Dr. Gregory Vartanov from AMD CorpAircraft recently submitted an article to Stainless Steel World about an interest new CRA which has great potential in the aircraft industry, as well as other applications. Landing gears and structures have stringent performance requirements. They are subjected to severe loading, corrosion, and adverse environmental conditions, and have complex shapes which vary in thickness. 300M steel is widely used for high stress aircraft landing gears and structures. This steel is not corrosion-resistant and requires protective coatings. Plating involves using expensive toxic materials which pollute the environment and pose higher health risks.
Cobalt-rich, carbide precipitation strengthened high strength corrosion resistant Ferrium S53 steel alloy (Ferrium S53) has been developed as a substitution of 300M steel. However, use of passive oxide film to provide corrosion resistance similar to 440C stainless steel, high cost of raw materials due to 14% concentration of Co, and complex and costly heat treatment limit applications of this steel alloy.
New cobalt-free, quenched and tempered high strength corrosion resistant steel alloy (“HSCR”) provides better corrosion resistance, the same strength, lower material cost, and lower heat treatment cost compared to the Ferrium S53.
Table 1 shows a comparison of the r.t. mechanical properties of the HSCR after solution annealing, oil quenching, refrigerating, and low tempering, and the Ferrium S53 after solution annealing, oil quenching, refrigerating, high tempering, oil quenching, refrigerating, reheating and the final high tempering.
HSCR showed no rust after the salt spray test in 5% NaCl solution at 95°F for more than 200 hrs.
HSCR is strengthened by solid solution and its microstructure consists of small packets of martensite laths, retained austenite, Ti carbides, Cr carbides, and complex carbides. Presence of Ti carbides inhibits grain growth during hot working and high temperature solution annealing. Cost of raw materials and heat treatment of the HSCR is at least 250% and 200% less than the cost of raw materials and heat treatment of Ferrium S53.
Ingots of the HSCR for aircraft landing gears and structures require vacuum induction melting and vacuum arc remelting. Further, the vacuum-melted ingots are homogenize annealed and hot forged. The forgings are normalized and stress relieved and the aircraft components are machined from them.
The components are subjected to the vacuum or in protective media heat treatment by solution annealing at 1925°F to 2000°F, oil or gas quenching, refrigerating at -80°F to -120°F, and tempering at 350°F to 500°F. Finally, the heat treated components are finish machined.
The combination of high strength, good general and pitting corrosion resistance, and low cost makes the HSCR applicable not only for the aircraft industry, but for the oil/gas, automotive, and shipbuilding industries as well.