Gas or plasma nitriding

[abri]

I'm a newbie to this, so seeking advice/opinions from guys with experience.

Which is best for a Mk1 S crank and why?

Thanks

[snoopy64]

Hi

EN40B?

Gas nitriding will be cheaper and quite adequate.

Plasma nitriding will be expensive and can be difficult to control especially keeping the temperature even over the crank.. ( there are two types of plasma too, hot wall is the best in this case!)a risk of distortion especially on a retreat process however bolt holes etc will be better protected.

pm me if you want a recommendation for a heat treater

[abri]

Snoopy, to throw a third spanner in the works.....liquid nitriding. Where does that fit into the equation?

One of the cranks is a EN40B and the other seems to be a Mk2 tuftrided crank.

Can both be treated to the same nitriding process after machining?

Strange, from what I've read on the web the plasma version is easier to control and to get a consistent application on the whole crank.

[snoopy64]

Hi

Liquid nitriding falls into the tuftriding category as a process and is inky suitable for some materials...it is a process with high heat transfer rates so you need to be sure your crank is very stable before attempting this on a retreat process...

Plasma nitriding is better for control of the type and depth of the layer certainly on a new crank where polishing is the only finishing operation or it can be controlled to reduce or eliminate polishing completely..

How much are you grinding off? You may still have enough depth there... The main nitriding benefit comes from the compressive stress that is caused in the surface of the material.. It is particularily beneficial in the fillets where they inhibit crack growth as a result of fatgue stresses. As the journals are groubd and polished the effect is reduced here so the benefit hete is wear resistance so its important to leave the fillets alone after nitriding...

[abri]

Thanks for the useful information Kevin. The one crank is currently standard and will be machined 10/10 the other one (EN40B) will be machined 20/10.

So, fair to say unless one has them crack tested one should not do liquid nitriding?

Plasma second best (I understand this is done at lower temp than gas nitriding)? Gas adequate?

Abri

[850man]

Its a long read but once you get to the hardening discussion there is some very good information here - http://www.ausmini.com/forums/viewtopic ... Crankshaft

[Spider]

Its a long read but once you get to the hardening discussion there is some very good information here - http://www.ausmini.com/forums/viewtopic ... Crankshaft

That is a long read indeed, but once you get over the personalities, some bloody good proven info there for sure.

Many thanks for the Link

[abri]

Thanks for the interesting link 850man. Now that I've learnt about what's involved it raises new questions. Please excuse my ignorance. I'm trying to make sure I get this right on my S.

Firstly, if you have your crank ground and balanced and it is then subjected to the nitriding or tuftriding process, do you send it back to the machinists afterwards to check that the crank is still straight and that the journal diameters are still correct? I've read that cranks can get slightly bent by the serious heat application during the nitriding process and that the case hardening can add a thou or two.

If the aim is a balanced crank with journals at 10 thou, would it not make more sense to tell the machinist to machine it to 8 or 9 thou, apply the nitriding treatment and then send it back to cut it to 10 thou and balance it?

[Spider]

The Cranks in the A Series engines were made by a Forging Process and then machined, heat treated (if any) and finally ground.

The Forging Process sets up stresses within the Crank, pulling and pushing. Any Heat Treatment will relieve those stresses, sometimes completely. This will change it's original shape to some degree (ie, warp it).

By Heat Treatment, this is not at all necessarily asscociated with Tuftriding and Nitriding, it can be just that, Heat Treatment, meaning it's basically shoved in an oven at relieving temperatures for a number of hours.

This is where the biggest gains will be made in making the crank last.

After heat treatment, it is then ground back to finished size and in the process, usually any warpage can be corrected.

Treatments, like Tuftriding (which is an ICI Patented process) or Nitriding only have effect where the steel has been ground, though there will be a very small effect elsewhere. The areas of the crank that still have a skin on in from the forging process the Nitriding or Tuftriding won't take to.

These treatments are done under heat (Heat treatment) and so that is where the biggest gains from Tuftriding or Notriding come from.

As that thread shows, these are only surface treatments and all they do is allow the cranks to go for longer before needing a grind. They do not make the cranks last longer and in fact, if the process is not done right, it will leave the crank weaker from enbrittlement. As I recall in fact, the factory didn't carry on with Nitriding for too long, only a few years.

Personally, I wouldn't bother having it re-done.

If you look at broken Mini cranks, as was pointed out to me from that thread, they nearlu always go in the same place. Co-incidence? I think not!

Just be sure that the Harmonic Damper is appropriate for the set up and that it's in good condition.

[snoopy64]

Thanks for the interesting link 850man. Now that I've learnt about what's involved it raises new questions. Please excuse my ignorance. I'm trying to make sure I get this right on my S.

Firstly, if you have your crank ground and balanced and it is then subjected to the nitriding or tuftriding process, do you send it back to the machinists afterwards to check that the crank is still straight and that the journal diameters are still correct? I've read that cranks can get slightly bent by the serious heat application during the nitriding process and that the case hardening can add a thou or two.

If the aim is a balanced crank with journals at 10 thou, would it not make more sense to tell the machinist to machine it to 8 or 9 thou, apply the nitriding treatment and then send it back to cut it to 10 thou and balance it?

Hi

En40b cranks are hardened and then tempered to get the correct core strength and stiffness, nitriding is performed after grinding.. The cranks will grow by up to 0.010mm per side due to the volume change that occurs...this is the good stuff that generates the compressive stress in the surface and therefore improves fatigue strength. The standard nitriding process for these cranks gives a total case hardness of around 0.5mm with and effective depth of 0.25-0.3mm ( this is the depth to which the useful hardness is maintained)

The hardening and tempering must not be repeated on a re grind crank! But they can be re nitrided if needed...

[Spider]

.. The cranks will grow by up to 0.010mm per side due to the volume change that occurs....

Some interesting info there snoopy.

How long a time in the tank would it take for the crank to grow this much?

[snoopy64]

.. The cranks will grow by up to 0.010mm per side due to the volume change that occurs....

Some interesting info there snoopy.

How long a time in the tank would it take for the crank to grow this much?

Gas nitriding in a furnace for 0.5 mm total case depth is a couple of days at temperature 500C ish.