Super alloy for fastener (marketing department of Shanghai HY Industry Co., Ltd)
(1) Features of Super alloy
-
Super alloy has high thermal strength and high thermal stability, and is an indispensable and important material in the manufacture of aero-engines. In a new turbofan engine, the amount of Super alloy material accounts for more than 50% of the weight of the engine. At present, aero-engines are developing in the direction of high thrust, low fuel consumption, high thrust-to-weight ratio and long life, which are mainly achieved by increasing the boost ratio of the compressor and the inlet temperature before the turbine and taking corresponding measures in design. The inlet temperature before the turbine has reached over 1300℃, and the outlet temperature of the compressor has exceeded 800℃. Therefore, at such a high working temperature, the heat resistance of the material is very important, otherwise it cannot meet the needs of the development of modern aero-engines. Since the 1960s, the working temperature of Super alloy has increased by an average of 10 °C per year, but it cannot meet the increasing demand of the inlet temperature before the engine turbine.
-
The composition of Super alloy is relatively complex, and the standard issued by the former Ministry of Metallurgy uses the Chinese pinyin “GH” to indicate. Super alloy is divided into iron-based, nickel-based, cobalt-based, etc. according to the matrix elements; according to the high temperature performance of the alloy, it is divided into thermally strong deformation Super alloy and thermally stable deformation Super alloy. Super alloy fasteners are mainly made of thermally deformed Super alloy. These alloys are characterized by high thermal strength and can be used for parts that are subjected to large loads and complex stresses under high temperature working conditions. However, cold deformation is difficult, and a hot forming process must be adopted. . In recent years, Super alloys with better heat resistance introduced from abroad include A286, Inconel718, Wasplloy, Rene41, Mp159, Nimonic80, etc. The room temperature strength of the thermally deformed Super alloy is also very good. Generally, the tensile strength exceeds 1000MPa, and the Mp159 alloy has the highest tensile strength, which can reach 1795MPa.
-
In addition to thermally strong deformation Super alloy, aerospace fasteners are also made of thermally stable deformation Super alloy materials, which have the following characteristics: strong anti-oxidation, anti-corrosion, and anti-scour ability, although the strength is not high (σ0.2 = 300～400MPa), but the plasticity is very good (δ=45%～60%), which can be deep drawn and cold heading. It is often used in the manufacture of aero-engine parts with low stress and high working temperature. parts, such as combustion chamber flame tube, afterburner and matching bolts, rivets, locking pieces, pins, etc. The specific alloy grades are GH1140, GH3030, GH3039 and so on.
(2) Forgeability of Super alloy and temperature control of hot forging (hot heading)
-
The content of alloying elements in Super alloy is relatively high, and carbides and intermetallic compounds are concentrated at the edge of dendrites, which affects the ductility and forgeability of the alloy. At the same time, the primary dendrite grain boundary is a weak link. When upsetting at a high speed is deformed, due to the high strength and poor plasticity of the alloy, intergranular cracks are easy to occur, and the mold is damaged; if the upsetting temperature is too high, it is easy to cause rapid growth of alloy grains and grain boundary oxidation, which will affect the strength of the alloy. Produce forging cracks. Therefore, the temperature of upsetting fasteners made of Super alloy materials should be strictly controlled.
(3) High temperature performance requirements of Super alloy fasteners
-
The commonly used high temperature mechanical properties of Super alloy are: high temperature instantaneous strength, high temperature durable strength and creep strength. The high temperature instantaneous strength is to measure the strength limit of the sample at a certain temperature by the ordinary tensile method. High-temperature endurance strength refers to the stress at which a material breaks after a specified time at a given temperature. It is generally measured by the axial tensile method. The endurance strength limit is generally expressed by σ and marked with time and temperature. For the material selection of high-temperature working parts And size design is a very important indicator. Parts working at high temperature will undergo plastic deformation below the elastic limit stress, and the longer the time, the greater the deformation, this phenomenon is called creep. The higher the temperature, the more serious this phenomenon is. Creep can cause parts to lose dimensional accuracy and even lead to safety incidents.
(4) Super alloy solid solution strengthening
-
Solid solution strengthening of Super alloy is to use some alloying elements (such as tantalum, niobium, tungsten, molybdenum, titanium, vanadium, aluminum, chromium, cobalt, etc.) added to dissolve them into the matrix (iron-based Super alloy is ferrite). Or austenite, nickel-based is austenite), a method of changing the atomic distribution of the matrix and the bonding force between atoms to improve the strength of the matrix. Typical Super alloys belonging to solid solution strengthening include GH3030, GH3039, GH3128, GH1140, etc. The alloying features are that the content of aluminum and titanium is low, and the content of nickel is high.
-
The effect of solid solution strengthening is not the main factor in Super alloys, and it is limited to improve thermal strength only by solid solution strengthening. Therefore, the use temperature of all Super alloys based on solid solution strengthening is not too high.
(5) Super alloy precipitation strengthening
-
The precipitation strengthening of super alloy is a method of improving the strength of the alloy by using the strengthening effect caused by the precipitation of the dispersed second phase from the supersaturated solid solution.
The strengthening phase in nickel-based alloys is not mainly carbides such as TiC, VC, M23C type carbides, etc., but intermetallic compounds, mainly Ni3 (Al, Ti), because it is also a face-centered cubic structure, only the lattice constant Different from the matrix (γ phase), it is often called the γ’ phase.
-
Titanium is the main component in the γ’ phase. When its content is greater than 1.4%, γ’ phase precipitation can occur. The higher the titanium content, the more γ’ phase, and the better the strengthening effect. The main function of aluminum is to increase the solid solution temperature of the γ’ phase to stabilize the γ’ phase so that it does not transform to the η (Ni3Ti) phase, so almost all precipitation hardening nickel-based alloys coexist with aluminum and titanium. However, when the content of titanium is too high and the content of aluminum is too small, the γ’ phase is unstable, and it is easy to transform into the close-packed hexagonal structure η phase (Ni3Ti). strength decreased. When the titanium-aluminum content ratio is adjusted to Ti/Al<2.5, the η phase can be avoided in the alloy. In other words, the aluminum content must increase when the titanium content increases, and the titanium content cannot exceed 2.5 times the aluminum content.
-
The reason why the γ’ phase can strengthen the matrix is that, on the one hand, the γ’ phase increases the motion resistance of dislocations in the matrix, and on the other hand, the γ’ phase maintains a coherent relationship with the matrix, thereby increasing the deformation resistance of the matrix. Therefore, the strengthening effect of γ’ phase is related to its size, shape, relative quantity, stability and lattice constant.
(6) Super alloy grain boundary strengthening
-
How to improve the grain boundary strength of Super alloy is of great significance to improve the performance of the alloy. Harmful impurities such as lead, arsenic, tin, sulfur, etc. brought into the alloy in the smelting process of Super alloy, often exist on the alloy grain boundary in the form of low melting point eutectic, and melt first at high temperature, reducing the strength of the grain boundary, such as sulfur and Ni3S2 formed by nickel, eutectic Ni+Ni3S2 begins to melt when the alloy is heated to 644℃, which makes the alloy show hot brittleness and decrease in high temperature strength. Therefore, these harmful impurities in the alloy are strictly controlled. The methods of strengthening grain boundaries include: adding trace elements such as boron, zirconium, rare earth elements, etc.; appropriate heat treatment; high temperature deformation heat treatment.
