Gas nitriding

For industrial use Nitriding processes are applied typically in the temperature range from 500 – 530°C. For the process chemically active, hence atomic, nitrogen is needed. Two processes have here procedurally established themselves, namely gas nitriding and plasma nitriding. The key difference is the production of the atomic nitrogen. In gas nitriding ammonia (NH3) serves as the nitrogen donor. During the process ammonia splits into its basic elements nitrogen (N) and hydrogen (H). The resulting atomic nitrogen has a high affinity to iron and forms iron nitride. Due to the concentration gradient it diffuses into the near surface areas of the component and subsequently forms the so-called diffusion zone also called the nitrided case (Nht).

Plasma nitriding on the other hand functions with the gases nitrogen (N2) and hydrogen (H2). These gases are fed in the millibar range into a vacuum retort. The wall of the retort is switched as an anode and the components are switched as a cathode. By applying voltage the gases are ionized. The positively charged atoms of the gas molecule are accelerated into the direction of the negatively charged components. Through the hereby arising high kinetic energy the molecules are split at the impact onto the component surface and atomic nitrogen is available. Simultaneously iron atoms are sputtered from the components and nitrogenized in the area of the so-called transition zone. Due to the positive charge they are drawn back to the component. The nitriding process then takes place in a similar way as described above.

The diffusion zone (nitrided case) consists of two layers. On the surface at first an iron nitride layer forms itself as the so-called compound layer (CL). One can distinguish between 2 different modifications. In the controlled gas nitriding and plasma nitriding the more nitrogen poor but relatively ductile Fe4N, also termed γ‘-nitride, is formed. It has ceramic character, reduces wear and has moderate anticorrosive qualities. The more nitrogen rich, harder and better corrosion resistant Fe₂₋₃N-VS, also termed ε nitride, is produced at nitrocarburization.

Link layers are barely etched in a metallographically primed cross-section polish and are therefore visible as a white layer. Hence the applied term “white layer“ in the English language for the layers which are referred to as compound layer compliant to the standard.

Beneath the CL the nitrogen forms special nitrides with the nitride forming elements (the most important are Al, Cr, Mo and V). Furthermore N stores itself in the interstitial gaps. This area of the diffusion zone is also called the precipitation layer

This combination of nitride precipitation and simultaneous distortion of atom lattice leads to considerable residual compressive stress and therefore increases the hardness in the nitrided cases. Induced by diffusion the nitrogen content decreases with increasing penetration depth and with it the residual compressive stress decreases. This results in the typical hardness profile of a nitrided case with a high surface hardness and constant sloping hardness in the deeper regions.

The gas nitriding can principally be applied to all non-alloy and low-alloy steels with Cr-contents of cast iron of up to 12%. Nitrocarburization should preferably be applied for non-alloy steels, lamellar and ferritic cast iron. Due to missing nitride forming elements no considerable residual compressive stress can be built up in the precipitation layer of these qualities. Furthermore it should be noted that the ferrite tends to brittleness during nitridation.

To be able to produce optimal nitrided cases it is necessary to use fine grained structures, hardened and tempered as far as possible. Lack of distortion can be guaranteed with stress relieve annealing. This annealing should be scheduled after the rough machining and the finishing to optimize the residual stress condition of the components.