Nitriding of Crankshafts

[RRoller123]

Latest area of education, involves the hardening of engine crankshafts, among many other items, gears, sprockets, etc. Good video, very involved. We may be faced with the decision as to whether to re-nitride harden the crankshafts, based upon inspection after disassembly and machining, if needed.

I have plenty of questions around this process, including:

1.) Which nitriding process did the factory use? There are 3 main ones available; Salt Immersion, Gas or Plasma. I think we can rule out plasma, as it was not available back then afaik. And the SI process involves cyanide, so that is unlikely. Most likely it was Gas, which is NH3 Ammonia gas, molecule broken into N and H3 by heat and pressure, and injected into the pressure vessel and absorbed by the steel over a period of time.

2.) How do they get rid of the H gas during processing? Being smaller and lighter than , it could easily disperse into the steel, where it is not wanted. Being lighter, maybe collected at the top of the pressure vessel and burned off? Yet the NH3 was constantly swirled to keep chemical density uniform. Curious how they do this.

3.) Thickness (or depth, actually) of treatment? This modern video, below indicates thathe process runs up to a maximum of about 6mm, or ~0.023" maximum. Yet it takes many hours to achieve this, approximately 48 hours or more, possibly 72 to get a really large depth. So it is likely that normal depths are far lower, like 0.010" to maybe 0.015".

Now, since process soak time is a human variable, and we know all too well what human nature is, it is not with high confidence that one would expect anything over, say 0.020" on a consistent basis. The only way to truly tell is either to do a cross section (destructive) or to do a Rockwell C hardness measurement after machining.

4.) Note the clear Eutectic point on the N-Fe phase diagram, like for solder.

https://www.youtube.com/watch?v=LSjHmZLfWco

5.) There is another, quite serious potential issue. There is a video of an Viper engine, ruined by the nitriding process, DUE TO HUMAN ERROR OF THE CUSTOMER, who did not measure the journal diameters again AFTER nitriding. The process expanded the journals a couple of thousandths and closed the gap. The engine was assembled and run, it froze, and threw a rod through the side of the block.

6.) Which of the 3 processes was used? I have not heard of this before. It implies that, if it is determined that machining of a crank is needed, that the nitriding be done before, assuming that a thick enough depth can be attained (longer process soak time) to allow for the removed material. Otherwise, the machining needs to take into account expansion, which can lead to "certain uncertainty".

7.) Is it true that the nitriding process can and does expand the material? Steel in this case. The answer has obvious implications to the process for machining any materials, crankshafts certainly, and adds the new variable of how MUCH machining is going to be needed, to determine whether to re-nitride, and when (before or after).

Viper experience video below.

https://www.youtube.com/watch?v=kiEEsJkNMVA

I find this a fascinating study, and may it be of use to all of us doing extensive engine work. Or just an interesting intellectual pastime. That is ok too.

Pete