Inert Gas Atomization Basics for Powder Tool Steels
Inert gas atomization of tool steels requires precise superheat control, supersonic argon nozzles, and closed-loop gas recycling to yield dense spherical powders.
Nitrogen Pick Up
Meaning
Metallurgical phenomenon involving the absorption of nitrogen gas into a molten metal or a solid surface during melting, atomization, or heat treatment. Controlling nitrogen pick up is necessary for maintaining the mechanical and chemical properties of sensitive alloys like stainless steel and titanium. The process occurs when the metal is exposed to air or contaminated process gasses at high temperatures, allowing nitrogen atoms to dissolve into the crystal lattice.
This can lead to the formation of nitrides which increase hardness but reduce the ductility and corrosion resistance of the material.
Absorption Mechanism
Solubility of the gas in the liquid phase depends on the temperature and the chemical composition of the alloy. In the case of nitrogen pick up, the rate of absorption increases when the metal surface is not protected by an inert atmosphere or a slag layer. High-energy processes like plasma melting or electric arc furnace operations can ionize the nitrogen in the air, making it even more reactive with the molten bath.
Certain alloying elements like chromium or manganese can increase the capacity of the metal to hold dissolved nitrogen. This makes the management of the atmosphere particularly challenging during the production of specialized battery components or aerospace parts.
Material Degradation
Presence of excessive nitrogen in the metal structure often leads to the loss of toughness and an increased risk of brittle fracture. When nitrogen pick up occurs, the resulting nitrides can act as initiation points for cracks under mechanical load. This is a concern for parts that operate in high-stress or low-temperature environments where the ability to absorb energy is a safety requirement.
The corrosion resistance of stainless steels is also compromised because the nitrogen can deplete the chromium at the grain boundaries. This localized depletion makes the material susceptible to intergranular attack and stress corrosion cracking.
Prevention Strategy
Maintenance of a high-purity inert gas environment is the primary defense against the contamination of the metal during processing. To minimize nitrogen pick up, producers use vacuum induction melting or high-pressure argon atomization to isolate the molten stream. The integrity of the seals and the quality of the gas recovery system are checked to prevent any air ingress.
If nitrogen is used as a cheap alternative to argon for certain stages, it must be carefully monitored to ensure it does not come into contact with the reactive stages of the melt. Refining the melting parameters and using specialized flux can also help to remove dissolved nitrogen from the bath before it solidifies. This control is fundamental for producing materials with predictable performance.