Nickel-based Cast Superalloy and Its Heat Treatment Process
With the development of science and technology, the industrial production requirements of aviation, automobile and other industries are constantly improving. In order to meet the requirements of overall industrial production, high-temperature components in the fields of gas turbines and turbine casings have also increased the performance requirements of aerospace raw materials. The method of casting high-temperature alloy parts and components can meet the requirements of industrial production in various fields for raw materials in terms of performance and processing methods. In addition, heat treatment is a process in the production process of superalloys, which can change the process part of the alloy to improve the heat resistance and corrosion resistance of the alloy. In the process of manufacturing nickel-based cast superalloys, the heat treatment process plays an important role . This article introduces the development history of nickel-based cast superalloys and the influence of heat treatment technology on the structure and properties, and expounds the development status and trend of heat treatment technology of nickel-based cast superalloys.
Nickel-based casting superalloys refer to high-temperature alloy materials that use nickel as the main component and directly prepare parts by casting. It has comprehensive mechanical properties such as excellent high-temperature strength, fatigue stability, fracture toughness, and good creep resistance stability. It can operate safely for a long time in a high-temperature environment and is widely used in aerospace, automotive industries, and engine turbine blades in the field of gas turbines. , blisk, casing and other high temperature components. With the development of aero-engine technology, the requirements for the performance of alloy materials have also been greatly improved. Improvements in raw material performance and preliminary processing have become major trends in Chinese industrial production. Heat treatment is a necessary process for improving the performance of nickel-based cast superalloys. By changing the morphology and distribution of the internal structure of the casting or changing the chemical composition of the casting surface, the comprehensive mechanical properties of the casting can be improved, the casting stress can be eliminated, or the corrosion resistance of the casting can be improved. . This paper mainly discusses the research process of nickel-based casting superalloy and its heat treatment technology, and summarizes and analyzes it, looks forward to the future, and provides reference for subsequent industrial production.
1 Development history of nickel-based cast superalloys
Nickel-based casting superalloys are an important part of the field of superalloys. They have outstanding advantages in the production of various precision castings. The main reason is the high temperature resistance, high oxidation resistance and corrosion resistance of this material. The early nickel-based superalloys were mainly deformed alloys. In the late 1950s, with the development of aero-engine technology, the requirements for the temperature bearing capacity of engine turbine components became higher and higher, which raised the requirements for the strength and service temperature of superalloys higher requirements. The way to improve the strength of nickel-based superalloys is to increase the degree of alloying of the alloys. As a result, nickel-based superalloys are increasingly difficult to deform or even unable to be deformed, so they can only be produced by casting processes. In addition, with the development of engine blade design technology, hollow air film hole cooling blades have appeared. Such a hollow blade with a complex inner cavity can only be produced by investment casting. Under this demand, a series of nickel-based cast superalloy grades with good high-temperature performance have been gradually developed at home and abroad.
Nickel-based superalloy materials were first discovered in England in the early 1940s, and their application in the field of jet aviation made many industrial manufacturers pay attention to the importance of alloy material properties. The United Kingdom first produced the nickel-based alloy Nimonic75 (Ni-20Cr-04Ti) in 1941. After a long period of time, developed countries such as the United States and the Soviet Union have also successfully developed similar alloy materials, while China in the late 1950s Some alloy materials with the same properties have also been developed one after another.
It is not difficult to find out by studying the development history of nickel-based cast superalloys that there are roughly two research paths: one is to study the influence of different element addition ratios on alloy properties through the adjustment and combination of alloy components, and then obtain the optimal The element and composition ratio; the second is to improve the alloy preparation technology through the optimization and improvement of the production process and production equipment of nickel-based cast superalloy. For example, in the early 1950s, scientists developed a vacuum smelting process through a large number of experimental studies. The emergence and development of this process technology provided a technical guarantee for the preparation of nickel-based cast superalloys; in the 1960s, the developed Through research, the country has proposed the investment casting process and produced a large number of casting alloys with good high temperature properties. In the following years, some single crystal cast superalloys with better high temperature performance have been developed successively. With the development of aerospace engine technology, there are higher requirements for nickel-based cast superalloys. Therefore, during the more than 40 years from the early 1950s to the end of the 1990s, with the continuous improvement of research and development capabilities, the performance and service temperature of nickel-based cast superalloys have also become higher and higher, and the application fields have become wider and wider.
2 Strengthening mechanism and structural characteristics of nickel-based cast superalloys
2.1 Solid solution strengthening
One of the ways to improve the performance of nickel-based cast superalloys is solid solution strengthening, which refers to the incorporation of alloying elements such as aluminum, titanium, niobium, and tantalum that can form a γ′ phase with the matrix into the alloy, resulting in a certain lattice distortion. Methods to increase the strength of alloys. This phenomenon of metal strengthening by incorporating certain solute elements to form a solid solution is called solid solution strengthening. This type of alloy has excellent anti-oxidation and anti-fatigue properties. The most obvious advantage is high structural stability and good thermoplasticity. According to the above characteristics, nickel-based cast superalloys can be used to manufacture metal material parts with large temperature changes, such as: fan blades of aero-engines, turbine casing parts, etc.
In 2005, Haynes Corporation of the United States (Haynes Alloy) developed an excellent nickel-based deformed superalloy Haynes282. content, so that the alloy has good creep strength, thermal stability, superior machinability and good welding performance, which is mainly due to the addition of various refractory metal components in the alloy, such as tungsten and Manganese and other trace components.
2.2 Precipitation strengthening
In recent years, researchers have mostly consistent methods for improving the strength and performance of nickel-based cast superalloys, even if the added materials are different, there is little change. In the process of improving the strength and performance of the alloy, researchers generally add a small amount of precipitation strengthening elements to the alloy, and precipitate the second phase (γ′, γ″, carbide, etc. from the parent phase of the alloy structure through the heat treatment process. ), and then greatly improve the strength of the alloy. The typical γ′ phase in nickel-based cast superalloys is Ni3Al or Ni3Ti. There are two ways to strengthen it: one is to increase the content of Ai and Ti elements in the alloy and then increase the γ′ phase The second is that Co, W, Mo and other elements can be added to increase the strength of the γ′ phase. Through the increase of some elements, the precipitated second phase can effectively inhibit the development and growth of metal material particles. Most of the alloys using this strengthening method It is used to make high-temperature parts of engines, such as: aero-engines, turbine blades of gas turbines, etc.
3 Development of heat treatment technology for nickel-based cast superalloys
Nickel-based cast superalloy heat treatment refers to a type of metal material thermal processing process that uses heating, heat preservation and quenching processing methods for nickel-based high-temperature alloy products in the as-cast state to achieve the expected microstructure and mechanical properties. Studying the effect of heat treatment on the microscopic composition of the alloy to explore a good heat treatment mechanism has positive significance for improving the high temperature properties of the alloy. Among them, solution treatment and aging treatment are the main heat treatment processes. Solution treatment refers to the heat treatment process in which the excess phase in the alloy microstructure is fully dissolved into the matrix phase, and then rapidly cooled to obtain a supersaturated solid solution. Solution treatment can strengthen the solid solution and improve the corrosion resistance of the matrix. At the same time, it can eliminate the residual stress of the matrix casting. It is generally used as a preliminary heat treatment to prepare for subsequent machining and subsequent aging treatment. Aging treatment refers to heating and maintaining for a period of time in the temperature range where the strengthening phase is precipitated, so that the strengthening phase of the superalloy is uniformly precipitated, thereby improving the strength of the casting
In recent years, Chinese researchers have also carried out more extensive and in-depth research on the heat treatment process of nickel-based casting alloys. Yang Heyang discussed the influence of different heat treatment methods on the structure and properties of a new type of nickel-based single-structure high-temperature composite material containing rare earth metals. He used differential thermal analysis to determine the solidus and liquidus temperatures of the composite material. The test method is used to measure the initial melting temperature of the composite material, and finally the heat treatment technology method of the composite material is worked out.
Matt et al. studied the influence of various heat treatment methods on the mechanical properties and structure of Inconel 718 alloy. The research results showed that when the solution treatment temperature is very low, the metal γ′ phase will be fully melted and produce uneven shape. The structure of γ′ phase coexistence can inhibit the growth of metal grains, thereby increasing the plasticity and hardness of the alloy. In the case of low ambient temperature and long aging time for a long time, the number of γ′ can be increased. , so the hardness of the alloy increases, but the plasticity decreases.
Wang Shusen et al. conducted different mechanical properties tests on the isothermally cast GH4169G alloy after thermal processing and creep. The results showed that after thermal processing by direct aging and primary solid solution and secondary aging, the δ phase in the alloy generally presents Granular or needle-like. Direct aging can reduce alloy stress concentration and delay the formation and expansion of cracks, while standard heat treatment can greatly reduce the bonding hardness of austenite grain boundaries, thereby promoting the formation and expansion of cracks in austenite grain boundaries.
Taking Inconel718 nickel-based alloy as the main research object, Dou Xuezheng conducted in-depth research on the relationship between the microscopic composition, thermodynamic properties and corrosion resistance of the alloy under various heat treatments and processing systems. High, the δ phase in the alloy is further dissolved, and when the solid solution temperature rises from room temperature to 1020°C, the δ phase is fully melted. In addition, the oxidation resistance of the Inconel718 alloy after solution treatment is slightly better than that of the alloy material after solution + aging treatment.
Taking the production of a nickel-based alloy for an internal combustion engine exhaust valve as the main scientific research goal, Zhu Yong et al. discussed three different thermal processing systems (T1: 850°C×4h, AC+730°C×4h, AC; T2: 704°C ×24h, AC; T3: 760℃×16h, AC), the effect of the indoor temperature dynamics of the alloy. The experimental results show that the alloy under the T1 system has the highest strength and toughness, reaching 347HV10; the tensile strength ratio of the alloy under the T1 and T3 systems at room temperature exceeds 1200MPa; the ductility ratio under the T2 system is the best, exceeding one-third, It is especially suitable for working environments that require high plasticity.
After discussing the effect of solid solution strengthening treatment time on the distribution of Re and Ru elements and their microscopic morphology in nickel-based high-iron alloys, Feng Yuehan believed that due to the obvious effect of solid solution strengthening treatment time on the distribution of Re and Ru elements, when solid solution When the strengthening time is less than 1h, the segregation phenomenon of Re and Ru components is obvious; when the solid solution strengthening time reaches 20h, the segregation phenomenon of the two components is obviously improved.
Gui Wan et al. conducted an in-depth study on the effect of solid solution treatment on the microscopic composition and segregation degree of a nickel-based single crystal cast superalloy, and found that the as-cast microstructure of the alloy has a significant component segregation phenomenon, and by optimizing the heat treatment system, the solid solution can be improved. Temperature can effectively reduce the segregation of alloy components.