M270M Gr.250 Preheat — H16, Mid HI, 20–40 mm: 175°F
Fracture-critical preheat requirement for M270M Gr.250 / M270 Gr.36 at 20–40 mm (3/4–1½ 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 175°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.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 20–40 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.250 / M270 Gr.36 at 20–40 mm (3/4–1½ in)
At 20–40 mm, Gr.250 (36) appears in floor beam stiffener plates and diaphragm gussets where loads transfer between main girder and secondary members. Weld details are typically partial-penetration groove welds or large fillet welds at connection brackets. The thicker section slows hydrogen escape, making preheat compliance more important than at thin plate. Fabricators often tack-weld stiffener clips without preheat and then apply full preheat for final welding — D1.5 requires preheat for ALL passes including tacks.
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 20–40 mm (3/4–1½ in) with 2.0–2.8 kJ/mm heat input, the 175°F (80°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 · 20–40 mm (3/4–1½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.345 / M270 Gr.50 | A | 175°F (80°C) |
| M270M Gr.345W / M270 Gr.50W | B | 225°F (110°C) |
| M270M HPS345W / M270 HPS50W | B | 225°F (110°C) |
| M270M HPS485W / M270 HPS70W | B | 225°F (110°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 20–40 mm (3/4–1½ in) thickness and 2.0–2.8 kJ/mm heat input, the minimum preheat is 175°F (80°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 175°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.