Nitriding Processes
Nitriding is a thermochemical case hardening process used to increase surface hardness, wear resistance, and fatigue life. Here we explore the two main types: Plasma and Gas Nitriding.
Nitriding Process Calculator
Estimate the approximate case depth based on material, temperature, and time. This is an estimation; actual results depend on precise material chemistry and process parameters.
Select parameters and calculate to see results.
Plasma (Ion) Nitriding
Ionized Gas (Plasma)
The process is conducted under a vacuum. A nitrogen-containing gas mixture is introduced, and a high voltage is applied between the workpiece (cathode) and the furnace wall (anode), creating a glow discharge plasma.
Ion Bombardment & Sputtering
Nitrogen ions from the plasma bombard the part's surface. This does two things: it cleans the surface by sputtering off contaminants and provides localized heating.
Nitrogen Diffusion
The bombardment and heating create an active surface that readily absorbs nitrogen, which then diffuses into the material to form a hard, wear-resistant nitride layer (the 'case').
- Lower processing temperatures (400-580°C) minimize distortion compared to other case hardening methods.
- Excellent control over the case structure, including the formation of a 'compound layer' and diffusion zone.
- Can be used on a wide variety of steels, including stainless steels without depassivation.
- Environmentally friendly process with no toxic byproducts like cyanide salts.
- Improved wear resistance, fatigue strength, and corrosion resistance.
Gas Nitriding
Ammonia Atmosphere
Parts are heated in a sealed furnace filled with ammonia gas (NH₃).
Thermal Dissociation
At the process temperature (typically 500-550°C), the ammonia breaks down on the steel surface into nascent (atomic) nitrogen and hydrogen.
Nitrogen Diffusion
The highly reactive atomic nitrogen is absorbed by the steel and diffuses into the surface to form the hard nitride layer.
Key Process Parameters
- Temperature: Typically 500-550°C.
- Time: Long cycles, often 24-96 hours, depending on required case depth.
- Ammonia Dissociation: The percentage of NH₃ that breaks down is monitored to control nitriding potential.
Advantages
- Good for large batches of small to medium-sized parts.
- Uses conventional furnace equipment (no vacuum or high voltage).
- A well-understood and widely available process.
Ferritic Nitrocarburizing (FNC)
Sub-critical Temperature
The process is performed below the lower critical temperature (around 570°C), meaning the base material remains in its ferritic state.
Gas or Salt Bath Medium
Can be performed in a controlled gas atmosphere (with ammonia and a carbon-source gas) or in a molten salt bath containing nitrogen and carbon compounds.
Compound Layer Formation
Forms a thin, dense, and highly lubricious compound layer (epsilon iron nitride) on the surface, which is extremely wear-resistant.
- Excellent scuffing, wear, and corrosion resistance.
- Very low distortion due to sub-critical process temperature.
- Applicable to a wide range of carbon and low-alloy steels.
- Relatively short process time compared to conventional nitriding.
Compound Layer (White Layer)
The outermost layer, consisting of iron nitrides (ε-Fe₂₋₃N and γ'-Fe₄N). It is very hard and provides excellent wear and corrosion resistance. Its thickness and phase can be controlled by process parameters.
Diffusion Zone
Beneath the compound layer, this zone contains dissolved nitrogen and fine nitride precipitates within the original steel matrix. It provides a gradual hardness transition to the core, which significantly improves fatigue strength.
Gas Composition
The mixture of nitrogen (N₂) and hydrogen (H₂) is critical. Hydrogen helps to clean the surface through sputtering and controls the nitriding potential. Methane (CH₄) can be added for plasma nitrocarburizing.
Temperature
Typically 400-580°C. Temperature controls the diffusion rate of nitrogen and influences the final case depth and hardness. Higher temperatures lead to deeper cases but may result in lower hardness.
Time
Process times can range from a few hours to 100+ hours. Time is the primary factor determining the case depth; the relationship is parabolic (depth ∝ √time).
Pressure
The vacuum level (typically 0.5-10 mbar) affects the plasma's characteristics and uniformity. It must be carefully controlled to ensure stable glow discharge.
Nitriding Steels
e.g., Nitralloy 135, 41CrAlMo7. These are specifically designed for nitriding, containing strong nitride-formers like Aluminum, Chromium, and Molybdenum for very high surface hardness.
Tool Steels
e.g., H13, D2, M2. Hot-work, cold-work, and high-speed steels benefit greatly, gaining surface hardness for wear resistance while maintaining core toughness.
Stainless Steels
e.g., 410, 17-4 PH, 316. Plasma nitriding can harden the surface without significantly compromising corrosion resistance, a major advantage over other methods.
Low Alloy Steels
e.g., 4140, 4340. Steels with Cr, Mo, and V show a good nitriding response, achieving significant increases in surface hardness and fatigue life.
Gears and Pinions
Crankshafts and Camshafts
Plastic Injection Molds and Extrusion Dies
Valves, Spindles, and Hydraulic Components
Forging Dies and Stamping Tools
- Vacuum Chamber:A robust, sealed vessel that can be pumped down to a vacuum.
- Power Supply:A high-voltage DC power supply to create and sustain the plasma.
- Gas Control System:Mass flow controllers for precise mixing of nitrogen, hydrogen, and other process gases.
- Temperature Control:Uses thermocouples and the plasma's heating effect to maintain a uniform temperature.
- Workpiece Fixturing:Parts are electrically isolated and act as the cathode in the electrical circuit.
