Stainless Steel
AMS 6425 (HY-TUF)
A high strength low alloy steel.
HY-TUF offers a tensile range of 1517-1655MPa as well as good ductility, fracture toughness and impact strength. It is vacuum arc remelted to provide optimum cleanliness and preferred ingot structure for superior mechanical properties.
Typical applications are within the aerospace industry, in particular landing gear, flap tracks and other structural applications.
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AMS 6425 (HY-TUF) Related Specifications
| System / Standard | Country / Region | Grade / Designation |
| UNS | International | K32550 (HY-TUF) |
| Trade name | International | Hy-Tuf |
| Trade name | International | Lescalloy HY-TUF Vac-Arc |
| AMS | USA / Aerospace | AMS 6425 (HY-TUF, Vac Melt) |
| AMS | USA / Aerospace | AMS 6418 (HY-TUF, N&T) |
| ASTM A579 | USA | Grade 31 (HY-TUF) |
| ASTM A322 | USA | Alloy steel, UNS K32550 |
| MIL-S | USA / Military | MIL-S-7108 |
| AMS-S | USA / Aerospace | AMS-S-7108 |
| DMS | USA / Aerospace | DMS-1841 Grade A |
| DMS | USA / Aerospace | DMS-1841 Grade B |
Properties
Chemical Composition
| Chemical Element | % Present |
| Chromium (Cr) | 0.00 - 0.30 |
| Manganese (Mn) | 0.00 - 1.35 |
| Silicon (Si) | 0.00 - 1.50 |
| Nickel (Ni) | 0.00 - 1.80 |
| Molybdenum (Mo) | 0.00 - 0.40 |
| Carbon (C) | Balance |
| Iron (Fe) | Balance |
Mechanical Properties
| Mechanical Property | Value |
| Tensile Strength | 1517-1655 MPa |
| Elongation A50 mm | 5 % |
General Physical Properties
| Physical Property | Value |
| Density | 7.77 g/cm³ |
| Thermal Expansion | 11.3 x 10-6/K |
| Thermal Conductivity | 37.49 W/m.K |
Applications of AMS 6425 (HY-TUF) Steel
AMS 6425 (HY-TUF / UNS K32550) is a vacuum-remelted, ultra-high-strength low-alloy steel developed specifically for landing gear and other heavily loaded aerospace structures. It combines very high strength with excellent toughness and fatigue resistance.
1. Aircraft Landing Gear and Support Structures
Main landing gear beams, trunnions and shock-strut components
Axles, drag braces, torque links and side-stay fittings
Nose gear struts, cylinders and attachment lugs
Components requiring very high strength, fracture toughness and excellent fatigue performance under spectrum loading
2. Flap Tracks, Slats and High-Lift System Hardware
Flap tracks and carriages carrying large cyclic loads during take-off and landing
Slat tracks, guide rails and associated fittings
High-lift system pins, levers and actuating links where compact size and minimum weight are critical
Parts exposed to severe vibration and variable amplitude fatigue in the wing high-lift system
3. High-Strength Shafts, Pins and Mechanical Links
High-load pins, pivot bolts and hinge pins in flight-control mechanisms
Actuator rods, clevises and levers for landing gear and control systems
Precision shafts in aerospace mechanisms where yield strength above typical 300M / 4340 levels is needed with better toughness
Structural joints where bolted/pinned interfaces must withstand very high bearing and shear stresses
4. Helicopter and Military Airframe Components
Undercarriage and support structure parts on helicopters and military aircraft
Highly loaded fittings in fuselage and wing/rotor pylon attachment points
Arresting-gear and catapult-system hardware on naval aircraft where impact and shock loading are severe
Structural components operating at low temperatures where high notch toughness is required
5. High-Integrity Forgings and Rings for Critical Structures
Closed-die forgings for compact high-strength lugs, joints and node fittings
Rolled rings and thick-section parts where through-thickness properties and cleanliness are essential
Components requiring vacuum-melt quality, low inclusion content and tight scatter in mechanical properties
Parts that must meet stringent aerospace fracture-control and damage-tolerance requirements
6. Comparison and Replacement of Other High-Strength Alloy Steels
Used as an upgrade over conventional ultra-high-strength steels (e.g. 300M, 4340M) where better toughness and fatigue crack-growth resistance are required
Selected where designers want to reduce section size and weight while maintaining or increasing load capacity
Applied in situations where both high hardenability in large sections and strict aerospace cleanliness requirements must be satisfied
Summary
AMS 6425 (HY-TUF) steel is primarily used for aircraft landing gear, flap tracks, high-lift hardware, high-strength shafts, pins and critical airframe fittings, especially where ultra-high strength must be combined with excellent toughness, fatigue resistance and vacuum-melt cleanliness to meet demanding aerospace damage-tolerance and weight-saving requirements.
Characteristics of AMS 6425 (HY-TUF) Steel
AMS 6425 (HY-TUF / UNS K32550) is a vacuum-remelted, ultra-high-strength low-alloy steel developed specifically for landing-gear and other highly loaded aerospace structures. It combines very high strength with exceptional toughness, fatigue resistance and high cleanliness.
1. Ultra-High-Strength Low-Alloy Steel (Not Stainless)
HY-TUF is a Ni–Cr–Mo low-alloy steel, not a stainless steel.
Designed to reach ultra-high tensile and yield strength after quenching and tempering.
Used where structural strength and fracture toughness are more important than intrinsic corrosion resistance (corrosion usually managed by coatings and sealants).
2. Very High Strength with Excellent Toughness
After appropriate quench-and-temper heat treatment, HY-TUF can reach:
Ultra-high tensile strength suitable for heavily loaded landing-gear and flap-track hardware.
High yield strength, allowing compact cross-sections and weight reduction.
Unlike many ultra-high-strength steels, HY-TUF is engineered to retain high Charpy impact toughness and good fracture toughness at these strength levels, reducing risk of brittle failure.
3. Outstanding Fatigue and Damage-Tolerance Performance
HY-TUF was developed specifically for spectrum-loaded aircraft structures.
It exhibits:
High fatigue strength under variable-amplitude loading (take-off, landing, taxi).
Slow fatigue-crack growth rates, supporting damage-tolerant design and inspection intervals.
This combination is a key reason it is widely used for landing gear beams, pins, tracks and critical fittings.
4. High Hardenability and Through-Thickness Properties
Alloy design gives high hardenability, so thick sections can be fully hardened and tempered.
Provides consistent properties from surface to core in large forgings, trunnions, beams and rings.
Good transverse and through-thickness toughness supports use in complex, heavily loaded joints and fittings.
5. Vacuum-Remelted, High-Cleanliness Steel
AMS 6425 requires vacuum melting / vacuum-arc remelting (VAR) or equivalent premium-quality routes.
Low levels of non-metallic inclusions and segregation mean:
Improved fatigue resistance and crack-initiation behaviour.
More uniform mechanical properties across large sections.
High cleanliness is essential for modern damage-tolerant landing-gear and airframe design philosophies.
6. Heat-Treat Response and Temper Resistance
HY-TUF is supplied for quench and temper heat treatment, allowing tuning of:
Strength level (tensile/yield).
Toughness and fatigue behaviour.
It shows good tempering resistance, so high strength can be maintained after tempering at temperatures that also provide adequate toughness and stress relief.
Properly chosen tempering conditions give a balanced strength–toughness profile tailored to each component and thickness.
7. Weldability and Fabrication Considerations
As an ultra-high-strength alloy steel, HY-TUF has limited weldability compared with lower-strength structural steels:
Welding must use strictly qualified procedures, controlled preheat / interpass temperatures and post-weld heat treatment.
For many primary landing-gear parts, design practice is to avoid structural welds and rely on closed-die forgings and machining instead.
HY-TUF can be forged and machined using conventional high-strength alloy-steel practices, with heat treatment integrated into the fabrication route.
8. Corrosion Protection Philosophy
HY-TUF itself does not provide stainless-level corrosion resistance.
In service, corrosion performance is achieved by:
Plating, coatings and primers (e.g. cadmium, chromium-free systems, paints).
Sealing, drainage and good design details to avoid moisture traps.
This approach allows designers to exploit HY-TUF’s mechanical excellence while still meeting corrosion-protection requirements via surface-engineering solutions.
Summary
AMS 6425 (HY-TUF) is a vacuum-remelted, ultra-high-strength low-alloy steel offering a rare combination of very high strength, excellent toughness, superior fatigue and crack-growth resistance, high hardenability and premium cleanliness, making it a benchmark material for landing gear, flap tracks, pins and other critical aerospace structures where damage tolerance and weight saving are essential and corrosion protection is provided by coatings rather than alloy chemistry.
