M270M HPS485W Preheat — H16, Mid HI, > 60 mm: 375°F
Fracture-critical preheat requirement for M270M HPS485W / M270 HPS70W at > 60 mm (> 2½ in) thickness with H16 hydrogen designation, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M HPS485W / M270 HPS70W
AASHTO M270M HPS485W (M270 HPS70W) is a high-performance weathering steel with 485 MPa (70 ksi) yield, used in long-span bridge girder flanges and heavily loaded members where weight reduction is critical. Produced as quenched-and-tempered plate up to 100 mm (4 in) thick. The high strength level places it in NFC Group 2 (Table 6.3) with higher minimum preheat than Group 1. FC preheat follows Tables 12.6/12.7 alongside the 345W grades but at higher temperatures reflecting the increased hardenability. Maximum interpass per Table 6.4 is 230°C (450°F) to protect the Q&T microstructure.
Understanding the FC Preheat for M270M HPS485W / M270 HPS70W
High-performance 485 MPa (70 ksi) weathering bridge steel. Under D1.5 fracture-critical requirements (Clause 12), the combination of H16 hydrogen designation and this heat input band requires 375°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 HPS485W / M270 HPS70W Is Used
Used in long-span bridge main girder flanges, cable-stayed bridge edge girders, and heavily loaded interchange ramp girders where weight reduction is critical. Enables 20–30% weight savings versus Gr.345 designs, allowing shallower girder sections that reduce embankment costs. Flange thickness often exceeds 50 mm (2 in), making preheat and interpass control critical at every CJP splice. Fabricators must use dual-readout temperature monitoring to ensure joints stay within the qualified range between minimum preheat and maximum interpass.
H16 Hydrogen Control for M270M HPS485W / M270 HPS70W
HPS485W (HPS70W) at H16 represents the highest-risk FC scenario for this grade — the combination of high strength, high hardenability, and high hydrogen demands the maximum preheat in Tables 12.6/12.7. Production scheduling must account for extended preheat soak times at every joint. Many fabricators refuse H16 on HPS485W, requiring H4 or H8 in their shop standards.
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 HPS485W / M270 HPS70W at > 60 mm (> 2½ in)
HPS485W (HPS70W) above 65 mm is the heaviest plate used in high-performance bridge construction. Flange widths reach 750–900 mm (30–36 in), with splice welds requiring 100+ passes over multiple shifts. Temperature management becomes the dominant production variable — too cold risks cracking, too hot risks softening the Q&T microstructure. Fabricators invest in automated temperature monitoring systems with data logging for FC traceability.
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 > 60 mm (> 2½ in) with 2.0–2.8 kJ/mm heat input, the 375°F (190°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 · > 60 mm (> 2½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 325°F (160°C) |
| M270M Gr.345 / M270 Gr.50 | A | 325°F (160°C) |
| M270M Gr.345S / M270 Gr.50S | A | 325°F (160°C) |
| M270M Gr.345W / M270 Gr.50W | B | 375°F (190°C) |
M270M HPS485W / M270 HPS70W 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 HPS485W / M270 HPS70W welded with H16-designated consumables at > 60 mm (> 2½ in) thickness and 2.0–2.8 kJ/mm heat input, the minimum preheat is 375°F (190°C) per D1.5 Table 12.6/12.7.
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 375°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.