirement for the 50 mm thickness tier.
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.345S / M270 Gr.50S at 20–40 mm (3/4–1½ in) thickness with 1.2–2.0 kJ/mm heat input, H4 consumables achieve the lowest preheat of 150°F (70°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.
Other Bridge Steels at H4 1.2–2.0 kJ/mm · 20–40 mm (3/4–1½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 150°F (70°C) |
| M270M Gr.345W / M270 Gr.50W | B | 200°F (90°C) |
| M270M HPS345W / M270 HPS50W | B | 200°F (90°C) |
| M270M HPS485W / M270 HPS70W | B | 200°F (90°C) |
M270M Gr.345S / M270 Gr.50S at H4 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 Gr.345S / M270 Gr.50S welded with H4-designated consumables at 20–40 mm (3/4–1½ in) thickness and 1.2–2.0 kJ/mm heat input, the minimum preheat is 150°F (70°C) per D1.5 Table 12.4/12.5.
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 150°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.