S53 was developed with Strategic Environmental Research and Development Program (SERDP) support as an alternative to the use of cadmium (Cd)-plated high-strength landing gear steel. QuesTek Innovations LLC developed the Materials by Design methodology in which alloys could be designed from first principles using a computational approach that avoided the need for the extensive trial-and-error formulation and testing that has been the mainstay of the alloy manufacturing industry since its inception. S53 was designed to be equivalent in mechanical properties to Cd-plated 300M ultrahigh strength steel used in landing gear and equivalent in corrosion resistance to the lower strength 15-5PH stainless steel used in actuators. It also replaces 4340 and 4340M steels and other high-strength steels (such as 4330 and HP9-4-30) used in landing gear and actuators. There is also potential for using S53 in place of lower strength corrosion-resistant (CRES) steels such as 15-5PH, 17-4PH, and PH13-8Mo, which are used in applications such as hydraulic actuators that require a combination of strength and corrosion resistance.

The regulatory drivers for this development are Cd and its concomitant hexavalent chromium (Cr⁶⁺) conversion. The Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for Cd is 5 μg/m³ per 8-hour shift, while the PEL for Cr⁶⁺ was also recently lowered to 5 μg/m³. In addition, both Cd and Cr⁶⁺ are highly restricted under the European Restriction of Hazardous Substances (RoHS) rules (although they currently exempt aerospace uses) and could to be ultimately banned under the European Registration, Authorization and Restriction of Chemicals (REACH) statute. By eliminating Cd and Cr⁶⁺, S53 removes all the environmental and health issues associated with these materials.

The demonstration objectives of this project were twofold:

1. To demonstrate and validate the Materials by Design methodology for developing new alloys.

2. To demonstrate and validate a CRES steel that would be mechanically equivalent to 300M ultrahigh strength landing gear steel, but with corrosion resistance equivalent to 15-5PH stainless steel used in modern aerospace actuators.

These objectives were met, with two minor exceptions: (1) the tensile yield strength of S53 is slightly lower than 300M (213 ksi minimum versus 230 ksi minimum), although ultimate tensile strength (to which landing gear is usually designed) was the same (280 ksi) and (2) under corrosion testing, S53 visually corrodes more rapidly than the target 15-5PH, although the pit growth rate is only a little higher (and it is pit depth that drives condemnation). Only service experience will show whether the difference is significant.

The demonstration showed that a new alloy could be accurately designed and optimized in a far shorter time by the computational method, while ensuring that both the thermodynamics and kinetics were correct. Other CRES steels designed in the traditional Edisonian manner have met the mechanical requirements but were not producible or scalable because the properties could only be obtained in small batches.

All the properties and performance relevant to qualifying for landing gear were measured. The alloy meets all the requirements but is more resistant to corrosion and corrosion-related failures such as stress corrosion cracking (SCC) and hydrogen embrittlement. In addition, it is more damage-tolerant because of its somewhat higher fracture toughness and is more resistant to grind burns and arc burns that can occur in depot maintenance. In addition, removal of Cd eliminates the Cd embrittlement that can occur when brakes and gear are overheated, which can occur on aborted takeoff. The mechanical properties are summarized below:

The axial, notch, and bending fatigue properties of S53 are better than or equal to 300M, while the corrosion fatigue is significantly better. S53 can be plated and high-velocity oxygen-fuel (HVOF) sprayed, although some plating does require the use of a Ni strike. The standard depot non-destructive inspection (NDI) methods such as fluorescent penetrant inspection (FPI), magnetic particle inspection (MPI), and Barkhausen work as well on S53 as on 300M. The only method that does not work is Nital etching for grind burns because the CRES steel cannot be etched in the same way. The alternative Barkhausen (Roll Scan) method does however work.

The major difference between S53 and 300M is cost, with 300M being about $3-5/lb and S53 being about $15-20/lb (about the same cost as the Aermet 100 used on most Navy gear). This does not mean, however, that S53 components are five times as expensive since most of the cost is in the fabrication. The cost premium for components examined varied from 40% to 80%. It was found that S53 was most cost-effective for components that are relatively complex (raw materials a smaller proportion of the cost) and are frequently condemned for corrosion-related causes or lead to corrosion-related service failures. C-5 roll pins and B-1 main landing gear (MLG) cylinders were particularly cost-effective to replace. For components that have serious service failure issues, replacement will be a judgment of risk rather than cost.

A new alloy cannot be used unless it has a commercial producer, aerospace specifications, and engineering allowables. S53 has, or will shortly have, all of these—two licensed manufacturers (Cartech and Latrobe), an Aerospace Materials Specification (AMS), AMS 5922, a Metallic Materials Property Development and Standardization (MMPDS) listing of Class A allowables, and an International Alloy Number (UNS S10500).

The Air Force is carrying out a full-scale rig test of an A-10 landing gear fabricated from S53. This is a critical test required before A-10 gear can be flight tested. Because S53 has not been used previously on other landing gear components, it must successfully pass the A-10 System Program Office (SPO) required tests and checks to ensure that it is flight worthy. Given that the existing gear is fabricated from 4330, a lower strength steel, S53 is a very low risk replacement.

A 12-month flight test is planned for one of the S53 A-10 cylinders manufactured during this project. The aircraft will proceed through its standard daily operations, and the S53 piston will go through periodic inspections for damage and corrosion throughout the 12-month evaluation period. A successful evaluation will show that the S53 piston can perform without any problems or failures in its designed manner in a typical environment.