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Invar alloy application (marketing department of Shanghai HY Industry Co., Ltd)
Invaralloy application (marketing department of Shanghai HY Industry Co., Ltd)
Fe-Ni alloy containing about 36% nickel has a very low expansion coefficient and its size hardly changes with temperature, so it is called Invar Alloy, also known as Invar steel. Invar was discovered in 1896 by the French metallurgical physicist Guilaume. In the Fe-Ni alloy, when the Ni content is 36%, the alloy has the lowest room temperature expansion coefficient, the length hardly changes with temperature, and the average linear thermal expansion coefficient from room temperature to the Curie temperature (T c ) is almost zero. As a result, a new generation of low-expansion materials represented by the classic Invar alloy Fe-36Ni was born. Its abnormal expansion phenomenon is called the Invar effect.
In recent years, the development and research featuring Invar alloys have been continuously deepened. A variety of Invar alloy materials have been developed outside China, which has continuously expanded its application fields and depth. The application of Invar alloys has expanded from the traditional precision instrument field to the fields of electrical conductors, electronic industry and special structural materials. Invar alloy is a single-phase austenitic structure alloy. When used as a low-expansion structural material, the strength of Invar alloy is generally low, about 500MPa, which seriously limits its large-scale industrial application. Therefore, the development of a new generation of high-strength and low-expansion Invar alloys has become the research focus of contemporary scholars from various countries.
1 Current status of international research
Since the invention of Invar alloy, it has become a hot spot in research and application due to its characteristics such as Invar effect. International research institutions and manufacturers in the United States, Japan, Russia and other countries have conducted extensive research on Invar alloys, which has greatly promoted the performance improvement and application of Invar materials and related products.
Mason and others from the United States added 1% to 4% Ti to the Fe-36Ni alloy, with a tensile strength of 800MPa, an elastic limit of 350MPa, and a linear thermal expansion coefficient α (20 to 300 ℃) <6.0×6.0×10^(-6) /℃. Thomas et al. studied the properties of Fe-(38～45)Ni-(3～15)Cr-(0.1～1.0)Ti series alloy. The tensile strength can reach 1000MPa and the thermal expansion coefficient is (8～10) ×10^( -6)/℃, and the processing performance is also improved. Carpenter Technology Company of the United States has developed an iron-nickel alloy of Fe-32Ni-5.5Co with an extremely small thermal expansion coefficient (α room temperature ≤0.1×10 ^(-6) /℃).
Hitachi Metals added 0.1 to 0.5% C and 1.0 to 4.0% Mo to the Invar alloy, and the carbide Mo2C precipitated after aging treatment. The tensile strength of the alloy is >1000MPa, and the expansion coefficient α is <3.2×10 ^(-6 )/℃. Mitsubishi Steel adds 0.1% C and 0.5% to 3.0% Mo (or W, or W+Mo) to the Fe-32Ni-4Co series alloy. The tensile strength of the developed alloy is >1100MPa, and the thermal expansion coefficient α (20~400 ℃)＞6.0×10^(-6)/℃. Japanese researcher Vinogradov studied the mechanical behavior and fatigue life of ultra-fine grain (UFG) Fe-36%Ni Invar alloy, and achieved a significant improvement in mechanical properties through equal channel angular extrusion (ECAP), compared with conventional grain sizes. Compared with ordinary Invar alloy, the yield stress and fatigue limit are increased by 3 times and 2 times respectively.
Russian materials researchers are also paying great attention to the research of high-strength Invar alloy application and have done related research work. Russia’s Central Ferrous Metallurgical Research Institute conducted research on V improving the physical and mechanical properties of Fe-Ni-C alloys. The results show that V can effectively improve the strength of Invar alloy and stabilize its thermal expansion coefficient. The alloy after cold deformation treatment The tensile strength reaches 1300 MPa and the thermal expansion coefficient α (20~100℃) <2.0×10^(-6)/℃. Nakama et al. found that solid solution treatment at 1150°C, high temperature deformation at 800-1000°C and subsequent aging treatment can increase the strength of Fe-36.2Ni-10.1Co-5.2Cr-2.4Ti Invar alloy to about 1400MPa. The thermal expansion coefficient did not change significantly.
2 Current status of research in China
For Invar alloy application , Chinese research institutions and manufacturers have followed the pace of international related research and conducted a large amount of research on Invar alloys. Lu Jiansheng et al. used Ni3 (Ti, Al) intermetallic compound precipitation phase to strengthen Invar alloy. Through secondary cold drawing processing and an aging treatment during the period, they can obtain Invar alloy wire rods with a tensile strength greater than 1500MPa; and at room temperature The excellent performance of linear expansion coefficient less than 3.5×10 -6 /℃ is maintained up to 100℃.
Based on Invar alloy Fe-36Ni-0.3C, Shanghai HY Industry Co., Ltd first used FactSage thermodynamics software to calculate the equilibrium microstructure of Invar steel by adding appropriate amounts of four alloying elements: Ti, Nb, Mo and Cr. , it was found that after slow cooling, primary carbides TiC, NbC, Mo2C, Mo6C and Cr23C6 were generated respectively, and their dissolution temperature ranges were 1200~1430℃, 1390~1430℃, 700~900℃, 1020~1220℃ and 700℃ respectively. ~1080℃. On this basis, experiments have found that after adding alloy elements Ti, Nb, Mo, and Cr, a second phase is generated in the material, and the grains are refined. After adding the above elements, the rolled mechanical properties of the steel are improved, but at the same time, the expansion coefficient of the material is increased.
Wang Chao et al. alloyed 4J36 with Mo element to prepare an alloy with low expansion coefficient and high strength. Through preparation processes such as smelting, forging and heat treatment, chemical analysis, metallographic and electron microscopy scanning, grain size, and display were performed. Based on the detection of microhardness and expansion coefficient, the mechanism is analyzed based on magnetostriction theory and fine grain strengthening theory. The results show that Mo alloying increases the strength of the matrix and maintains a low expansion coefficient of the matrix.
This article studies the effect of high-temperature annealing on the microstructure and thermal expansion coefficient of cold-drawn Fe-Ni alloy wire. The alloy contains approximately 61.0wt%Fe and 36.2wt%Ni. The structure of the original sample after annealing at 950°C for 3 hours was a single γ phase. After testing, the thermal expansion coefficient of the original cold-drawn Fe-Ni wire at room temperature is about 5.5×10^(-6)/℃, and the thermal expansion coefficient of the alloy treated at 950℃×3h near room temperature is only 0.2×10^(-6 ) /°C, exhibiting almost zero thermal expansion rate. Research suggests that the reason for the high room temperature thermal expansion coefficient of the original alloy is that the presence of a large number of dislocations promotes the segregation of solute atoms and forms Cottrell air clusters, which inhibits the flipping of magnetic domains. Annealing at 950°C can effectively reduce the dislocation density or the content of solid solution atoms, limiting the formation of Cottrell air clusters, thus obtaining a low room temperature thermal expansion coefficient.
3 Invar alloy application in transmission lines
At present, transmission conductors are all made of aluminum conductors. Due to the material characteristics of aluminum conductors, the transmission capacity of the conductors is limited. The operating transmission capacity of the line depends on the conductor’s ability to withstand temperature. Due to power grid security restrictions, the temperature of ordinary steel-core aluminum stranded wires can only reach 70-90°C during long-term use, and the transmission capacity is affected. When transmission corridors are restricted, capacity-increasing wires need to be used to increase the power transmission capacity to meet power demand. Due to the low expansion coefficient of Invar alloy, Invar alloy wire can replace ordinary steel core and be twisted with heat-resistant aluminum alloy wire to form a conductor. It is called aluminum-clad Invar core heat-resistant aluminum alloy stranded wire, which can operate at high temperatures for a long time, and The wire strength increases and the sag decreases.
3.1 Development of aluminum-clad invar core heat-resistant aluminum alloy stranded wire
In the early 1980s, aluminum-clad invar core heat-resistant aluminum alloy stranded wire was born in Japan. Afterwards, the United States, Europe and South Korea conducted a large number of experiments on its commercial operation. For example, Japan’s JPS Company and South Korea’s LG Company began to put it into commercial operation on a large scale in the 1980s. Only Japan’s Tokyo Electric Power Company has put aluminum into operation. Nearly 4,000km of heat-resistant aluminum alloy stranded wires with Yinhua steel core are used to improve the increasingly tense power network.
China’s conductor research institutions and manufacturers have conducted research and production on aluminum-clad Invar core heat-resistant aluminum alloy stranded wires. In 2003, State Grid Liaoning Electric Power Co., Ltd. developed low-sag aluminum-clad invar core heat-resistant aluminum alloy stranded wire, which was applied on the entire 220kV Beigeng line in 2004 and has been operating with good results so far. Later, some Chinese cable manufacturers successively researched and developed this type of products and applied them in some transmission lines. At present, Chinese foreign conductor manufacturers have developed aluminum-clad Invar core heat-resistant aluminum alloy stranded wires with a heat-resistant temperature of 210 to 230°C, which are used in capacity expansion and transformation of transmission lines. However, the current price of China’s aluminum-clad Invar core heat-resistant aluminum alloy stranded wire is relatively high and has not yet been widely promoted and applied.
The technical feature of aluminum-clad Invar core heat-resistant aluminum alloy stranded wire is that it can achieve “double capacity” transmission capacity under the “same outer diameter and same sag”. It has the advantages of large carrying capacity, low sag and long life. The long-term operating temperature of the aluminum-clad Invar core heat-resistant aluminum alloy stranded wire can reach 210~230℃, and the transmission current can reach more than 2 times the current carrying capacity of the same cross-section steel core aluminum stranded wire at 70℃; during operation, the The tension of the aluminum alloy wire part decreases with the increase of temperature. After reaching the migration point temperature, the tension of the conductor is transferred to the aluminum-clad invar core. At this time, the linear expansion coefficient of the stranded wire is the linear expansion coefficient of the aluminum-clad invar core. Aluminum-clad invar core heat-resistant aluminum alloy stranded wire can ensure that the sag is basically the same as steel-core aluminum stranded wire with the same outer diameter at high temperatures. Since the outer surface of the Invar core is covered with aluminum, there is no potential difference with the outer aluminum alloy wire, which improves the corrosion resistance of the wire. It can be used in various environments and has a working life of more than 40 years.
4 Conclusion
As a low-expansion alloy material, Invar is widely used in many fields, and it has great application prospects as a core material for power transmission conductors. However, the current high price of Invar alloy limits its large-area application. Future research on Invar alloys should take into account low expansion properties and high strength while also considering the issue of low cost, thereby accelerating the application of Invar alloys, especially its promotion and application in transmission lines, which is very important for China. The development of power transmission technology is of great significance
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