How to correctly distinguish LPC IV – LPCN

I already wrote about carbonitriding  under reduced pressure last year. https://www.jstconsultancy.cz/nitrocementace-jak-na-to-v-lpc/. My conclusion that the LPCN process is feasible is valid, but I have been instructed that it is a little different.

I had the honor to meet Ben Kahle, who developed this technology for ALD and later for ECM. His position is as follows:

“During the eight years that I have been deeply involved in low-pressure carbonitriding (LPCN), new surprises and challenges have constantly emerged. Unlike conventional atmospheric carburizing, LPCN is a process that is still not fully developed or universally mastered by anyone in the world”

So much for the quote. It was a bit shocking, but such a Přenosil has been dealing with this process all his life. Unlike conventional carbonitriding,  the fundamental problem with LPCN is that acetylene and ammonia cannot be admitted at the same time, due to the formation  of HCN according to the scheme below.

C (graphite) + NH₃ + C₂H₂ → HCN + other products (e.g. H₂, N₂)

Ben describes the experiment:

“steel 16MnCr5, process at 870 C, 90 min, graphite chamber, total active area 1 m², only NH₃ is allowed. Despite this fact, the resulting layer contains 0.6–0.7% nitrogen and about 0.75% carbon. But where did the carbon come from, when we only allowed NH₃? The answer is the formation of HCN in the contact of ammonia with graphite elements of the heating chamber”

In short, HCN is a critical and unavoidable part of the process in LPCN, but probably also in classical carbonitriding, if we had a graphite heater and graphite lining. The fact that we don’t have it is unlucky. Because if that were the case, Přenosil would already know about this problem.

But why is this such a problem? As we have explained in the previous chapters, if the reaction of C₂H₂ → 2 C + H₂ takes place, then all the carbon from the catalytic reaction is consumed as part of diffusion into the steel. But this is not the case with ammonia. There, the reaction C (graphite) + NH₃ → HCN + H₂ is again a catalytic reaction, but not only from contact with the batch surface, but also from contact with graphite heating elements, furnace wall, CFC fixtures or from soot deposits in the hot zone. And because this area is basically still the same, the amount of HCN will be the same, even if the batch surface area will be different. This will significantly complicate the control of the carbonitriding process with different parts surfaces. If we fine-tune the carbonitriding  process to a reference batch of 1 m2, the result will be completely different for another batch with other parts and an surface area of 20 m2. And that’s what Ben claims.

In that last post, I was a supporter of admitting ammonia in the first step. I don’t think so anymore. The reason is not only the different nitrogen and carbon content from catalytic reactions, but the problem is also in the reverse diffusion of nitrogen. Because as soon as I turn off the ammonia, I have a two-sided gradient, i.e. both towards the steel and into the atmosphere, and the nitrogen will slowly disappear from the layer with the formation of N2. Again, I quote Ben:

• At 900 °C, approximately 0.1% by weight of nitrogen was lost from 20MnCr5 within 15 minutes.
• At 980 °C, the loss increased to approx. 0.3 % by weight in the same timeframe.

A partial salvation lies in the fact that specially alloyed steels for carburizing have been developed for these purposes, with microalloying with aluminum and niobium. These stabilize the nitrogen in the layer in the form of expelled nitrides, which is then difficult to get back out of the layer. Another advantage of this microalloying is the stabilization of the austenitic grain, which makes it possible to work with higher temperatures up to 1000 C.

So, what is the result of these considerations? Ammonia will be admitted only in the last step, at a temperature of 850-900 C, with a diffusion delay of up to 90-120 minutes depending on the required depth. Ammonia must be supplied in short pulses and with low pressure to minimize the possibility of reaction on the graphite elements of the heating zone. For the actual carbonitriding of the steel surface, we only need p_N2 < 10 Pa, i.e. less than 0.1 mbar.

If we want to have constant nitrogen potential in the furnace, then we need to add a sensor that will control the ammonia supply. Here we offer either a mass spectrometer or an OPTIX gauge from Gencoa, both very expensive devices.

In any case, it is necessary to consider the formation of HCN, under certain conditions it can make up tens of percent of the atmosphere. This is not so much at a total pressure of up to 10 mbar, but  HCN is a very toxic and corrosive substance, and even a small amount has significant consequences for both the service life of the equipment and the safety of the operator.

Expected impacts on different elements in the furnace:

It looks quite tragic, doesn’t it? However, if I buy LPC equipment, and I want to perform LPCN in it, I have to be aware of all the above.

If I buy LPCN equipment, then the equipment supplier will handle all this for us. But we have to think about it at the moment when I prepare the technical assignment.

At the end, I will mention Ben. A publication has been published for the carbonitriding of automotive parts, together with FVA, ZF and IWT Bremen, which shows the carbon and nitrogen depth profiles expected after carbonitriding .  Ben claims that the ALD device meets these conditions even with 20 m2 charges. This is good news. Since Ben worked for ECM for some time, I believe that ECM devices will achieve the same results. And the others? Rather than believe it, I’d rather test it beforehand. A properly designed device for LPCN must also solve all of the above, it’s not just about having only one extra mass flowmeter.

Jiří Stanislav

November 11, 2025