David Pye's series on surface treatments continues.

Nitriding

Nitriding is a thermal-chemical diffusion treatment that forms stable nitrides within the surface of a component. The purpose of the procedure is to generally transform the steel by diffusing nitrogen into the surface. This is accomplished with a carburizing/ferritic nitrocarburizing process at low thermal-processing temperatures, which are below the A1 transition line of the iron-carbon equilibrium diagram (Fig. 4).

The nitriding procedure generally takes place at processing temperatures that are below the A1 line from 840-1130°F and, generally, without a quench procedure. Nitrogen is diffused into the steel surface. It is first dissolved into the iron matrix. If the nitrogen concentration exceeds the solubility limit of 2.5% by weight, a single or multiphase nitride layer is formed.

If only molecular nitrogen diffuses into the surface of the steel, the process is referred to as nitriding. If at the same time carbon diffuses into the surface of the steel as a result of the addition of a carbon source into the nitriding process gas, however, the process is now known as nitrocarburizing and generally known as ferritic nitrocarburizing.

Derivative Nitriding Processes

The derivative nitriding processes are generally applied to low-alloy steels – ferritic nitrocarburizing and a combination treatment of nitriding followed by controlled surface oxidation. Figure 6 is a metallurgical processing method of improving both surface hardness and corrosion resistance.

Because of the addition of carbon into the process-gas stream, the epsilon compound layer is formed quite rapidly. With the addition of an oxygen source at the end of the nitrocarburizing treatment, a homogenous layer of iron oxide will be formed that will assist in corrosion resistance.

When using the plasma-assisted process technique, the pre-cleaning (commonly known as sputter cleaning or simply explained as surface cleaning by ionic bombardment) could be likened to atomic shot blasting.

Thin-Film Hard Coating of Cutters and Engineering Tooling

This procedure can work extremely well combined with the nitriding procedure.

• Pre-harden and temper of the cutter to produce a cross-sectional hardness traverse of approximately 64 HRC (800 HV) up to 66 HRC (830 HV)
• Nitride (by diffusion) to 68 HRC (880-940 HV)
• Thin-film hard coat (deposition treatment and not diffusion) to a surface-hardness value of approximately 1,800-2,100 HV