Iron-based superalloy is a kind of material with certain strength and oxidation resistance and gas corrosion resistance at high temperature of 600~800 degrees Celsius. Austenitic alloy mainly based on iron and containing a certain amount of chromium and nickel.
Development
Iron-based superalloys are developed from austenitic stainless steels. In the 40s, it was found that the addition of molybdenum, niobium, titanium and other elements to 18-8 stainless steel can improve the durable strength of this steel under the condition of 500~700 °C, so as to make a work-hardened austenitic heat-resistant steel represented by the American grade 16-25-6 (Fe-25Ni-16Cr-6Mo). In order to meet the needs of the aviation industry for high-temperature resistant materials, a series of precipitation-strengthened Fe-Ni-Cr and Fe-Ni-Co-Cr superalloys have been developed, such as A286, Incoloy 901, etc. During World War II, Germany, Japan and other countries were forced by the need for war and the lack of nickel resources to develop Fe-Cr-Mn series and Fe-Ni-Cr-Mn series superalloys. In this way, a series of iron-based superalloys is gradually formed. In the 50s, in order to save nickel resources, the United States also developed nickel-free AF-71 (Fe-Cr-Mn series) alloy for the manufacture of gas turbine components. Combined with its own resource conditions, China began to develop iron-based alloys at the end of the 50s, and developed a series of Fe-Ni-Cr solid solution strengthening and precipitation strengthening superalloys, such as GH140, GH130, GH135, K13, K14, etc.
Ingredients and properties
Nickel in iron-based superalloys is the main element for the formation and stabilization of austenite, and the Ni3 (Ti, Al) precipitated strengthening phase is formed during aging treatment. Chromium is mainly used to improve oxidation resistance and gas corrosion resistance. Molybdenum and tungsten are used to strengthen solid solutions. Aluminum, titanium, niobium are used for precipitation strengthening. Elements such as carbon, boron, and zirconium are used to strengthen grain boundaries. Iron-based superalloys can be divided into deformation superalloys and cast superalloys according to the manufacturing process, and can be divided into work-hardened type, solution strengthening type and precipitation-strengthened superalloy according to the strengthening method (see Strengthening of metals). The composition and properties of some typical iron-based superalloys are shown in the table. The matrix of iron-based superalloy is austenite, and the main precipitation strengthening phases are γ'[Ni3(Ti, Al)] and γ" (Ni3Nb) phases. In addition, there are trace carbides, borides, Laves (such as Fe2Mo) phases and δ equals. Compared with the nickel-based superalloy structure, the phase structure of iron-based alloy is more complex, the stability is poor, and it is easy to precipitate harmful phases such as η (such as Ni3Ti), σ (such as FexCry), G (such as Fe6Ni16Si7), μ (such as Fe7Mo6) and Laves (see alloy phase). The organization of several typical alloys is seen.
Heat treatment
The heat treatment of the alloy mainly includes solution treatment and aging treatment to obtain appropriate grain size, reasonable distribution and size of strengthening phase, favorable grain boundary state, and make the alloy have good comprehensive properties. For example, the grain size of materials used to manufacture turbine discs is generally 4-5; γ' The phase size is about 100~500, which is evenly distributed in the matrix; There are spheroidized precipitates (such as carbide, Laves, etc.) distributed uniformly at the grain boundary.
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