Liquid Salt Bath Nitriding

Nitriding is a popular case hardening technique renowned for the qualities it delivers at relatively low process temperatures. Salt bath nitriding - also known as ferritic nitrocarburizing FNC)- is one of the most popular ways to achieve these results.

It remains a popular case hardening technique because it offers:

• Increased hardness and wear resistance at part surfaces while cores remain softer and more ductile
• Increased corrosion resistance
• Significantly reduced risk of distortion due to a comparatively lower treatment temperature leading to reduced risk of distortion during treatment.

Salt bath nitriding is a thermochemical process in which nitrogen and carbon are diffused simultaneously into the surface of the material. The high concentration of nitrogen chemically combines with iron and other nitride forming elements to produce an outer layer of epsilon iron nitride (Fe₃N) which is thin, hard and ductile.

Before nitriding, the components have to be thoroughly cleaned and degreased. Any surface contamination from grinding particles, oil or metal chips will result in an uneven formation of the nitrided layer. This can cause cracks in the coating which leads to flaking and corrosion. After cleaning, the parts are dried and preheated and then transferred to the actual nitriding environment. The various nitriding processes can be differentiated mainly by their nitrogen source and the energy supply. Salt bath-, gas- and plasma nitriding have different advantages regarding investment cost, process time, environment, safety and quality. The properties of the resulting nitrided or nitrocarburised surface are in many cases independent of the production process. The required case depth is determined by the application of the nitrided component and can be regulated through the nitriding temperature and time.

The competitiveness of titanium alloys is due to their high strength to weight ratio, heat and corrosion resistance. At the same time, the low surface hardness and wear resistance along with poor high-temperature oxidation resistance are seen as their major disadvantages. Nitriding is one of many treatments aimed at improving their tribological characteristics.

In principle, all major nitriding techniques are applicable to titanium. A disadvantage of gas nitriding is the high temperature of 650-1000°C required, long time of up to 100 h and reported fatigue life reduction. For Ti-6Al-4V alloy a typical compound layer of 2-15 µm forms with a surface hardness between 500 and 1800 HV. The plasma nitriding of titanium alloys is conducted at temperatures of 400-950°C and substantially shorter time from 0.5 to 32 h, generating a compound layer with a thickness of approximately 50 μm. A reduction in fatigue strength may be eliminated by lowering the nitriding temperature. The ion beam nitriding, using nitrogen at temperatures of 500-900°C for up to 20 h, produces 5-8 μm thick compound layer with microhardness of 800-1200 HV on Ti-6Al-4V alloy. Also laser nitriding is applicable to titanium but surface case has a tendency to cracking. An attempt was made to apply the diode laser gas nitriding technique to Ti6Al4V alloy, commonly used for rotors and blades of engines in power generation. The laser surface melting of the substrate surface in a mixture of nitrogen and argon leads to an increase in surface hardness up to 1300 HV0.2 although the outcome depends on process parameters.

The formation of the nitrided layer on titanium involves several reactions taking place at the gas/metal interface and within the metal. At the nitriding temperature, below the Ti polymorphic transformation, the α-Ti phase exists. First, the nitrogen absorbed at the surface diffuses inward titanium, forming the interstitial solution of nitrogen in the hcp titanium phase α-Ti(N) and building the nitrogen concentration gradient. After exceeding the solubility limit, the Ti₂N phase is formed. During further increase in the nitrogen concentration at the gas/metal interface, TiN is formed as specified below:

α - Ti → α – TiN → Ti₂N → TiN

After slow cooling, the precipitation in the diffusion zone is possible. The simplified morphological schematic, emphasizing the growth sequence, is shown in Figure 1.

Figure 1: Schematics of the morphology development during nitriding of titanium