Nickel-based alloys refer to a class of alloys that have high strength and a certain degree of oxidation and corrosion resistance at a high temperature of 650 to 1000 ℃. According to the main performance, it is subdivided into nickel-based heat-resistant alloys, nickel-based corrosion resistant alloys, nickel-based wear-resistant alloys, nickel-based precision alloys and nickel-based shape memory alloys. High-temperature alloys are divided into iron-based superalloys, nickel-based superalloys and cobalt-based superalloys according to the different substrates. Among them, nickel-based superalloys are referred to as nickel-based alloys.
The main alloying elements are copper, chromium and molybdenum. It has good comprehensive performance and can resist various acid corrosion and stress corrosion. The earliest application (produced in the United States in 1905) is nickel-copper (Ni-Cu) alloy, also known as Monel alloy (Monel alloy Ni 70 Cu30); in addition, there is nickel-chromium (Ni-Cr) alloy (nickel-based heat-resistant alloy) , The heat-resistant corrosion-resistant alloy in the corrosion-resistant alloy), nickel-molybdenum (Ni-Mo) alloy (mainly refers to Hastelloy B series), nickel-chromium-molybdenum (Ni-Cr-Mo) alloy (mainly refers to Hastelloy C Series) etc. 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 various corrosion-resistant environmental parts such as petroleum, chemical industry, and electric power.
Nickel-based corrosion resistant alloys mostly have austenitic structure. In the state of solid solution and aging treatment, there are intermetallic phases and metal carbonitrides on the austenite matrix and grain boundaries of the alloy.
The common elements in nickel-based alloys mainly include:
Nickel Ni
Provide metallurgical stability, improve thermal stability and weldability, improve corrosion resistance to reducing acids and caustic soda, and improve stress corrosion cracking resistance, especially in chloride and caustic soda environments.
Chromium Cr
Solid solution strengthening effect, passivation effect; improve corrosion resistance and high temperature oxidation resistance; excess chromium easily forms chromium carbide and chromium boride hard phases with carbon and boron to improve alloy hardness and wear resistance.
Mo
The large atomic radius will cause large distortion of the crystal lattice after solid solution, significantly strengthen the alloy matrix, improve the high-temperature strength and red hardness of the matrix; it can cut off and reduce the network structure in the coating; improve the resistance to cavitation and erosion.
Iron Fe
Improve resistance to high temperature carburizing environment, reduce alloy cost, and control thermal expansion.
Copper Cu
Improve the corrosion resistance to non-oxidizing acids.
Al
Improve high temperature oxidation resistance and age hardening.
Titanium Ti
Combined with carbon, it reduces intergranular corrosion caused by chromium carbide precipitation during heat treatment and improves aging strengthening.
Niobium Nb
Combined with carbon, it reduces intergranular corrosion caused by chromium carbide precipitation during heat treatment, improves pitting and crevice corrosion resistance, and improves high-temperature strength.
Tungsten W
Improve the resistance to reducing acid and local corrosion, improve strength and weldability.
Nitrogen N
Improve metallurgical stability, improve pitting and crevice corrosion resistance, and increase strength.
Cobalt Co
Provides enhanced high temperature strength, improved resistance to carbonation and vulcanization.
Many of these alloying elements can combine with nickel in a wide range of composition to form a single-phase solid solution, ensuring that the alloy has good corrosion resistance under many corrosive conditions. The alloy also has good mechanical properties in a fully annealed state, without worrying about harmful metallurgical changes during manufacturing or hot working. Many high nickel alloys can increase their strength through solid solution hardening, carbide precipitation, precipitation (ageing) hardening, and dispersion strengthening.