M270M HPS690W Preheat — 2.8–3.6 kJ/mm, 20–25 mm: 150–300°F
Fracture-critical preheat requirement for M270M HPS690W / M270 HPS100W at 20–25 mm (3/4–1 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. Table 12.8 specifies a preheat RANGE of 150–300°F for this thickness and heat input combination. Unlike other FC grades, HPS690W requires both minimum AND maximum preheat control because the Q&T microstructure is sensitive to both excessive and insufficient heat. Many thickness-heat input combinations are prohibited ("—" in the table) because no safe preheat window exists — the steel cannot be reliably welded under those conditions.
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 20–25 mm (3/4–1 in)
At 20–40 mm, HPS690W (HPS100W) appears in cable-stayed bridge edge girder flanges and heavily loaded truss members. The preheat range requirement (both min and max) becomes more restrictive — the window between required minimum and maximum narrows as thickness increases, leaving less margin for production variation. Fabricators must plan weld sequences to maintain temperature within the narrow permitted band throughout multi-pass welds.
Min–Max Preheat Control for M270M HPS690W / M270 HPS100W
Highest-strength 690 MPa (100 ksi) bridge steel with restricted FC combinations. Table 12.8 specifies a preheat RANGE of 150–300°F for this thickness and heat input combination. Unlike other FC grades, HPS690W requires both minimum AND maximum preheat control because the Q&T microstructure is sensitive to both excessive and insufficient heat. Many thickness-heat input combinations are prohibited ("—" in the table) because no safe preheat window exists — the steel cannot be reliably welded under those conditions.
M270M HPS690W / M270 HPS100W at 2.8–3.6 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
D1.5 Table 12.8 requires a preheat range of 150–300°F (60–160°C) for HPS690W at 20–25 mm (3/4–1 in) with 2.8–3.6 kJ/mm heat input. Both minimum AND maximum must be controlled.
HPS690W achieves its 690 MPa (100 ksi) strength through quenching and tempering. Excessive preheat can soften the tempered microstructure in the heat-affected zone, reducing the steel's strength below the design minimum. The maximum limit of 300°F (160°C) protects this Q&T condition.
D1.5:2025 reference data. Not affiliated with AWS or AASHTO.