M270M HPS690W Preheat — 1.6–2.0 kJ/mm, > 50 mm: N/A
Fracture-critical preheat requirement for M270M HPS690W / M270 HPS100W at > 50 mm (> 2 in) thickness, per AASHTO/AWS D1.5:2025, the Bridge Welding Code.
M270M HPS690W / M270 HPS100W
AASHTO M270M HPS690W (M270 HPS100W) is the highest-strength bridge steel at 690 MPa (100 ksi) yield, reserved for the most demanding long-span and cable-stayed bridge applications. It follows a completely different FC preheat table (Table 12.8) with min-max ranges and many prohibited thickness-heat input combinations marked with “—”. Table 12.8 requires H4 or H8 designated consumables; H16 is not permitted. NFC preheat per Table 6.3 Group 2; maximum interpass per Table 6.4 is 205°C (400°F) for material up to 40 mm, 230°C (450°F) above.
Understanding the FC Preheat for M270M HPS690W / M270 HPS100W
Highest-strength 690 MPa (100 ksi) bridge steel with restricted FC combinations. This thickness and heat input combination is marked "—" (not permitted) in Table 12.8. No preheat temperature exists that simultaneously prevents hydrogen-induced cracking and avoids degrading the 690 MPa Q&T microstructure. The fabricator must adjust either the heat input (by changing welding parameters) or use a different joint geometry that allows a thinner effective section.
Where M270M HPS690W / M270 HPS100W Is Used
Reserved for the most demanding long-span and cable-stayed bridge applications where maximum weight reduction justifies the material cost premium and fabrication complexity. Table 12.8 imposes both minimum AND maximum preheat limits, and many thickness-heat input combinations are outright prohibited. Fabricators must plan weld procedures around the permitted combinations before cutting plate. Filler metals must carry the H4 or H8 supplemental designator — H16 is not permitted for HPS690W FC applications.
M270M HPS690W / M270 HPS100W at > 50 mm (> 2 in)
HPS690W (HPS100W) above 65 mm pushes the boundary of what can be reliably welded in FC bridge service. Most heat input combinations are prohibited at this thickness, leaving only narrow windows where welding is possible. Production planning must account for weather, ambient temperature, and wind conditions that could push joint temperature outside the narrow permitted range. Pre-production mockup testing per D1.5 Clause 12 is advisable.
Why This Combination Is Prohibited
Highest-strength 690 MPa (100 ksi) bridge steel with restricted FC combinations. This thickness and heat input combination is marked "—" (not permitted) in Table 12.8. No preheat temperature exists that simultaneously prevents hydrogen-induced cracking and avoids degrading the 690 MPa Q&T microstructure. The fabricator must adjust either the heat input (by changing welding parameters) or use a different joint geometry that allows a thinner effective section.
M270M HPS690W / M270 HPS100W at 1.6–2.0 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
No — this combination is marked "—" (not permitted) in D1.5 Table 12.8. No preheat temperature exists that simultaneously prevents hydrogen cracking and avoids degrading the 690 MPa quenched-and-tempered microstructure at these parameters. Adjust the heat input or joint geometry.
Table 12.8 permits only specific thickness-heat input combinations. Check the calculator for permitted ranges at > 50 mm (> 2 in). Generally, thicker sections require higher heat input to access permitted combinations.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.