A new ultra-high-strength, corrosion-resistant steel, developed with SERDP and ESTCP support, is making its way to real-world aerospace applications. A recently completed U.S. Air Force field service evaluation of landing gear made from Ferrium S53 has shown that the new alloy exceeds conventional steels in terms of corrosion protection and other properties, without the need for a hazardous cadmium coating.

Ultra-high-strength steels are commonly used in structural aerospace applications; however, because these steels lack adequate corrosion resistance, they are typically electroplated with cadmium. Exposure to this toxic chemical creates an occupational safety and health risk, and additionally many cadmium-plated items are susceptible to hydrogen embrittlement and stress corrosion cracking.

Using advanced computational materials modeling and systems engineering methods, known as Materials by DesignTM, SERDP researchers from QuesTek Innovations LLC designed and characterized the new corrosion-resistant steel for use in aircraft landing gear. The model used in the Materials by Design™ approach rapidly evaluated and tailored the characteristics of a large suite of new metal alloys to create the novel stainless steel, Ferrium S53, with mechanical characteristics equivalent to those of the industry standard ultra-high-strength alloys 4340 and 300M and corrosion performance equivalent to stainless steel. A follow-on SERDP project was undertaken to investigate how to manufacture components using the S53 alloy and provide test data for the steel’s mechanical behavior, corrosion resistance, embrittlement resistance, and life-cycle cost. With the scale-up capability of S53 established, Ferrium S53 was successfully demonstrated and validated by ESTCP for landing gear as a mechanical equivalent to 4340 and 300M, with the added corrosion resistance of stainless steel used in modern aerospace actuators.

Following the landing gear study, a field service evaluation was conducted by the U.S. Air Force on two main landing gear pistons made from the Ferrium S53 steel. The pistons were primed and painted, but not coated with cadmium. They were tested on a T-38 aircraft and compared to cadmium-plated, primed, and painted 4340 pistons. The 19-month evaluation, encompassing more than 500 landings, showed that in addition to providing general corrosion protection, the S53 steel also was more resistant to stress corrosion cracking, fatigue, corrosion fatigue, and grinding burn than the 4340 steel. Although redesigned 4340 pistons were ultimately selected for the T-38 landing gear, this field service evaluation demonstrates the significant potential for use of S53 steel on future aircraft components.

Ferrium S53 is commercially produced and sold by Latrobe Specialty Metals Company and Carpenter Technology Corporation, under license from QuesTek. S53 is covered by SAE Aerospace Material Specifications (AMS) 5922 and is included in the Metallic Material Properties Development and Standardization (MMPDS) handbook. 

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