Crank hardening

[Flieger]

Interesting, too was that the factory ordered several crankshaft prototypes for the 917 engine, made from different alloys and some made in pieces with the gear a different alloy. Time was more important than money at that point but they decided the case-hardened, one-piece crank/gearshaft was best.

[jcge]

[QUOTE=chris_seven;5463183]

Originally Posted by jcge

Do you know when the crankshaft material was changed?

No Chris - that's why I indicated the date of publication of Feb 1972...but no hard date or model info....perhaps Ludvigson's Excellence or some of Frere's texts have some further detail.

[QUOTE=chris_seven;5463183]

Originally Posted by jcge

It is lso interesting that the 917 used a 17CrNiMo6 alloy as I would typically describe this as a gear steel. It has a realtively low carbon content so it can be case hardened but straightening it after carburising and quenching must have been a real trick.

As Max indicates above - there were 2 versions - solid crank and built up crank. (I'm assuming the 17CrNiMo6 is referring to the solid crank version, given its central gear for driving both the "output" and "intermediate" shafts)

The built up crank was welded together like this...(Ferdinand Piech is credited as the inventor)

[zioo]

Ion tank exposure can achieve a Rockwell hardness of 55 on a Porsche crank. Stress relieving can realign the grain. The straightening process and jig check can be done after grinding. Check the aluminum plugs or putting in new ones is a good idea. My crank is being ground by Castillo's Crank Specialty Service in La Mirada California. His web site has a primer on crankshafts that is most educational. He's on the web for address and phone, etc. His prices are very competitive and he appears to have dedication to the craft and good references. zioo

[chris_seven]

Originally Posted by zioo

Ion tank exposure can achieve a Rockwell hardness of 55 on a Porsche crank. Stress relieving can realign the grain.

I haven't read this site as I can't find them on Google but theoretically Ion Nitirding for cranks seems really scarey and I am not sure I would like to do this on my race car components.

This process is usually used for the production of hard, wear resistant surfaces rather than the fatigue life improvement associated with conventional nitriding, although I would accept is has a small effect.

The workpiece is treated in a chamber at a pressure of a few Torr. A process gas which is a mixture of Nitrogen and Hydrogen is used and a voltage (either RF or DC) applied to the workpiece causing a corona discharge.

There are two main issues:

1.If the partial pressure of hydrogen is not precisely controlled then hydrogen diffusion, at the surface temperatures involved, can occur which will embrittle the crank. (Hydrogen Embrittlement that occurs in plated fasteners)

2. There is also the chance that as the process is carried out using 'glow' discharge it is possible for instabilities to occur and for loacl melting to arise. - I would agree that with a well controlled process this may be unlikely but it will NEVER happen with conventional nitiriding.

The other downside is that the 'diffusion' layer that is present with Tenifer type treatments does not develop very well with Ion techniques so although hard surfaces are produced the fatigue enhancement is limited.

It is also better to use a nitriding steel for this process.

The Hardness quoted at '55 Rockwell' must refer to a Rockwell C Scale. I must say that this isn't the most appropriate way to measure the hardness of surface treated parts.

A Rockwell C Test involves measuring the depth to which a 120 degree pointed, conical diamond will penetrate a metal surface when a load of 150 kg (330lbs) is applied.

Clealry this indentor will punch its way through a thin hard shell and cause damage and give meaningless results.

It is common to use a 'Superficial' Rockwell test and then convert to an HRC value and the scatter in results produced by these techniques render the result6s quite poor and IMHO unreliable.

Much better to use Vickers Hardness tests which are much more controllable and can be applied with relatively light loads.

I really don't understand the statement that stress relieving can realign the grain!! Metallurgically very shakey ground.

To modify the grain structure of any ferritic steel implies that it must be heated to a sufficiently high temperature to allow recrystallisation to occur.

The idea of stress relief is to relax the small residual stresses that are introduced my machining, welding etc.

It is always carried out much below recrystallisation temperatures.

I can agree that Ion Nitirding may produce residual stresses but these are undesirable and stress relief may be needed but it is not a specific benefit.

I do realise that I may seem negative about many of these new and improved techniques but to understand the reasons does need detailed explanation.

Ion beam methods were developed to replace conventional Gas Nitiriding where surface hardness and cost were the most important factors.

Ion Nitriding reduces cycle times from 20 hours to 4-5 hours with significanlty lower energy costs. This doesn't mean it is the berst replacement in all circumstances.

zioo, can you post up the link please.

[BURN-BROS]

Originally Posted by chris_seven

I haven't read this site as I can't find them on Google but theoretically Ion Nitirding for cranks seems really scarey and I am not sure I would like to do this on my race car components.

Hi Chris,

Ion nitriding is standard practice on the billet cranks for Nascar and have held up well.

Top fuel however, requires a more tradition heat treat as there is too much torsional movement. A crank that has been nitrided lives for 1 pass and one that has been treated normally can get three passes before it is thrown out. The cranks regardless of the type of heat treat, exhibit cracking after the first pass The nitrided cranks exhibit strange radial cracking. I do not remember which heat treatment they ultimately chose. This is on the extreme end of the spectrum but clearly shows a limitation.

Personally, as long as the crank is designed properly or you are not asking it to go much farther than it's designed parameters then Ion Plasma is a great choice.

[chris_seven]

Aaron,

Very interesting, I have a pal that I was at college with that did about 10 years research in Plasma Nitiriding and now runs a specialist Heat Treatment company, I guess i had better call him and learn more.

It is interesting that the traditional gas nitirded parts seem to have a better fatigue life and I agree with your comments about the torsional behaviour of cranks. I would be interested to see some photographs of the radial cracking.

I completely agree that if the torsional stresses are low enough then fatigue life shouldn't be a problem but I guess this results in a fairly high inertia crank.

If I understand Nascar regs correctly the counterweights and hence the mass of the crank is pretty much specified and doesn't give much scope for change and torsional stresses are quite well controlled.

I used to be involved with torsional analysis and used some very good Greman Software that allowed the interia torques and the gas torques to be modelled seperately and then combined the outputs.

It seems to me that we do tend to muck about with the mass-elastics of rotating assemblies without too much analysis and it does bite from time to time.

ARLA Maschinentechnik GmbH


I also looked at the Lunati web site and it seems that the have a 'non-twist' steel available , which is an amusing idea.

[BURN-BROS]

Originally Posted by chris_seven

Aaron,

Very interesting, I have a pal that I was at college with that did about 10 years research in Plasma Nitiriding and now runs a specialist Heat Treatment company, I guess i had better call him and learn more..

Love to hear more.

Originally Posted by chris_seven

I also looked at the Lunati web site and it seems that the have a 'non-twist' steel available , which is an amusing idea.

LOL, too funny!