Nitrogen in Metal Alloy Industry — Comprehensive Applications
Nitrogen plays multiple essential roles in the metal alloy industry — as a protective atmosphere, a carrier gas, a cooling medium, and even as an alloying element that improves material properties. With the advancement of materials science and manufacturing technology, nitrogen-based processes have become indispensable in modern metallurgy.
1. Heat Treatment Atmospheres
Heat treatment is the largest application of nitrogen in metal processing. Different heat treatment processes require different atmosphere compositions:
Annealing
- Bright annealing of stainless steel: Pure nitrogen or nitrogen + 2-5% hydrogen at 1050-1150°C prevents oxidation and produces a bright surface finish. Requires nitrogen purity of 99.99% with dew point below -60°C.
- Spheroidize annealing of tool steels: Nitrogen atmosphere at 700-800°C promotes the formation of spheroidal carbides, improving machinability.
- Stress relieving: Nitrogen atmosphere at 550-650°C for welded components and castings prevents surface oxidation.
Carburizing and Carbonitriding
Gas carburizing uses nitrogen as a carrier gas for the carbon-bearing atmosphere (typically endothermic gas or methanol). Nitrogen-based carburizing atmospheres provide precise carbon potential control (±0.05% C) and eliminate the need for endothermic generators. Carbonitriding adds ammonia to the nitrogen atmosphere, introducing nitrogen into the case for improved hardenability.
Nitriding and Nitrocarburizing
In these processes, nitrogen is not just a protective gas but an active alloying element:
- Gas nitriding: Ammonia dissociates in the furnace to provide active nitrogen that diffuses into the steel surface, forming hard nitrides (Fe₂N, Fe₃N, Fe₄N) with surface hardness of 800-1200 HV.
- Plasma nitriding: Nitrogen plasma (ionized nitrogen gas) bombards the workpiece surface, providing faster nitriding rates and better control of the compound layer composition.
- Nitrocarburizing: Combined nitrogen and carbon diffusion creates a tough, wear-resistant surface layer with excellent corrosion resistance.
2. Powder Metallurgy and Sintering
Nitrogen-based atmospheres are essential in powder metallurgy sintering:
- Iron and steel powder sintering: Nitrogen + 5-10% hydrogen atmosphere at 1120-1150°C prevents oxidation and ensures complete metallurgical bonding between powder particles.
- Stainless steel powder sintering: Pure nitrogen or nitrogen + hydrogen atmosphere at 1200-1300°C produces fully dense sintered parts with corrosion resistance equivalent to wrought stainless steel.
- Metal injection molding (MIM): Nitrogen atmosphere during debinding and sintering of MIM parts prevents carbon loss and oxidation.
- Hardmetals (WC-Co): Nitrogen-controlled atmosphere during vacuum sintering of cemented carbide tools prevents decarburization and cobalt pooling.
3. Brazing and Soldering
Nitrogen atmosphere brazing produces clean, flux-free joints with superior strength:
- Continuous furnace brazing: Nitrogen atmosphere with dew point below -50°C for brazing aluminum heat exchangers, steel assemblies, and carbide tool tips.
- Vacuum brazing with nitrogen backfill: Nitrogen backfill after vacuum brazing cycles prevents oxidation during cooling and reduces cycle time.
- Aluminum brazing: Nitrogen atmosphere in CAB (Controlled Atmosphere Brazing) furnaces for aluminum automotive heat exchangers — the largest brazing application globally.
4. Additive Manufacturing (3D Printing of Metals)
Metal additive manufacturing processes require inert atmospheres to prevent oxidation of reactive metal powders:
- Selective Laser Melting (SLM): Nitrogen atmosphere in the build chamber with oxygen below 1000 ppm for stainless steel, tool steel, and aluminum alloys. Titanium and reactive alloys require argon instead.
- Electron Beam Melting (EBM): Helium atmosphere with nitrogen for cooling in the powder bed.
- Directed Energy Deposition (DED): Nitrogen shielding gas for the melt pool during deposition of steel and nickel alloy powders.
- Metal powder production: Nitrogen atomization for producing spherical metal powders, where molten metal is disintegrated by high-pressure nitrogen jets.
5. Non-Ferrous Metal Processing
- Aluminum: Nitrogen degassing of molten aluminum removes hydrogen and reduces porosity. Nitrogen fluxing also removes inclusions and alkali metals.
- Copper: Nitrogen atmosphere during oxygen-free copper (OFC) casting prevents oxygen pickup.
- Titanium: Nitrogen in titanium alloys (e.g., Ti-6Al-4V) as an interstitial strengthener improves creep resistance at elevated temperatures.
- Magnesium: Nitrogen + SF₆ (or SO₂) protective atmosphere during magnesium melting prevents rapid oxidation and burning.
6. Nitrogen as an Alloying Element
Nitrogen is increasingly used as an intentional alloying addition in steels:
- Austenitic stainless steels: Nitrogen addition (0.05-0.5%) increases yield strength by 30-60% without reducing ductility, stabilizes austenite, and improves pitting corrosion resistance.
- High-nitrogen steels (HNS): Steels with >0.4% nitrogen exhibit exceptional strength (up to 2000 MPa) combined with excellent corrosion resistance and non-magnetic properties.
- Tool steels: Nitrogen alloying in powder metallurgy tool steels improves wear resistance and hot hardness.
- Maraging steels: Nitrogen contributes to age-hardening reactions in nickel-cobalt-molybdenum maraging steels.
7. Metalworking and Cutting
- Laser cutting: High-pressure nitrogen (10-25 bar) for clean, oxidation-free cutting of stainless steel, aluminum, and non-ferrous metals.
- Plasma cutting: Nitrogen as the plasma gas for high-definition plasma cutting of carbon and stainless steel.
- Cryogenic machining: Liquid nitrogen cooling in turning and milling of difficult-to-machine alloys (titanium, Inconel) improves tool life by 200-500%.
Hongbo Gas Solutions
Hongbo Gas supplies a complete range of nitrogen generation systems for the metal industry, from small-scale units (10 Nm³/h) for laboratory furnaces to large-scale systems (5000 Nm³/h) for continuous heat treatment lines. All systems include purity monitoring, flow control, and backup nitrogen storage for uninterrupted operation.