Nickel-based alloy is the most widely used superalloy with the highest high temperature strength.
The main reasons are:
- More alloying elements can be dissolved in the nickel-based alloy, and the structure stability can be maintained.
- The coherent ordered A3B intermetallic compound γ'[Ni3(Al, Ti)] phase can be formed as the strengthening phase, so that the alloy can be effectively strengthened, and the high temperature strength is higher than that of iron and cobalt based superalloys.
- 3. The chromium-containing nickel-based alloy has better oxidation resistance and gas corrosion resistance than the iron-based superalloy.
Nickel-based alloys contain more than ten elements, of which Cr mainly plays an anti-oxidation and anti-corrosion role, and other elements mainly play a strengthening role.
According to their strengthening mode of action can be divided into: solid solution strengthening elements, such as tungsten, molybdenum, cobalt, chromium and vanadium;
Precipitation strengthened elements such as aluminum, titanium, niobium and tantalum; Grain boundary strengthening elements such as boron, zirconium, magnesium and rare earth elements.
Nickel-based superalloys have solid solution strengthened alloy and precipitation strengthened alloy according to the strengthening method.
» Solid solution strengthened alloy
It has certain high temperature strength, good oxidation resistance, thermal corrosion resistance, cold resistance, thermal fatigue performance, and good plasticity and weldability, and can be used to manufacture parts with high working temperature and little stress, such as the combustion chamber of gas turbines.
» Precipitation-strengthened alloy
Generally, three strengthening methods of solid solution strengthening, precipitation strengthening and grain boundary strengthening are integrated, so it has good high temperature creep strength, fatigue resistance, oxidation resistance and thermal corrosion resistance, and can be used to make components with high stress under high temperature, such as turbine blades and turbine disks of gas turbines.
Nickel-based alloy has high strength and oxidation resistance at 650~1000℃. According to the main properties, it is subdivided into nickel-based heat resistant alloy, nickel-based corrosion resistant alloy, nickel-based wear resistant alloy, nickel-based precision alloy and nickel-based shape memory alloy.
» Nickel-based corrosion resistant alloy
The main alloying elements are copper, chromium and molybdenum. It has good comprehensive properties and can withstand various acid corrosion and stress corrosion. The earliest application is nickel copper (Ni-Cu) alloy, also known as Monel alloy (Monel alloy Ni 70 Cu30); In addition, there are nickel-chromium (Ni-Cr) alloy, nickel-molybdenum (Ni-Mo) alloy (mainly refers to the Hastelloy B series), nickel-chromium molybdenum (Ni-Cr-Mo) alloy (mainly refers to the Hastelloy C series) and so on. At the same time, pure nickel is also a typical representative of nickel-based corrosion resistant alloys. These nickel-based corrosion resistant alloys are mainly used in the manufacture of components for various corrosion resistant environments such as petroleum, chemical, and electric power.
Nickel-based corrosion resistant alloys mostly have austenitic structures. In the state of solid solution and aging treatment, there are intermetallic phases and metal carbon nitrides on the austenitic matrix and grain boundaries of the alloy, and various corrosion resistant alloys are classified by composition and their characteristics are as follows:
Ni-Cu alloy has better corrosion resistance than nickel in reducing media, and better corrosion resistance than copper in oxidizing media, and it is the best material to resist high temperature fluorine gas, hydrogen fluoride and hydrofluoric acid in the absence of oxygen and oxidants .
Ni-Cr alloy is also nickel-based heat resistant alloy; It is mainly used in oxidizing medium conditions. It is resistant to high temperature oxidation and corrosion of gases containing sulfur and vanadium, and its corrosion resistance increases with the increase of chromium content. This kind of alloy also has good resistance to hydroxide (such as NaOH, KOH) corrosion and stress corrosion resistance.
Ni-Mo alloy is mainly used under the conditions of corrosion by reducing media. It is one of the best alloys to resist hydrochloric acid corrosion, but in the presence of aerobic and oxidizing agents, corrosion resistance will be significantly reduced.
Ni-cr-mo (W) alloy has the properties of the above Ni-Cr alloy and Ni-Mo alloy. It is mainly used under the conditions of oxidizing and reducing mixed media. This kind of alloy has good corrosion resistance in high temperature hydrogen fluoride gas, in hydrochloric acid and hydrofluoric acid solution containing oxygen and oxidant, and in wet chlorine gas at room temperature.
Ni-Cr-Mo-Cu alloy has the ability to resist both nitric acid and sulfuric acid corrosion, and also has good corrosion resistance in some oxidation-reducing mixed acids.
Application field
Nickel-based superalloys are used in many fields, such as:
1. Ocean: Marine structures in the Marine environment, seawater desalination, mariculture, seawater heat exchange, etc.
2, environmental protection: thermal power generation of flue gas desulfurization equipment, wastewater treatment.
3. Energy field: nuclear power generation, comprehensive utilization of coal, tidal power generation, etc.
4. Petrochemical field: oil refining, chemical equipment, etc.
5. Food field: salt production, soy sauce brewing, etc.
» Nickel-based wear resistant alloy
The main alloying elements are chromium, molybdenum, tungsten, but also contain small amounts of niobium, tantalum and indium. In addition to wear resistance, its oxidation resistance, corrosion resistance, welding performance is good. It can be used as a coating material, which can be coated on the surface of other base materials by surfacing welding and spraying process.
Nickel base powders include self-fluxing alloy powder and non-self-fluxing alloy powder.
The non-self-fluxing nickel-based powder refers to the nickel-based alloy powder which does not contain B, Si or B, Si content is low. This kind of powder is widely used in plasma arc spraying coating, flame spraying coating and plasma surface strengthening. It mainly includes :Ni-Cr alloy powder, Ni-Cr-Mo alloy powder, Ni-Cr-Fe alloy powder, Ni-Cu alloy powder, Ni-P and Ni-Cr-P alloy powder, Ni-Cr-Mo-Fe alloy powder, Ni-Cr-Mo-Si high-wear-resistant alloy powder, Ni-Cr-Fe-Al alloy powder, N i-Cr-Fe-Al-B-Si alloy powder, Ni-Cr-Si alloy powder, Ni-Cr-W based wear resistance alloy powder, etc.
The nickel base self-fluxing alloy powder was formed by adding proper amount of B and Si to the nickel alloy powder. The so-called self-melting alloy powder is also known as low eutectic alloy, hard surface alloy, which is a series of powder materials formed by adding low-melting eutectic alloy elements (mainly boron and silicon) to nickel, cobalt and iron-based alloys. Commonly used nickel-based self-fluxing alloy powders are Ni-B-Si alloy powder, Ni-Cr-B-Si-Mo, Ni-Cr-B-Si-Mo-Cu, high molybdenum nickel base self-fluxing alloy powder, high chromium molybdenum nickel base self-fluxing alloy powder, Ni-Cr-W-C base self-fluxing alloy powder, high copper self-fluxing alloy powder Gold powder, tungsten carbide dispersion type nickel based self-fluxing alloy powder, etc.
The role of various elements in the alloy:
● The role of boron and silicon elements: significantly reduce the melting point of the alloy, expand the solid-liquid phase temperature zone, and form low-melting eutectic; Deoxidation reduction and slagging function; Hardening and strengthening of the coating; Improve operation process performance
● The role of copper element: improve the corrosion resistance to non-oxidizing acids
● The role of chromium element: solid solution strengthening, passivation; Improve corrosion resistance and high temperature oxidation resistance; The excess chromium is easy to form the hard phase of chromium carbide and chromium boride with carbon and boron, thus improving the hardness and wear resistance of the alloy
The role of molybdenum element: the atomic radius is large, the crystal lattice is distorted after solution, significantly strengthen the alloy matrix, and improve the high temperature strength and red hardness of the matrix; It can cut off and reduce the reticular tissue in the coating; Improve the resistance to cavitation and erosion.
» Nickel-based precision alloy
Including nickel-based soft magnetic alloy, nickel-based precision resistance alloy and nickel-based electric heating alloy. The most commonly used soft magnetic alloy is about 80% nickel permalloy, its maximum permeability and initial permeability is high, coercivity is low, is an important core material in the electronics industry. The main alloying elements of nickel-based precision resistance alloy are chromium, aluminum and copper. This alloy has high resistivity, low resistivity temperature coefficient and good corrosion resistance, and is used to make resistors. Nickel-based electrothermal alloy is a 20% chromium nickel alloy with good oxidation resistance and corrosion resistance, and can be used for a long time at 1000~1100℃.
» Nickel-based memory alloy
Nickel alloy containing 50(at)% titanium. The recovery temperature is 70℃, and the shape memory effect is good. A small change in the proportion of nickel-titanium components can make the recovery temperature change in the range of 30~100℃. It is widely used in the manufacture of automatic opening structural parts used in spacecraft, self-stimulating fasteners used in aerospace industry, artificial heart motors used in biomedicine, etc.
Tissue
In addition to the austenite matrix, there are γ’ phase dispersed in the matrix, secondary carbides on the grain boundary and primary carbides and borides precipitated during solidification. With the increase of alloying degree, the microstructure changes have the following trend: the number of γ’ phases gradually increases, the size gradually increases, and the size of the γ’ phase becomes a cube, and the size and shape of the same alloy appear different γ’ phases. In the casting alloy, γ+γ’ eutectic is also formed during the solidification process, and the grain boundary precipitates discontinuous granular carbide and is surrounded by γ’ phase thin films. These changes in microstructure improve the properties of the alloy.
The chemical composition of modern nickel-based alloys is very complex, and the saturation of the alloy is very high, so it is required to strictly control the content of each alloy element (especially the main strengthening element), otherwise it will be easy to precipitated harmful phases during use, such as σ, µ Phase, damage the strength and toughness of the alloy. Directional crystal turbine blades and single crystal turbine blades have been developed in nickel-based cast superalloys.
The directional crystal blade eliminates the transverse grain boundaries which are sensitive to holes and cracks, and makes all grain boundaries parallel to the direction of the stress axis, thus improving the service performance of the alloy. The single crystal blade eliminates all grain boundaries and does not need to add grain boundary strengthening elements, so that the initial melting temperature of the alloy is relatively increased, thus improving the high temperature strength of the alloy and further improving the comprehensive properties of the alloy.
Production Technology
Nickel-based alloys, especially precipitation-strengthened alloys, contain high alloying elements such as aluminum and titanium. It is usually melted by vacuum induction furnace and remelted by vacuum consumable furnace or electroslag furnace. Hot processing using forging, rolling process, for high alloyed alloy, due to poor thermoplasticity, the use of extrusion after rolling or with mild steel (or stainless steel) sheath direct extrusion process. The cast alloy is usually melted by vacuum induction furnace, and the parts are made by vacuum remelting and precision casting.
Deformed alloy and part of the cast alloy need to be heat treated, including solution treatment, intermediate treatment and aging treatment, taking Udmet 500 alloy as an example, its heat treatment system is divided into four stages: solution treatment, 1175℃, 2 hours, air cooling; Intermediate treatment, 1080℃, 4 hours, air cooling; One time aging treatment, 843℃, 24 hours, air cooling; Secondary aging treatment, 760℃, 16 hours, air cooling. In order to obtain the required organizational state and good comprehensive performance.
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