M270M Gr.250 Preheat — H16, Mid HI, ≤ 20 mm: 125°F
Fracture-critical preheat requirement for M270M Gr.250 / M270 Gr.36 at ≤ 20 mm (3/4 in) thickness with H16 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M Gr.250 / M270 Gr.36
AASHTO M270M Gr.250 (M270 Gr.36) is the metric/US customary designation for the basic structural bridge steel with 250 MPa (36 ksi) minimum yield. It is the direct bridge equivalent of ASTM A709 Gr.36, procured under AASHTO M270 with mandatory Charpy V-notch testing per temperature zone. Used for secondary bridge members — lateral bracing, diaphragm plates, floor beam stiffeners, and bearing components where Gr.345 strength is not needed. Non-fracture-critical preheat follows Table 6.3 Group 1; fracture-critical follows Tables 12.4/12.5 with hydrogen and heat input as additional variables.
Understanding the FC Preheat for M270M Gr.250 / M270 Gr.36
Basic 250 MPa (36 ksi) bridge steel for secondary members. Under D1.5 fracture-critical requirements (Clause 12), the combination of H16 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 Gr.250 / M270 Gr.36 Is Used
Specified for secondary bridge members under AASHTO LRFD — lateral bracing angles, diaphragm plates, floor beam stiffeners, bearing seat components, and railing posts. Gr.250 (36) requires CVN testing per AASHTO temperature zone, distinguishing it from plain ASTM A36 by guaranteeing fracture resistance at the bridge design service temperature. Transverse stiffener fillet welds and floor beam web-to-flange joints are high-frequency fabrication details. Temperature zones (1 through 3) determine CVN test temperature: Zone 1 at 21°C (70°F) for moderate climates, Zone 2 at 4°C (40°F) for cold, Zone 3 at -12°C (10°F) for severe cold.
H16 Hydrogen Control for M270M Gr.250 / M270 Gr.36
H16 designation (16 mL/100g max) on Gr.250 (36) is the highest hydrogen level permitted for FC bridge welding. For Gr.250 secondary members, H16 is sometimes specified when using self-shielded FCAW (E71T-8) for field connections where gas shielding is impractical — the resulting higher preheat is accepted as the cost of field weldability.
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.250 / M270 Gr.36 at ≤ 20 mm (3/4 in)
Gr.250 (36) at up to 20 mm is the lightest bridge fabrication scenario — cross-frame angles, stiffener clips, and lateral bracing gussets. Hydrogen diffusion through this thin section is rapid, so preheat has the least influence on cracking resistance. Most fabricators use E7018 SMAW for short fillet welds at this thickness because the joint lengths are too short for mechanized FCAW setup to be economical. Interpass monitoring is minimal at this tier because heat builds slowly in thin plate.
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 2.0–2.8 kJ/mm heat input, the 125°F (50°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 2.0–2.8 kJ/mm · ≤ 20 mm (3/4 in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.345W / M270 Gr.50W | B | 125°F (50°C) |
| M270M HPS345W / M270 HPS50W | B | 125°F (50°C) |
| M270M HPS485W / M270 HPS70W | B | 125°F (50°C) |
| M270M Gr.345 / M270 Gr.50 | A | 125°F (50°C) |
M270M Gr.250 / M270 Gr.36 at H16 2.0–2.8 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.250 / M270 Gr.36 welded with H16-designated consumables at ≤ 20 mm (3/4 in) thickness and 2.0–2.8 kJ/mm heat input, the minimum preheat is 125°F (50°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 2.0–2.8 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.