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The nitriding may be accomplished by either a single-stage or double-stage process. 

In the single-range process, the brittle gamma iron nitride at the temperature in the 

range of about 500 to 525

 o

C is produced. The dissociation rate of ammonia ranges 

from 15 to 30%. The double-stage process is known as the Floe process [23] by using 

a two-stages process. The first stage of the double-stage process is a duplication of the 

single-stage process. The principal purpose of the second stage is to minimize the 

depth of the white layer. The dissociation rate of ammonia is increased to 65-85% 

with the temperature ranges from 550 to 565

  o

C. Since the NH

3

 content of the 

atmosphere is reduced, the iron nitride does not grow as rapidly and in fact dissolves 

as it supplies nitrogen into the interior of the steel. Nitriding times are quite long, 

anywhere from 10 to 130h depending on the application, and the case depths are 

relatively shallow. [23] 

 

2.4 Effects of contaminants on the heat treating process 

Heat treatment processes are facing increasing specifications with reference to process 

quality, safety and results in terms of reproducibility and repeatability. It is well 

known that the surface contamination may hinder surface modification processes, so 

they can be met only if the surface condition is controlled during manufacturing and, 

especially, prior to the heat treatment. Thus, cleaning to provide a residue free surface 

before the gas nitriding is important. It has been shown that surfactant cleaners which 

consist of anionic or non-ionic surfactants tend to decrease the rate of nitrogen 

acceptance. [34] Alternatively, hydrocarbon cleaners can be used which do not form 

non-volatile adsorption layers and are ideal for removing water insoluble oils and fats. 

[35]

 

Not all types of surface contaminants interfere with the nitriding or 

nitrocarburizing process. Generally, the more volatile components are desorbed or 

vaporised, whereas others can react with the metal to form stable surface films 

preventing diffusion. If the nitriding process does not lead to satisfactory results in 

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terms of surface hardness, nitrogen depth profile, diffusion or compound layer 

thickness, etc., the overall reaction rate was too slow, or the reactivity of the surface 

was low. This surface condition is termed as "passive". [36]   

 

Residual coolants and lubricants from cold or hot work, cutting or machining 

operations can act as passive layers and prevent or hinder a thermochemical diffusion 

process. If the contaminants are nonvolatile and do not vaporize completely before the 

nitriding temperature in the furnace, there will be a passive layer formed. Residues 

from cleaning must be removed completely from the steel surfaces, so that the rinsing 

stage with rinsing water composition surveillance and control is an important part of a 

cleaning installation, especially if aqueous cleaning agents are used. 

 

Not all contaminants are equally effective, which can be seen from Figure 5. The pure 

paraffin mineral oil does not hinder the formation of the compound layer. In contrast 

to this, a commercial cutting oil leads to reduced surface hardness and a thinner, 

non-uniform compound layer, even if its composition is about 95% mineral base oil.

 

[36] 

 

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Figure 5 Nitriding results of 42CrMo4 specimens with reference to surface hardness 

and white layer mophology. [36] 

 

If the cleaning agent residues are not removed by diligent rinsing, they can lead to 

passive layers. They are adsorbed on the steel surface in contact with the cleaning 

solution as shown in Figure 6. Their negative effect of surface hardness after nitriding 

depends on their volatility; inorganic salts like silicates and phosphates have high 

melting points and do not vaporize at nitriding temperature. However, if a cleaning 

installation provides rinsing stages, they can be removed completely. 

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Figure 6 Adsorption of cleaning agent components on a steel surface after rinsing in 

solutions of different concentrations (FAE=fatty alcohol ethoxylate) [36] 

 

During cold or hot work, as well as during machining, the cooling lubricants, 

hydraulic oils, and machine grease may form stable reaction layer at the elevated 

temperatures and pressures which occur during the manufacturing process. Reaction 

layer can work as barrier layers and inhibit surface modification reactions. Reaction 

film from lubricant additives can affect the white layer formation and surface 

hardness, which makes white layer too thin and non-uniform.    However, the different 

additives react differently. Whereas a fatty ester appeared to promote the formation of 

the hard layer, additives containing sulfur of phosphorous reduced the achievable 

surface hardness. The most defective result achieved using commercial cutting oil. It 

should be stressed that the effect is enhanced by vigorous cutting conditions which 

lead to a great amount of cold work. 

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Figure 7 Surface hardness of soiled C15 specimens after nitriding (DBS = 

dodecylbenzene sulfonate)[36]

 

 

Even cleaning agent residues can lead to passive surface layers if not removed by 

diligent rinsing. They are adsorbed on the steel surface in contact with the cleaning 

solution as can be concluded from Figure 7. Their negative effect of surface hardness 

after nitriding depends on their volatility; inorganic salts like silicates and phosphates 

have high melting points and do not vaporize at nitriding temperature. Furthermore, 

they form glassy films which can completely prevent diffusion, or nitrogen uptake. 

However, if a cleaning installation provides rinsing stages, they can be removed 

completely. 

 

 

 

 

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