Ion nitriding

Dr. Dmitri Kopeliovich

Ion nitriding (plasma nitriding) is a surface Hardening heat treatment, in which Nitrogen is delivered to the workpiece surface in form of ionized gas (plasma).

Ion nitriding produces high surface hardness, good wear resistance, increased fatigue strength and toughness.

• Ion nitriding process
• Advantages of ion nitriding
• Applications of ion nitriding

Ion nitriding is performed in a vacuum chamber at a pressure 1-10 torr.

A DC voltage 100-700 V is applied between the workpiece and the chamber wall. The workpiece is connected to the negative terminal (cathode). The chamber wall is the anode (positive terminal). The wall is usually grounded.

The air is first evacuated from the chamber to 0.1 torr, which is then backfilled with a mixture of nitrogen, a Hydrogen containing gas (eg., methane) and an inert gas. The gas mixture is continuously supplied to the chamber, pressure of which (1-10 torr) is controlled by the gas flow rate and the vacuum system.

Electrostatic field between the cathode and anode ionizes the gas forming a glow discharge plasma at the workpiece surface.
Positively charged ions of hydrogen and nitrogen are attracted by the negatively charged workpiece. They are accelerated by the electric field.
The nitrogen ions move towards the cathode and bombard its surface where they dissolve and chemically react with the steel components (iron, chromium, aluminum, vanadium, molybdenum).

Most of nitrides formed in the surface layer (case) are iron nitrides Fe₂N, Fe₃N, Fe₄N.

Hydrogen contained in the gas mixture is required for cleaning the metal surface from the oxides. Oxide-free and activated surface easily reacts with the nitrogen ions.

The steel workpiece is heated by the glow discharge plasma to a temperature 700-1200°F (370-650°C). Titanium alloys are treated at higher temperatures (up to 1600°F/870°C).

The temperature determines the speed at which the depth of the white layer (case depth) increases.
Typically the case depth is within the range 0.004-0.025” (0.1-0.6 mm).
The treatment time is commonly 10-50 hrs.

to top

Advantages of ion nitriding over ammonia nitriding are as follows:

• Shorter (by 20-50%) treatment cycle.
• Better process control and automation. The process parameters (pressure, voltage, temperature, gas flow, DC current) are easily controlled.
• Higher surface hardness may be achieved due to lower process temperature (up to 1200 HV).
• Better dimensional stability (lower distortions) due to lower process temperature and uniform heating.
• Cases with uniform depth are formed even over parts with complex shapes.
• Easier masking for selective nitriding.
• Lower energy consumption due to lower temperature and shorter treatment cycle.
• Reduced gas consumption.
• Safer operation.
• Lower environment pollution.

to top

Ion nitriding is used for Case hardening of Alloy steels, Stainless steels, Titanium alloys.

Ion nitriding is effectively used when high surface hardness, good wear resistance, increased fatigue strength and toughness are required.

The following parts are processed by ion nitriding:

• Cutting tools
• Forging dies
• Drawing dies
• Molds for Transfer molding and Compression molding of polymers
• Machine and automotive parts subject to wear under friction (gear wheels, valves, lifters, cams, rocker arms, crankshafts)

to top

• Annealing and stress relief
• Normalizing
• Hardening
• Case hardening
• Tempering
• Precipitation hardening
• Cryogenic treatment of steel
• Salt bath heat treatment
• Furnaces for heat treatment
• Basic principles of heat treatment
• Iron-carbon phase diagram

to Metals
to Heat treatment technologies