In many structural aerospace applications, ultra-high strength steels are used because they provide the lightest weight for highly loaded systems. These steels, however, lack adequate corrosion resistance and are commonly coated with cadmium. The environmental problems caused by cadmium create occupational safety and health risks and raise maintenance costs throughout the life of all cadmium-plated parts. Many items that are cadmium plated, such as landing gear, are damage intolerant and sensitive to hydrogen embrittlement and stress corrosion cracking. This sensitivity causes stress corrosion cracking to be the primary failure mechanism for landing gear. The development of an ultra-high strength stainless steel with corrosion resistance and strength to meet the requirements for landing gear has been the focus of traditional alloy development efforts for many years without success. A new technique, using advanced computational materials modeling and systems engineering methods, known as Materials by Design™, was used to design an innovative new prototype stainless alloy during an earlier SERDP Exploratory Development (SEED) project (WP-1149). This prototype, termed S53, proved to be more compelling than any previously developed stainless alloy for this application.
The overall technical objectives of this project were twofold. The first objective was to develop appropriate processing standards for alloy production processes, component manufacturing processes, and overhaul and repair processes to provide the information required for manufacture of components of the S53 alloy. The second objective was to provide adequate test data for mechanical behavior, corrosion resistance, embrittlement resistance, and life-cycle cost to prove the ability of the alloy to replace cadmium-coated aircraft structural steels using standard manufacturing techniques.
The S53 alloy concept is based on secondary hardening steel technology. The alloy can be described as a system that includes the process path used to produce the alloy and manufacture components from it, the microstructural subsystems that denote the structure of the alloy, and the property requirements for the application. This system was optimized using computational methods to produce the first prototype. Of particular importance, the strengthening was optimized using cobalt (Co) for dislocation recovery resistance leading to very efficient secondary hardening via heterogeneous nucleation of M2C steel carbides. A highly stable passive oxide surface film maximizes corrosion resistance due to very high chromium chemical activity, also promoted by high Co levels. Other microstructural systems are optimized to promote strength, corrosion resistance, and fracture toughness. The implementation of S53 will be expedited using the same modeling techniques previously employed and relying on the inherent predictability of a designed material. By streamlining the optimization process and reducing the experimental requirements, this project has positioned the S53 alloy for insertion into demonstration and validation efforts.
This project demonstrated desired goals for ultimate tensile strength, ductility, fracture toughness, and fatigue. Corrosion and stress corrosion tests met project goals but identified a higher sensitivity to pitting corrosion attack. Adequate weldability, compatibility with coating and surface modification processes, and machinability in the fully hardened state were determined. The current alloy design did not meet the yield stress goal of 230 ksi, demonstrating typical values of about 215-220 ksi. A yield stress deficit will not affect the design of a great majority of landing gear components since ultimate tensile stress generally sets the design. Of greater concern is lower fatigue due to the low yield stress. The fatigue studies did not show a fatigue debit over the 300M baseline alloy. Machinability evaluations of the S53 alloy in the annealed state indicate that additional annealing process development will be needed to reduce tool wear and increase stock removal rates to commercially viable levels. Annealed S53 contains a significant amount of retained austenite and will likely require cryogenic treatment in the annealing cycle as is common for many commercial high-strength stainless alloys such as Custom 465. This project demonstrated the scale-up capability of the S53 alloy to ingot sizes greater than 17 inches in diameter. Alloy producibility was investigated, and optimal processing conditions were evaluated. Efforts are now under way to demonstrate and validate the S53 alloy for structural applications and in rotary-geared actuators under ESTCP projects WP-200304 and WP-200619, respectively.
The greatest impact of the S53 alloy will be the reduction of life-cycle cost and toxic waste in Department of Defense squadrons and maintenance depots. Derivatives of the new steel will also be valuable replacements in actuators and for sustainment of legacy systems. In addition, this project provided a clear demonstration of the Materials by Design™ methodology itself, which holds promise for faster and less expensive development of alloys to meet the needs of higher performance, lower cost of ownership, and environmental cleanliness. (Project Completed - 2005)