Vacuum/Surface Treatments

Using Nitrogen Availability as a Nitriding Process Parameter

September 12, 2012
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In gas nitriding processes, different types of input atmospheres are used such as a single-component atmosphere composed of ammonia (NH3) and two-component atmospheres, which are diluted by pre-dissociated ammonia (NH3/NH3diss) or by molecular nitrogen (NH3/N2).





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In order to control the nitriding process and the growth kinetics of the nitrided layer, the nitriding potential (KN) and/or the ammonia dissociation rate (α) is used.[1] Growth kinetics of the nitrided layer are decided by the correlation between the flux of nitrogen derived from the gas phase and the flux of nitrogen that diffuses into the steel core.

The first of these fluxes is described by the laws of transportation of mass in the gas phase, while the second is described by the laws of diffusion. During the nitriding process at a constant temperature, it is possible to directly control only the flux of nitrogen from the gas phase to the nitrided surface.[2] A measure of this flux is the nitrogen availability of the nitriding atmosphere (mN2), which ties the ammonia dissociation rate (α) with the flow rate of the input atmosphere (FIn).[3]

Reaction (1; left) describes nitriding conditions in which 1 mole of ammonia generates the formation of 0.5 mole of nitrogen and 1.5 moles of hydrogen.                      

Knowing the volume of the dissociating ammonia, the mass of the mole of nitrogen (28.016 g) and its volume (22.414 dm3), it is possible to calculate the mass of nitrogen obtained for reaction (1).             

where FIn is the flow rate of the input atmosphere in liters/minute and a is the rate of dissociation of ammonia in the nitriding atmosphere.

The article discusses two methods of changing the nitrogen available to the nitriding atmosphere. The first (process 1) is by changing the value of the nitriding potential in a two-component input atmosphere comprised of ammonia and pre-dissociated ammonia (NH3/NH3diss). The second (process 2) is by changing the content of nitrogen in a two-component input atmosphere comprised of ammonia and nitrogen (NH3/N2) while maintaining a constant value of the nitriding potential.



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Experimental

Nitriding processes were carried out on the X37CrMoV5-1 steel grade. The composition of the steel used in the investigations and the conditions of heat treatment are given in Table 1.

Nitrided layers were generated by the NITREG controlled-gas nitriding method, employing the Nx609 furnace supplied by Nitrex. Process parameters are given in Table 2. Upon completion of the nitriding process, the thickness of the compound layer was measured on metallographic mounts as the total case depth. Investigations of phase composition of the nitrided layers were carried out by the X-ray method, employing CoKα radiation.

Figure 1 shows variations of the nitriding potential in processes 1 and 2 (Fig. 1a) and corresponding variations of nitrogen availability of the nitriding atmosphere (Fig. 1b) as a function of time.



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Fig. 1. Variations of value of the nitriding potential (a) and corresponding variations of nitrogen availability of the nitriding atmosphere (b) for processes carried out in a two-component input atmosphere NH3/NH3diss (process 1) and a two-component input atmosphere containing nitrogen NH3/N2   (process 2). KNε/γ' is the value of nitriding potential at the ε/γ' interphase boundary.

 

Processes were carried out in two stages, with a lowering of the nitrogen availability during the second stage. The first stage of both processes was carried out in a nitriding atmosphere obtained from a single-component input atmosphere of ammonia only, achieving values of the nitriding potential that were contained within the range of the ε phase in accordance with the Lehrer diagram.[5] 

During the second stage of process 1, a two-component input atmosphere was utilized, which was comprised of ammonia and pre-dissociated ammonia in the ratio 30%NH3/70%NH3diss. This caused a lowering of the value of the nitriding potential to the γ' phase range (Fig. 1a, process 1). This lowering of the nitriding potential was accompanied by a lowering of nitrogen availability of the nitriding atmosphere (Fig. 1b, process 1).

During the second stage of process 2, a two-component input atmosphere was utilized, which was comprised of ammonia with nitrogen in the ratio 20%NH3/80%N2. This also caused a decrease of nitrogen availability in the nitriding atmosphere (Fig. 1b, process 2). But contrary to process 1, the decrease of nitrogen availability was not accompanied by a decrease of the value of the nitriding potential (Fig. 1a, process 2).



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Fig. 2. Variations of thickness of superficial compound layer vs. process time for processes 1 and 2 obtained on X37CrMoV5-1 steel

Results of Investigations and Discussion

Figure 2 shows the variation of thickness of the compound layer versus time obtained on X37CrMoV5-1 steel in processes 1 and 2, while Figure 3 shows diffraction patterns of the nitrided steels.

Because the values of nitrogen availability during the first stage of the process differed insignificantly (Fig. 1b), the same thickness of the superficial compound layer was obtained in both processes after the first stage. The constant nitriding potential of the first stage of both processes (Fig. 1a) resulted in the formation of a biphase (ε + γ' ) compound layer with a significant dominance of the ε phase (Fig. 3).

Lowering the nitriding potential (Fig. 1a) during the second stage of process 1 from 5.8 (range of stability of the ε phase) down to 0.75 (range of stability of the γ' phase) caused a gradual lowering of the surface concentration of nitrogen from CN  = 9.1% to CN  = 5.8%. This resulted in a gradual drop of the ε phase content and growth of the content of the γ' phase in the superficial compound layer. Under conditions of the second stage of process 1, the γ' phase grew as the result of both gradual breakdown of the e phase into the γ' phase as well as the formation of the γ' phase at the surface.[6] The ε phase dropped from 66% to 35% (Fig. 3a) during the second stage of process 1.

The growth kinetics of the superficial compound layer slowed during the second stage of process 1. Lowering the nitrogen availability of the nitriding atmosphere during the second stage of process 2 (Fig. 1b) caused a delay of the growth kinetics of the compound layer similar to the outcome of process 1 (Fig. 2). Unlike process 1, lowering of nitrogen availability in process 2 was not accompanied by a drop in the nitriding potential (Fig. 1a). Therefore, the phase composition of the superficial compound layer was unchanged (Fig. 3b). 

Figure 4 shows the variation of total case depth obtained in both processes on X37CrMoV5-1 steel. As can be seen in Figure 4, the growth kinetics of the diffusion case during the first stage of both processes is the same because the development of the diffusion case during the first stage of both processes proceeded in the presence of a biphase (ε + γ') compound layer (Fig. 3).

During the second stage of each process, very distinct differences occur in the growth kinetics of the diffusion case. In process 1, the growth kinetics of the diffusion case are lower than that of the diffusion case in process 2 (Fig. 4). The formation of the nitrided case consists of three main stages:  

  • Formation of a solution case and saturation of the steel matrix to the point of attaining a state of quasi-equilibrium of nitrogen concentrations to a value corresponding to the nitriding potential according to the Lehrer diagram
  • Nucleation of nitride phases
  • Growth of nitride compound layer


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Fig. 3. Comparison of diffraction patterns of nitrided X37CrMoV5-1 grade steel after 4 hours and 10 hours in process 1 (a) and process 2 (b)

 

The growth of the diffusion (solution) case continues during the second and third stage of formation.

During the first and second stage (until the moment of formation of a continuous compound layer), the growth kinetics of the diffusion case depend on the value of the nitriding potential. During the third stage, the growth kinetics of the diffusion case does not depend on the value of the potential but rather on phase and chemical composition of the compound layer.

During the second stage of process 1 and 2, the growth of the diffusion case continues to proceed in the presence of a biphase (ε + γ') compound layer. But because the nitriding potential decreases during the second stage of process 1, the ε phase gradually transforms into the γ' phase. The growth of the γ' phase, which has greater homogeneity in comparison with that of the ε phase, constitutes an effective obstacle for the nitrogen flux in the direction of the diffusion case.

A relatively small limitation of the growth kinetics of the diffusion case may stem from the failure of the ε phase to transform to the γ' phase (Fig. 3a). During the second stage of process 2, the growth of the diffusion case continued to proceed in the presence of a biphase (ε + γ' ) compound layer, but there was no change to the chemical and phase composition (Fig. 3b). For this reason, the growth kinetics of the diffusion case are the same as during the first stage of the process.



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Fig. 4. Variation of total case depth vs. process time for processes 1 and 2 obtained on X37CrMoV5-1 grade steel

Summary

For the single-component input atmosphere comprised of ammonia only as well as the two-component atmosphere containing pre-dissociated ammonia (NH3/NH3diss), the nitriding potential is a sufficient parameter to describe the process in full. The nitriding potential of the nitriding atmospheres unequivocally describes the nitrogen availability. A change of nitrogen availability requires a change of the value of the nitriding potential.

For atmospheres derived from a two-component input atmosphere containing molecular nitrogen (NH3/N2), it is possible to change the nitrogen availability by changing the composition of the input atmosphere without simultaneously changing the nitriding potential. This is a characteristic of these atmospheres. The utilization of nitrogen availability as a process parameter creates the possibility of a broader use of atmospheres diluted by nitrogen. IH  

Project funded by research funds for 2009-2012.  

For more information: Contact Jerzy Michalski Dr Eng, Dr-habil, Institute of Precision Mechanics, Duchnicka 3 Street, 01-796 Warsaw, Poland; tel: (48) (22) 5602940; e-mail: michalski@imp.edu.pl; web: www.imp.edu.pl  

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