MIL-S-24645
MIL-S-24645 is the US military specification governing HSLA-80 (Grade A) and HSLA-100 (Grade B) high-strength low-alloy structural steel plate for naval ship and submarine construction. Unlike the quench-and-tempered HY-grade steels, both grades under MIL-S-24645 achieve their strength through precipitation hardening of fine copper-rich particles during controlled rolling — eliminating the post-rolling quench and enabling field welding without mandatory preheat in most plate thicknesses.
Grade A (HSLA-80) provides 80,000 psi minimum yield with excellent Charpy toughness at −60°F; Grade B (HSLA-100) provides 100,000 psi minimum yield with toughness at the same temperature. The dramatically improved weldability versus HY-80 and HY-100 (for equivalent yield classes) makes MIL-S-24645 material the preferred structural steel for modern US Navy surface combatant hull construction where high heat-input welding processes are used.
Mechanical Properties — MIL-S-24645
- Grade A (HSLA-80): Min yield 80,000 psi (552 MPa), Min UTS 90,000–110,000 psi, Min elongation 18%, CVN ≥ 60 ft-lb at −60°F
- Grade B (HSLA-100): Min yield 100,000 psi (690 MPa), Min UTS 115,000 psi, Min elongation 16%, CVN ≥ 60 ft-lb at −60°F
- Thickness range: 0.25–2.50 inch (Grade A), 0.25–3.50 inch (Grade B)
- Width: up to 144 inches
- Condition: As-rolled or controlled-rolled (no post-rolling quench required)
MIL-S-24645 plate achieves its mechanical properties through controlled thermomechanical rolling and copper precipitation aging — not through post-rolling quench and temper. Carbon equivalent is controlled to enable welding with high heat input (>100 kJ/in for Grade A in thicknesses below 1.5 inch) without mandatory preheat. Grade B HSLA-100 permits single-pass high heat-input welding where HY-100 would require controlled inter-pass temperature. Both grades are NAVSEA-approved for naval ship construction; NAVSEA Technical Publication requirements govern qualification and procurement.
- US Navy surface combatant hull structural plating (destroyers, cruisers, carriers)
- Naval vessel deck plating for aircraft carriers and amphibious ships
- Structural frames and longitudinals in naval vessel construction
- Submarine non-pressure-hull structural members and fairwater panels
- Marine crane structures and shipboard structural steel fabrications
- Offshore platform topsides requiring high yield and subzero toughness
HSLA-80 Steel
80 ksi yield Grade A plate per MIL-S-24645 — age-hardening weldable alternative to HY-80 for surface combatant hull construction without mandatory preheat.
HSLA-100 Steel
100 ksi yield Grade B plate per MIL-S-24645 — weldable alternative to HY-100 for modern US Navy destroyer and cruiser hull structure.
MIL-S-16216 — HY-80/HY-100
Quench-and-tempered HY-grade naval steels — predecessor grades requiring preheat for welding, used for submarine pressure hulls.
Certified Material for Critical Service
MIL-S-24645 mill test reports document heat chemistry (including copper content 1.00–1.75%), yield strength, tensile strength, elongation, and Charpy V-notch impact energy at −60°F for each plate. Mechanical testing is performed per ASTM A370. NAVSEA qualification and authorization is required for all material used in naval ship construction. Material must be procured from NAVSEA-qualified producers on the Qualified Products List.