Nitrocarburizing and ammonia quality
Nitrocarburizing of steels belongs to a group of progressive technologies that enable surface improvement of low-alloy steels. The advantage of this process is that it is a single-operation process that takes place at relatively low temperatures up to 600 °C. The disadvantage is the need to use ammonia.
When we once started with these processes with low-pressure nitriding from Fours BMI (LPN), we started with NH3 quality, purity 2.8. That was a mistake. It is essentially ammonia for cooling systems according to DIN 8960, with a water content of up to 400 ppm.
Is it a little or a lot? 1 ppm is 1 millionth of the content. If we have an atmospheric pressure of 1 bar = 105 Pa, then 1 ppm = 0.1 Pa. 400 ppm will therefore be 40 Pa of the partial pressure of water in the system. But to nitride or nitrocarburize, we only need 4-7 Pa of partial pressure of nitrogen where aN ~ pN2 holds. The partial pressure of H2O will therefore be higher than the partial pressure of nitrogen pN2. Since LPN takes place at a total pressure of 300-400 mbar NH3, we will have around 160 ppm of water in the process, i.e. 12 to 16 Pa. That is still too much.
And what was the problem? It simply stopped nitriding. If we look at the equation for the nitridation number Kn, then we have the formula
As the hydrogen content increases, the nitriding number decreases. If we do not want to nitride, Kn must be close to zero. If we want to nitride to a solid solution of nitrogen in the alpha iron matrix α-Fe(Nx), then it must be < 1. If we want to nitride ε-Fe(2-3)Nx, then Kn must be > 2.
Fig. 1 – Dependence of the nitriding number Kn on the degree of ammonia dissociation
Since the system has stopped nitriding, this is a sign that the proportion of hydrogen in the equation for calculating Kn is increasing. And where does this hydrogen come from? Well, from water, according to the equation
2 * H2O → 2 * H2 + O2
If we have too much water in ammonia, we endanger both the nitriding and nitrocarburizing processes because the thermal decomposition of water will significantly reduce the nitriding number Kn. In addition, the oxygen released from the dissociation of water reacts with carbon to form CO and CO2, and at the same time decarburizes the steel surface.
To eliminate this, we must get rid of the water in NH3. Grade 2.8 contains 400 ppm H2O, grade 3.8 only 200 ppm H2O and grade 4.0 only 100 ppm H2O.
Of course, it is not only a question of the purity of ammonia, but also of the purity of the packaging, bottles or tanks. If we really want to have the processes under control, it is usual for the NH3 supplier to reserve packaging for us that rotates only for us and in a system that guarantees the impossibility of internal contamination.
Fig 2– On the left the mass spectrum of the residual atmosphere and on the right the mass spectrum of NH3 with a high water content. The peak at mass m/e = 18 gives us the amount of water content, H2O+ ions.
Fig 3 – – Comparison of individual spectra a) residual atmosphere, b) NH3 flow rate 100 ml/min c) theoretical NH3 spectrum
Therefore, increased attention needs to be paid to the purchase of ammonia for nitriding and nitrocarburizing. However, since only GHC Invest supplies quality 4.0, the choice is clear.
Deliveries are possible in the entire range of containers from 2.6 kg bottles to large drums weighing 475 kg.
If you are interested in trying ammonia in quality 4.0 from GHC, you can contact me at any time. I will arrange everything.
Jiří Stanislav
January 18, 2025
