Nickel has a silvery white metal appearance, similar in many aspects to iron metal, but with good oxidation and corrosion resistance.
It is used as an alloying element mainly to improve high temperature strength, corrosion resistance and other properties in a wide range of ferrous and non-ferrous alloys.
Other properties that stand out are the thermal and electrical conductivities, as well as the magnetic property. Properties that make nickel and its alloys very valuable metals.
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Representing about 14% of the nickel used, nickel alloys are mainly used in services subjected to high temperatures and corrosion.
Alloys containing Chromium show good oxidation resistance at elevated temperatures and also resist corrosion. Varieties containing appropriate amounts of aluminum and titanium are precipitation hardenable and exhibit high mechanical strength at elevated temperatures.
Nickel alloys are divided into several families: commercially pure nickel; binary alloys such as Ni-Cu and Ni-Mo; ternary alloys such as Ni-Cr-Fe and Ni-Cr-Mo; complex alloys such as Ni-Cr-Fe-Mo-Cu (with the possibility of additional elements); and the super leagues.
Pure or in metallic alloys, nickel has many and varied applications.
It is a very corrosion resistant metal, it is not surprising that 65% of the world’s nickel production is destined for the production of stainless steel. Stainless steel is an alloy consisting mainly of iron, with 18% chromium and 8% nickel. This steel is used in the most diverse applications from simple kitchen material to construction material for train tracks or the construction of offshore oil platforms.
Some other nickel-containing alloys have interesting applications.
Nickel/copper alloys (Ex: MonelTM ) are extremely resistant to corrosion especially in salt water and are therefore used in the naval and oil industry. As it is resistant to acidic environment, it is also used in the food industry.
Nickel/chromium/molybdenum alloys (Ex: InconelTM ) are also resistant to stress corrosion cracking in chloride-containing media due to their high nickel content; and this element also provides resistance to basic media (such as caustic soda) and diluted acid reducing media, however it does not prevent pitting corrosion or the formation of deposits on the part’s surface.
Nickel and chromium alloys, containing between 11% and 22% of chromium and small amounts of other elements, are common constituents of electrical resistances in toasters and ovens.
Another use of nickel alloys is coin minting. The most popular example is the 5 cents coin, which is called nickel, but which, in fact, only contains 25% nickel in its composition. This coin began to be produced in 1865. On the other hand, in Europe, coins with nickel were introduced in Belgium in 1860. Even today, coins in cupronickel alloy, an alloy of copper and nickel, continue to be minted in Portugal.
Applications in the pre-salt: Solutions to reduce corrosion, using forged steel coated with nickel alloys on a large scale. (oil transport lines and BX rings, placed between oil pipe flanges)
Handling caustic soda; Pump parts, propeller shafts, equipment for pickling and chemical processes.
The development of so-called nickel, cobalt and iron superalloys began in the United States in the 1930s, but over the years nickel superalloys became the most used.
In addition to jet turbines, nickel superalloys find varied applications at high temperatures, such as in rocket engines and space vehicles in general, nuclear reactors, submarines, thermoelectric plants, petrochemical equipment, for example. However, the main application of these alloys remains their use in aviation jet turbines.
Other materials, such as chromium alloys, other metals with a higher melting point, and refractory ceramics, have been studied as possible alternatives to the use of nickel superalloys, but so far, a better combination of properties has not been found in these materials. required for this type of application than what is currently obtained with nickel superalloys.
Nickel Superalloy Properties
Pure nickel has a density of 8.9 g/cm3, melting point 1455 ºC, CFC (face-centered cubic) crystal structure. The density of most nickel superalloys is between 7.79 and 9.32 g/cm3.
For example, the density of Inconel 100 (contains about 60% nickel) is 7.79 g/cm3, due to the high aluminum and titanium contents, while superalloys with high tungsten and tantalum contents reach densities on the order of 9.07 g/cm3. Density is an important property for nickel superalloys, as reducing jet turbine component density leads to increased centrifugal stresses, reducing component life.
The thermal conductivity of pure nickel is on the order of 0.089 (W/mm2)/(ºC/mm), therefore higher than that of pure iron (CCC: body-centered cubic), which only reaches 0.072 (W/mm2)/(ºC /mm). However, the thermal conductivity of superalloys is much lower, on the order of 10% of these values, due to the addition of many alloying elements at high levels. The ideal would be to obtain superalloys with higher thermal conductivity, as this would be important to dissipate heat and thus minimize temperature gradients, thus reducing thermal stresses and thus the tendency for thermal fatigue failure to occur.
Thermal expansion in nickel superalloys is lower than in austenitic ferrous alloys and this is important from an application point of view in jet turbines, as these components are designed with close dimensional tolerances to perform well in service, in addition to a low Thermal expansion coefficient contribute to minimizing thermal stresses, thus minimizing the occurrence of warping and thermal fatigue.
The primary reason for the existence of nickel superalloys with different chemical compositions is their excellent mechanical strength over a wide temperature range.
The face-centered cubic (FCC) compact crystal structure of the austenitic matrix of nickel superalloys has already been proven, has a great capacity to maintain tensile strength, breaking strength and good creep properties at homologous temperatures much higher than the alloys. of body-centered cubic matrix (BCC) because of several factors, including the excellent modulus of elasticity and the high diffusivity that the secondary elements have in this type of matrix.
It is of great importance the great solubility of many alloying elements in the austenitic matrix and the ability to control the precipitation of intermetallic phases such as the gamma line, which confer high mechanical strength. Hardening can also be increased by forming carbides and also by dissolving some elements in the matrix (solid solution hardening).
This hardening ability of these austenitic nickel, cobalt and iron alloys makes them suitable for applications in jet turbines and rocket engines, which require high mechanical strength at medium and high temperatures.
However, not only the mechanical strength/hardness is important in these types of applications.
Ductility under service conditions is also important, and most superalloys have good ductility.
Superalloys in general also have good resistance to impact, high and low cycle fatigue and thermal fatigue.