How to correctly distinguish LPC III

As I have already described in the previous sections, in order for the LPC to work properly, both with regard to the layers and the condition of the furnace, we must follow certain principles. From my personal experience, I know that it is not such a big problem to carburize a few pieces in the furnace, I also did that in 1989 on RDVA furnaces, which were started by ZEZ Praha at that time. Another experience of mine came at a time when we needed to select LPC equipment for Poclain Hydraulics in 2000. We have already tested these on professionally made devices. But I became vigilant when I was told at one test furnace that they had to remove carbon deposits and tar from the carburizing chamber every week.  And not a small amount.

How is this possible? I have described part of it in the previous chapters. The use of gases other than C2H2 does not make sense, because unlike acetylene, which decomposes catalytically, they decompose thermally, in the volume of the furnace. They are therefore a source of carbon that has no consumption and it is deposited in the furnace. The fact that at that time companies offered ethylene, propane, gas mixtures resulted from patent protection. I remember negotiations about the purchase of equipment with LPC. When I asked why they use propane and not acetylene, the answer was: it’s patented, but when we deliver it to you, feel free to put a bottle of acetylene in there.

Times have changed. Basically, everyone is only on pure acetylene.

We also already know that if the saturation period lasts longer than 2 minutes, it loses its meaning. In the furnace, the activity of the atmosphere and the surface of the steel is compared, and with the zero gradient I only supply gas with carbon, which again has no consumption. Other rules apply to the total pressure, flow velocity, flow direction and its optimization inside the chamber, etc.

If the LPC system is perfected, secondary ethylene is not formed in the furnace from the C2H2 + H2 reaction, and we do not have a problem with the excretion of soot.

But even that is not enough. Another source of contamination of the heating chamber is quenching oil. On the one hand, due to the penetration of oil vapors from the quenching chamber into the heating chamber, on the other hand, due to the clogging of oil through the transport mechanism. Thus, physical contamination of the chamber with hydrocarbon vapors from the quenching oil occurs. These two phenomena often add up  and together cause the formation of sticky carbon condensates (tar, soot, polymerized carbon) on walls, heating elements and diffusers.

After quenching, oil vapours (CnHm) are released in the quenching chamber, which are returned by diffusion or flow into the heating chamber. These vapors, e.g. from mineral oil, polyalkylene glycols, esters, have  a lower decomposition point, 300–600 °C, than acetylene.

On the hot surfaces of graphite heating elements, or on the surfaces of ceramics, these heavy hydrocarbons are pyrolysed and tar/soot is formed. When these layers are overlapped with a layer of ethylene pyrolysis, they form a conductive, insulatingly unstable coating that destroys the heating elements or thermocouples.

Typical manifestations are black paint deposits on the insulation, sticky coatings on the furnace walls, or reverse contamination of the quenching oil.

To prevent these phenomena, we must respect certain principles when designing a furnace for LPC

• We must have a high-quality vacuum seal between the heating and quenching chambers, preventing contamination of the heating chamber with quenching oil
• Separate feeding system into the heating chamber from the elevator, which dips the hot charge into the oil. With each capture of the transport system, contaminated with quenching oil, we can carry the oil residue into the heating chamber
• Flushing the quenching chamber with nitrogen so that the transfer of oil vapours is limited before opening the heating chamber vacuum seal.
• During the quenching itself, there must be a certain nitrogen pressure above the oil level, improving both the cooling curve of the oil and reducing oil evaporation
• Oil condensers must be installed on vacuum pumps to capture oil vapours, preventing their recirculation into the process chamber.
• Inerting of gas supply pipes with nitrogen should prevent air back flow
• Periodic firing of the heating chamber should be applied at a temperature above 900 C and a pressure of less than 10 pascals.
• Hardening oils with minimal vapour tension should be used to prevent evaporation

From the point of view of hot zone maintenance, it is recommended:

• Every 100-150 cycles: visual inspection of the interior, measurement of heating resistance.
• Every 500 cycles: “burn-off” cleaning.
• Every 1000 cycles: cleaning the suction pipe of the vacuum pump.
• Once a year: replacement of filters and sealing elements in contact with the furnace atmosphere and replacement of nozzles

This problem is completely eliminated with devices such as ECM Flex or Jumbo, or ModulTherm from ALD with oil quenching. Why? Because the oil quenching chamber is not connected to the carburizing cells, the transport system is always 100% separate from the elevator that dips the charge into the oil.

Our problem is therefore more tied to two- or three-chamber devices, where there is a vacuum connection on the one hand, and the chambers are connected by a transport system. Here we can say that everyone has it differently. So if you want to choose a new device for LPC, you probably have to ask the right questions when entering. Not everything that looks the same is the same.

Even if I am accused of bias, from what I know today, I am sure that ECM has thought this through best. Everything I have stated also follows from discussions with customers or specialists from ECM. The problem arises when the customer is supposed to believe it. And here usually a sudden reversal occurs. The customer prefers to buy a furnace just because the neighbor has one too. After all, LPC is the same everywhere. And the fact that the neighbor completely throws out the heating twice a year, well, that is probably some kind of emergency that will not concern us. And the neighbor, well, he has long since come to terms with the fact that he will live with this problem for the next 20 years.

So, leaving aside all the above, ECM is now approaching the number of installed carburizing celle to 2000. That speaks for itself.

Jiří Stanislav

November 7, 2025