M270M Gr.345S Preheat — H16, Low HI, ≤ 20 mm: 150°F
Fracture-critical preheat requirement for M270M Gr.345S / M270 Gr.50S at ≤ 20 mm (3/4 in) thickness with H16 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 H16 hydrogen designation and this heat input band requires 150°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 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.
H16 Hydrogen Control for M270M Gr.345S / M270 Gr.50S
Gr.345S (50S) at H16 applies primarily to field-erected cross-frame connections using self-shielded FCAW. The higher preheat requirement at H16 can be challenging to maintain on thin shape flanges that lose heat rapidly to the surrounding air — electric resistance heating pads help maintain uniform temperature.
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 Gr.345S / M270 Gr.50S at ≤ 20 mm (3/4 in)
Gr.345S (50S) shapes under 20 mm flange thickness include WT sections for K-frame cross-frames and MC channels for diaphragms. The rolled section geometry concentrates stress at the flange-to-web fillet, making proper preheat at gusset connections critical for fatigue performance. SMAW E7018 is the typical process for shape-to-plate connections in bridge subassemblies because of limited access around shape flanges.
Higher Preheat at H16 Designation
H16 consumables allow up to 16 mL of diffusible hydrogen per 100g — the highest level permitted for FC bridge welding. At ≤ 20 mm (3/4 in) with 1.2–2.0 kJ/mm heat input, the 150°F (70°C) preheat compensates for the higher hydrogen potential. Switching to H8 or H4 consumables would reduce the required preheat for this joint.
Other Bridge Steels at H16 1.2–2.0 kJ/mm · ≤ 20 mm (3/4 in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.345W / M270 Gr.50W | B | 150°F (70°C) |
| M270M HPS345W / M270 HPS50W | B | 150°F (70°C) |
| M270M HPS485W / M270 HPS70W | B | 150°F (70°C) |
| M270M Gr.250 / M270 Gr.36 | A | 150°F (70°C) |
M270M Gr.345S / M270 Gr.50S at H16 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 H16-designated consumables at ≤ 20 mm (3/4 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.
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.