Liquid Salt Bath Nitriding


Also known as ferritic nitrocarburizing (FNC), liquid salt bath nitriding is a commonly used case hardening technique which provides advantages such as increased hardness and corrosion resistance whilst maintaining core ductility.
As a thermochemical reaction the end result of the nitriding process is a thin outer layer of epsilon iron nitride (Fe3N) formed by a high concentration of nitrogen combining with iron on the surface of the material.

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


1. The salt bath nitriding process and its safer alternative, Paulo, Accessed FEB 2019;

2. Salth Bath Nitriding, Accessed FEB 2019;

3. Metallographic preparation of nitride and nitrocarburised components, Struers, Accessed FEB 2019;

4. Frank Czerwinski: Thermochemical Treatment of Metals, INTECH 2012.

Caută în baza de date

Introduceţi frază de căutare pentru:

Caută după

Text complet
Cuvinte cheie


The Total Materia database contains many thousands of titanium materials across a large range of countries and standards.

Where available, full property information can be viewed for materials including chemical composition, mechanical properties, physical properties, advanced property data and much more.

Using the Advanced Search page, define the search criteria by selecting ‘Titanium’ in the Group of Materials pop-up list. It maybe that you need to further narrow the search criteria by using the other fields in the Advanced Search page e.g. Country/Standard.

Then click Submit.

solution img

A list of materials will then be generated for you to choose from.

solution img

After clicking a material from the resulting list, a list of subgroups derived from standard specifications appears.

From here it is possible to view specific property data for the selected material and also to view similar and equivalent materials in our powerful cross reference tables.

solution img

Click on the property data link of interest to you to view specific property data.

solution img

For you’re a chance to take a test drive of the Total Materia database, we invite you to join a community of over 150,000 registered users through the Total Materia Free Demo.