M270M Gr.250 Preheat — H4, Mid HI, > 60 mm: 275°F
Fracture-critical preheat requirement for M270M Gr.250 / M270 Gr.36 at > 60 mm (> 2½ in) thickness with H4 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 H4 hydrogen designation and this heat input band requires 275°F minimum preheat at > 60 mm (> 2½ 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.
H4 Hydrogen Control for M270M Gr.250 / M270 Gr.36
For Gr.250 (36), the H4 restriction to 4 mL/100g diffusible hydrogen provides the widest margin against cold cracking in secondary bridge members. The low carbon equivalent of Gr.250 already makes it one of the least crack-sensitive bridge steels, so the practical benefit of H4 over H8 is primarily preheat reduction — often 10–20°C (20–40°F) lower, which speeds production on high-volume stiffener and bracing fabrication.
Why Preheat Matters at > 60 mm (> 2½ in)
Material over 65 mm (2-1/2 in) includes the heaviest bridge girder flanges and box-section walls. Table 6.3 requires 110°C (225°F) for both groups at this thickness. Extended preheat soak time is necessary to achieve uniform through-thickness temperature. FC preheat for the heaviest sections reaches 180–200°C (350–400°F) at the H16 hydrogen level.
M270M Gr.250 / M270 Gr.36 at > 60 mm (> 2½ in)
Gr.250 (36) above 65 mm is rare — typically only in large bearing assemblies or rehabilitation projects repairing older bridges where original plate was overspecified. Preheat soak-through at 225°F requires flame heating on both sides for 15–20 minutes on plate this thick. The cost of preheat compliance at this thickness often triggers a design review to consider switching to Gr.345 which achieves the same capacity with thinner, lighter sections.
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.250 / M270 Gr.36 at > 60 mm (> 2½ in) thickness with 2.0–2.8 kJ/mm heat input, H4 consumables achieve the lowest preheat of 275°F (140°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.
Other Bridge Steels at H4 2.0–2.8 kJ/mm · > 60 mm (> 2½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.345 / M270 Gr.50 | A | 275°F (140°C) |
| M270M Gr.345W / M270 Gr.50W | B | 325°F (160°C) |
| M270M HPS345W / M270 HPS50W | B | 325°F (160°C) |
| M270M HPS485W / M270 HPS70W | B | 325°F (160°C) |
M270M Gr.250 / M270 Gr.36 at H4 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 H4-designated consumables at > 60 mm (> 2½ in) thickness and 2.0–2.8 kJ/mm heat input, the minimum preheat is 275°F (140°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 275°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.
Table 6.3 converges at 110°C (225°F) for both groups at this thickness because the dominant factor becomes hydrogen diffusion distance through the thick section rather than the steel’s hardenability. Even Group 1 steels need substantial preheat at 65+ mm to keep cooling rates slow enough for safe hydrogen escape.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.