M270M HPS485W Preheat — H4, High HI, 20–40 mm: 150°F
Fracture-critical preheat requirement for M270M HPS485W / M270 HPS70W at 20–40 mm (3/4–1½ in) thickness with H4 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 H4 hydrogen designation and this heat input band requires 150°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 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.
H4 Hydrogen Control for M270M HPS485W / M270 HPS70W
For HPS485W (HPS70W), specifying H4 consumables is a strategic choice that significantly reduces preheat costs on thick flange splices. At 40+ mm flange thickness, every 25°F reduction in required preheat saves approximately 15–20 minutes per splice in heating time, directly impacting the critical-path schedule of bridge girder fabrication.
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 HPS485W / M270 HPS70W at 20–40 mm (3/4–1½ in)
At 20–40 mm, HPS485W (HPS70W) serves as the primary flange material for long-span bridge girders where weight reduction is the design driver. The 70 ksi yield enables 20–30% thinner flanges versus Gr.345 designs, reducing dead load and allowing shallower girder depths that save embankment costs. CJP flange splices at this thickness require precise heat input control to stay within the FC table parameters.
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 HPS485W / M270 HPS70W at 20–40 mm (3/4–1½ in) thickness with > 2.8 kJ/mm heat input, H4 consumables achieve the lowest preheat of 150°F (70°C) in the FC tables. This is the preferred hydrogen level when preheat reduction is a priority.
Other Bridge Steels at H4 > 2.8 kJ/mm · 20–40 mm (3/4–1½ in)
| Steel | Table | Preheat |
|---|---|---|
| M270M Gr.250 / M270 Gr.36 | A | 100°F (40°C) |
| M270M Gr.345 / M270 Gr.50 | A | 100°F (40°C) |
| M270M Gr.345S / M270 Gr.50S | A | 100°F (40°C) |
| M270M Gr.345W / M270 Gr.50W | B | 150°F (70°C) |
M270M HPS485W / M270 HPS70W at H4 > 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 H4-designated consumables at 20–40 mm (3/4–1½ in) thickness and > 2.8 kJ/mm heat input, the minimum preheat is 150°F (70°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 > 2.8 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.
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.