Nickel-based alloys are metallic materials with nickel as the primary matrix element, combined with one or more alloying elements to achieve superior mechanical and chemical properties.
These alloys offer excellent corrosion resistance, oxidation resistance, high-temperature strength, toughness, weldability, and processability, making them indispensable in demanding industries such as:
Petrochemical
Power generation
Marine engineering
Aerospace
Oil & Gas
Nuclear energy
Chemical processing
Depending on their performance characteristics, nickel-based alloys can generally be classified into three categories:
Nickel-based corrosion-resistant alloys
Nickel-based superalloys (high-temperature alloys)
Nickel-based functional alloys with special physical properties
Producing high-quality nickel alloys requires strict control throughout the melting process due to their unique metallurgical behavior.
Nickel and nickel-based alloys have relatively high melting temperatures.
During melting, the molten metal readily absorbs gases and reacts with refractory materials. Compared with copper alloys, nickel exhibits stronger oxidation and gas absorption tendencies.
Oxygen dissolves in molten nickel primarily as nickel oxide (NiO). Oxygen, moisture, and carbon dioxide in the furnace atmosphere can all oxidize nickel during melting.
Other harmful gases include:
Hydrogen
Nitrogen
Carbon monoxide
Carbon dioxide
Sulfur dioxide
Hydrocarbon gases
At elevated temperatures, sulfur dioxide reacts with nickel to form nickel sulfide (Ni₃S₂), which significantly reduces ductility and causes hot brittleness.
To minimize contamination, manufacturers commonly adopt:
Vacuum melting technologies
Electroslag remelting (ESR)
Magnesia-based refractory linings
Protective slag covering
Desulfurization and deoxidation refining processes
These measures help maintain alloy purity and improve final product quality.
Modern nickel alloy production employs various melting technologies depending on alloy requirements and quality standards.
Single melting uses only one melting method to produce the final ingot.
Typical processes include:
Air Arc Melting (AAM)
Air Induction Melting (AIM)
Vacuum Induction Melting (VIM)
Plasma Arc Furnace (PAF)
Plasma Induction Furnace (PIF)
Double melting is the most widely used production route for high-performance nickel alloys.
The first melting stage is mainly used to obtain the required chemical composition, while the second remelting stage improves cleanliness by reducing:
Gas content
Non-metallic inclusions
Segregation
Internal defects
Common remelting technologies include:
Vacuum Arc Remelting (VAR)
Vacuum Arc Double Electrode Remelting (VADER)
Vacuum Induction Remelting (VIR)
Non-consumable Electrode Melting (NAV)
Plasma Melting (PMV)
Electroslag Remelting (ESR)
Vacuum Induction + ESR (VIM + ESR)
Plasma Arc Remelting (PAR)
Electron Beam Remelting (EBR)
Vacuum Induction + Electron Beam Cold Hearth Melting (VIM + EBCFM)
For aerospace-grade and premium nickel superalloys, triple melting processes are frequently adopted to achieve the highest cleanliness.
Typical process routes include:
VIM + VAR + ESR
VIM + ESR + VAR
NAV + EBR + VAR
The additional remelting stage further reduces gas content and inclusions, resulting in superior mechanical properties and fatigue resistance.
Even under vacuum induction melting conditions, certain active alloying elements can react with refractory materials at high temperatures.
Typical oxides formed include:
Al₂O₃
Cr₂O₃
ZrO₂
NiO
Rare-earth oxides
These oxides accumulate as surface films and become non-metallic inclusions in the molten alloy.
In addition, remelting may introduce:
Oxidation contamination
Nitride inclusions
Refractory erosion products
Entrapped slag particles
Nitrogen is particularly difficult to remove during vacuum melting and may form stable nitrides with elements such as:
Titanium
Chromium
Vanadium
Aluminum
Zirconium
These inclusions reduce:
Fluidity of molten metal
Machinability
Fatigue life
Mechanical performance
To improve melt cleanliness, many foundries employ high-temperature ceramic foam filtration, which effectively removes inclusions before casting.
For many nickel superalloy castings, the finished component may account for only 10–30% of the total melted alloy.
Consequently, recycled materials—including runners, risers, gates, rejected castings, and process scrap—represent an important source of raw materials.
After sorting and inspection, recycled materials are generally reused in three ways:
Recycled materials are remelted together with virgin materials for producing high-quality components.
Recycled materials are mixed with virgin materials at specified ratios to produce alloys meeting composition requirements.
Typical international mixing ratios include:
1:1
3:2
3:1
Recycled materials may be used for lower-grade alloys or non-critical components, although this practice is becoming less common due to increasing quality requirements.
Proper charge calculation is essential for achieving accurate chemical composition while minimizing production costs.
General alloying guidelines include:
Manganese is typically charged at approximately 0.2 wt.% when no lower specification exists, with an estimated melting loss of 6%.
Chromium is usually charged around the nominal composition with an expected loss of 1–3%.
Niobium, tungsten, and vanadium are generally controlled toward the upper specification limit. Estimated melting losses are approximately 4–6% for vanadium and 3–4% for tungsten.
Molybdenum is typically charged near the nominal composition.
Aluminum and titanium additions require careful adjustment according to alloy grade, melting practice, and production experience.
A well-designed charge calculation not only reduces raw material costs but also minimizes repeated chemical analysis and improves production efficiency.
The following principles are generally followed:
Select raw materials with purity equal to or higher than the target alloy.
Add expensive alloying elements near the lower or nominal specification whenever possible.
Fully consider melting losses during charge calculation.
Prepare highly reactive or volatile alloying elements as master alloys whenever appropriate.
Properly balance virgin materials with recycled materials while maintaining alloy consistency.
Charge calculations are commonly based on producing 100 kg of finished alloy, after which the result is scaled according to actual production requirements.
The typical procedure includes:
Determine the target chemical composition.
Estimate melting losses for each alloying element.
Define available raw materials, including virgin metals, master alloys, and recycled materials.
Calculate alloying elements already contained in recycled materials.
Determine the additional amount of each alloying element required.
Calculate the required quantities of master alloys or pure metals.
Determine the remaining nickel required after accounting for recycled materials.
Verify impurity levels and finalize the charge sheet.
Nickel-based alloys play a critical role in modern high-performance engineering applications. Achieving excellent mechanical properties and corrosion resistance depends not only on alloy design but also on advanced melting technologies, proper charge calculation, and strict process control.
By combining vacuum melting, multiple remelting technologies, optimized alloy charge design, and effective inclusion control, manufacturers can consistently produce nickel-based alloys that meet the demanding requirements of aerospace, energy, petrochemical, and other critical industries.