HY-TUF Steel
HY-TUF is a silicon-nickel-chromium-molybdenum ultra-high-strength alloy steel (1.35–1.85% Si, 1.65–2.00% Ni, 0.65–0.90% Cr, 0.30–0.45% Mo, 0.23–0.28% C) originally developed in the 1950s as a high-toughness alternative to 4340 for aircraft structural applications. The high silicon content (similar in concept to 300M) stabilizes the martensite against softening during tempering, allowing the steel to be heat treated to 220,000–240,000 psi while maintaining fracture toughness (K₁₂) values of 75–100 ksi√in. This combination exceeds the toughness of 4340 or 300M at comparable strength.
HY-TUF is governed by AMS 6418 and is produced by both air-melt and vacuum-arc-remelt practice. It is weldable by standard aerospace welding procedures with appropriate preheat and post-weld heat treatment, making it suitable for both machined forgings and welded structural assemblies requiring strengths above 200,000 psi with demanding fracture toughness requirements.
Product Forms & Size Range of HY-TUF Steel
HY-TUF bar and forging stock per AMS 6418 is available in round bar diameters from 0.625 inch to 14.000 inches and as billet for forging. VAR grades are available for the most demanding aerospace fracture-critical applications. Bar is supplied in the annealed or normalized condition for machining. Forging quality grades support landing gear main fitting and structural bracket production.
HEAT TREATMENT OF HY-TUF STEEL
HY-TUF is austenitized at 1600°F (871°C), oil quenched, and tempered at 450–600°F (232–316°C) for two to four hours to achieve 220,000–240,000 psi. Double tempering is recommended for critical applications to ensure full transformation of retained austenite and maximum fracture toughness. The high silicon content allows tempering at these intermediate temperatures while maintaining a strength level that would require lower, more embrittlement-prone tempering temperatures in standard 4340.
300M Steel
Si-V modified 4340 at 280 ksi minimum — the most specified ultra-high-strength steel for aircraft landing gear worldwide.
HP 9-4-30 Steel
Co-Ni ultra-high-strength weldable steel at 220–250 ksi with outstanding fracture toughness for large aerospace structural applications.
Certified Material for Critical Service
HY-TUF steel is supplied to AMS 6418 (bars and forgings). Material certifications document composition including silicon content, mechanical properties at the specified temper condition, and fracture toughness where required. For landing gear applications, hydrogen embrittlement relief after plating and cadmium plating baking must be performed per applicable plating specifications.