Our discussion continues.
The case is always a similar formation for gas nitriding. An illustration is shown in Figure 1. The case will always form with the surface compound layer, and below that will be the diffusion layer, the transition zone and then into the core of the material.
The compound layer is always a dual-phase surface metallurgy (also known as the white layer), as shown in Figure 1 (but not in the proportions shown). Figure 2 shows the formed gas-nitrided metallurgy illustrating the white layer.
The general process temperature is usually 935°F (500°C) up to 1000°F (537°C), using a gas dissociation (decomposition) of 30% to produce results similar to what is shown. Note the white layer, then the diffusion zone and then into the transition zone and finally the core. The amount of gas dissociation that will occur will be dependent on the gas flow into the process vessel.
The gas dissociation is measured by the use of water in a reservoir and a graduated flask that is purged with the exhaust process gas. Water will absorb 70 times its own volume of ammonia in solution. Based on this fact, the graduated vessel is purged with the exhaust gas. The reservoir valve is then opened, and the water from the reservoir will flow into the graduated flask. The water will absorb the spent ammonia gas with the exception of the undissolved nitrogen and undissolved ammonia. Thus, the process is measuring volumetrically the undissolved nitrogen and hydrogen, which is the amount of dissociated gas. It is generally run at 70% water and 30% dissociated gas, measured volumetrically.
The compound layer will always form, and it will generally be in a ratio of 10% of the total case depth. It will also be a dual-phase layer comprising 50% gamma prime and 50% epsilon phase. The epsilon is formed as a result of the carbon in solution in the steel or an added hydrocarbon gas such as methane or carbon monoxide. The epsilon nitride is extremely hard and brittle. The gamma prime is softer but more flexible.
There are three methods of reducing the compound layer: grind it off, use the two-stage process to reduce the thickness or use what is commonly known as the dilution method.
The reduction of the compound layer will be determined by: part application, steel chemistry, process temperature, time at process temperature, process gas composition, and pre-cleaning method and effectiveness of the cleaning.
There is no “magic” temperature number that can be every time and all the time. It will be dependent on the above process conditions.
The carbon content of the steel will also affect the immediate surface composition of the compound layer. Lower-carbon steels will tend to form the gamma-prime phase of the compound layer. Depending on the steel chemistry, higher-carbon steels will tend to form the epsilon phase, which is extremely hard.
So, a study of the component and its designed load conditions should be made in relation to the available steel chemistries from the steel merchant or the mill.
From this acquired information, one can then begin to determine: process equipment selection, process temperature, process time at temperature, case depth required and surface metallurgy, core hardness, surface hardness and pre-heat treatment procedures.
This concludes our discussion on gas nitriding.