1.1 Ferritic Stainless Steel This stainless steel has a matrix composed primarily of ferrite (α phase) with a body-centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. It contains 11-30% Cr, and may contain small amounts of Mo, Nb, and Ti. It is essentially Ni-free. It offers high strength and excellent resistance to chloride stress corrosion, pitting, and crevice corrosion, but is sensitive to intergranular corrosion and has poor low-temperature toughness. Key grades include 06Cr13Al, 10Cr17, 10Cr17Mo, 008Cr27Mo, and 008Cr30Mo2.
Structural steels include 4130, 4140, 4340, 300M, and Hy-Tuf, primarily used in front and rear landing gear. Stainless steels include 301, 302, 303, 321, 17-4PH, and 17-7PH, primarily used in engine nacelles, pylons, and thrust reversers, with smaller amounts also found in various parts of the fuselage.
Because both ferrite and retained austenite are resistant to corrosion, they appear white under a microscope, making them often confusing without careful observation.
Nitrogen is a common element in steel, originating from high-temperature absorption from the charge or molten steel. Nitrogen stabilizes the austenite structure and significantly improves the strength and toughness of steel after solution treatment. Nitrogen can improve the macrostructure of high-chromium and high-chromium-nickel steels, making them dense and strong. Therefore, replacing nickel with nitrogen in stainless steel and heat-resistant steels has significant economic value and development prospects. Nitrogen has a strong affinity for alloying elements such as titanium and aluminum in steel, combining with them to form highly stable nitrides. These disperse within grain boundaries and act as precipitation strengthening, inhibiting creep deformation at high temperatures and improving creep and long-term strength. Surface penetration methods such as nitriding and carbonitriding can form nitrides and cyanides on the steel surface, thereby increasing the hardness, strength, wear resistance, and corrosion resistance of the surface layer. In addition to ferritic stainless steel, almost all types of stainless steel, especially austenitic and duplex stainless steel, have been widely alloyed with nitrogen in the past decade, bringing austenitic and α+γ duplex stainless steel into the modern stainless steel era. Stainless steel includes controlled nitrogen ([N]≤0.10% or [N]＜0.12%), medium nitrogen ([N]≤0.40% or [W]＜0.50%) and high nitrogen ([N]＞0.4% or [N]≥0.50%) stainless steel, super austenitic stainless steel and super martensitic stainless steel containing low nitrogen. Although nitrogen cannot prevent the precipitation of σ phase in duplex stainless steel, it can inhibit the formation of σ phase. The addition of nitrogen to duplex stainless steel led to the emergence of second- and third-generation duplex stainless steels (super duplex and economical duplex), establishing a family of duplex stainless steels alongside the three major categories of stainless steel: martensitic, ferritic, and austenitic. Because nitrogen has extremely low solubility in ferritic stainless steel and high solubility in austenite, its beneficial effects in duplex stainless steel are primarily reflected in improving the properties of the austenitic structure.
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