一: Classification and Current Applications of Nickel and Nickel-Based Alloys
1. Classification by Alloying Elements
(1) Industrial Pure Nickel:
Industrial pure nickel exhibits a slightly yellowish hue compared to silver, with a lustrous finish. It possesses excellent plasticity and toughness, alongside resistance to atmospheric corrosion, alkalis, and fresh water. In industrial production, pure nickel is predominantly used in rolled sheet form for structural components. Its nickel content exceeds 99%, with carbon content not exceeding 0.3%. It exhibits relative stability at elevated temperatures and possesses certain thermal strength properties. Currently, structural components made from industrial pure nickel are extensively employed in combustion chambers for aerospace aircraft, combustion turbines, and heat exchangers for nuclear reactors.
(2) Nickel-based alloys:
Nickel-based alloys are created by adding alloying elements such as Cu, Cr, Mo, Nb, and W to pure nickel. Examples include the Ni-Cu, Ni-Cr-Fe, Ni-Cr-Mo, and Ni-Cr-Mo-Cu series.
2. Classification by alloy strengthening method
(1) Non-heat-treatable industrial pure nickel:
This category comprises pure nickel metal without any alloying elements, whose strength cannot be enhanced through heat treatment. Examples include the Ni200 series of pure nickel. Due to its inherent properties, this industrial-grade pure nickel finds extensive applications in rocket engine components, corrosion-resistant evaporators, sonar equipment elements, spark plug electrodes, and electron tube cathodes.
(2) Solution-strengthened alloys:
These alloys incorporate appropriate minor amounts of alloying elements such as Al, Cr, Cu, Fe, Mo, Ti, W, V, Nb, and rare earth elements. High-temperature solution treatment is employed to enhance the alloy’s strength.
(3) Precipitation-Hardening Nickel-Based Alloys:
These alloys achieve strength enhancement through a sequence of solid solution treatment followed by ageing treatment after alloying element addition. Their development primarily targets high-temperature, high-stress operating conditions.
(4) Dispersion-Strengthened Alloys:
These alloys are predominantly nickel-based alloys strengthened by dispersed thorium oxide, such as TD-Ni and DS-Ni. Containing approximately 2% thorium oxide and 98% nickel, the oxide is uniformly dispersed throughout the alloy matrix, significantly enhancing tensile strength. Compared to TD-Ni alloys, the Ni-Cr type TD-NiCr alloy, containing around 20% chromium, exhibits superior strength and corrosion resistance.
3. Classification by Alloy Forming Method
(1) Deformable Nickel-Based Alloys:
These alloys are primarily nickel-based alloys formed through pressure processing, capable of being rolled into thin sheets and other small rolled components. Consequently, these alloys exhibit high thermal stability and thermal strength. Nickel-based alloys undergo solution treatment to achieve good plasticity and toughness, enabling them to withstand high-temperature dynamic loads and undergo stamping processes. They are commonly used in production to form welded structural components.
(2) Cast Nickel-Based Alloys:
These alloys are cast into components of specific shapes and dimensions using foundry techniques. Production often employs precision casting methods. The alloys retain good thermal strength and weldability. However, due to the cast microstructure and susceptibility to casting defects, cast alloys are less widely used than wrought alloys.
二: Alloying of Nickel and Nickel-Based Alloys
(1) Influence of Sulphur:
When welding nickel alloys, sulphur exerts a more pronounced effect on the weld metal than in other alloy materials. The primary concern arising from this influence is the issue of crystallisation cracks in the weld metal.
(2) Effect of Magnesium:
Magnesium forms sulphides in nickel and nickel alloys, possessing a significantly higher melting point than typical sulphides. Consequently, magnesium promotes the solidification of sulphur, thereby mitigating its detrimental effects.
(3) Effect of Niobium:
The addition of niobium to nickel alloys markedly reduces the occurrence of hot cracks.
(4) Influence of Lead:
Lead is an undesirable element in nickel alloys, primarily due to its tendency to induce weld heat cracks – a phenomenon termed lead embrittlement.
(5) Effect of Phosphorus:
Phosphorus affects nickel alloys similarly to sulphur and lead. Though present in minimal quantities, its detrimental effects should not be underestimated. Within the alloy, phosphorus primarily forms low-melting-point eutectic compounds with nickel, segregating at grain boundaries. This increases the width of the partially molten zone, thereby enhancing crack propensity.
(6) Effect of Boron:
Boron improves high-temperature mechanical properties and strength in nickel alloys, yet possesses adverse effects. In weld metal and heat-affected zones, boron exceeding permissible limits forms low-melting-point eutectics with nickel. It may also participate in alloying elements to form low-melting-point compounds, thereby increasing hot cracking susceptibility.
(7) Effect of Zirconium:
In nickel-based alloys, zirconium behaves similarly to boron. Small additions enhance high-temperature mechanical properties and fracture toughness. However, excessive amounts, particularly under welding conditions, promote segregation in weld metal, thereby increasing hot cracking susceptibility.
(8) Effect of Carbon:
Carbon acts as an interstitial strengthening element in nickel alloys, forming carbides with carbide-forming elements such as Mo, Cr, and Nb to enhance strength. In chromium-free nickel alloys, carbon precipitates as granular graphite at grain boundaries within the heat-affected zone under high-temperature welding conditions, reducing the plasticity of this zone. Additionally, in alloys such as Ni-Cr and Ni-Cr-Fe, carbon increases intergranular corrosion susceptibility. However, this tendency is mitigated if titanium and niobium are added as stabilisers. Employing ultra-low-carbon nickel-chromium alloys achieves a similar effect.
(9) Influence of Molybdenum:
Molybdenum acts as a carbide-forming element in nickel and nickel-based alloys. It significantly contributes to solution strengthening and age hardening, while also enhancing the alloy’s high-temperature strength, making it a highly beneficial element. Consequently, a certain amount of molybdenum is added to both Ni-Cr and Ni-Cr-Fe alloys.
(10) Effect of Silicon:
Silicon content is maintained at low levels in nickel-based alloys. This is because silicon readily induces hot cracking in nickel alloys, with high Ni-Cr alloys being particularly sensitive to its effects. Concurrently, excessively high silicon content markedly reduces ductility and toughness. To counteract silicon’s tendency to increase hot cracking susceptibility, niobium is often added to offset its detrimental effects. During the development of NiCrFe-1 nickel alloy welding rods, the author controlled silicon content while incorporating niobium, significantly enhancing crack resistance.
(11) Influence of Aluminium:
Aluminium is incorporated into nickel alloys as a deoxidiser. It forms oxides and contributes to age-hardening effects. Consequently, aluminium in high-nickel alloys is primarily added for its age-hardening properties. During nickel alloy welding, aluminium also functions as an additive in the welding material, enhancing the weld metal’s properties to match those of the base metal. However, excessive aluminium content must be avoided, as it degrades weld metal performance.
(12) Effect of Titanium:
Titanium is an indispensable alloying element in nickel alloys. Similar to aluminium, it deoxidises to form oxides and induces age hardening, while also reducing defects such as porosity and gas holes in weld metal. Its content must remain minimal, constituting only trace amounts.
(13) Influence of Rare Earths:
Rare earth alloys typically involve the addition of rare earth oxides such as cerium, lanthanum, and calcium to nickel alloys. These act as purifiers, proving highly effective in removing gases like hydrogen, oxygen, and nitrogen. They also improve the distribution of oxides at grain boundaries, significantly enhancing the creep strength of high-nickel alloys.