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 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.
Advantages of ion nitriding over ammonia nitriding are as follows:
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:
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