Are 911 Crankshafts Nitrided?

[jluetjen]

Specifically, I'm concerned with a 2.2 counterweighted crank, but I'm sure that others would be interested in the other variaties also.

- John

[Eagledriver]

I've read that they are "tenefer" (sp) hardened. I don't know what that is but it seems to be important according to books I've read. I think it's some kind of hot acid bath. According to the 911 performance handbook the only place that can do this correctly is the factory.

-Andy

[tadd]

Tefner harding uses a bath of molten cyanide salts to form a thin, hard wearing surface layer. Other places than the factory can do this process and do it well, but they are few in number. There used to be a place in Nashville that did this process, and were as old as the hills, but finally closed their doors.

tadd

[davidppp]

Hello all.

The current equivalent is tuftriding.

It is still legal in UK...if anyone is interested, we could get a Pelican Group batch done..

The process involves a first grind, hardening, straightening if needed, then regrind and polish.

The process produces a swelling of the skin of the crank..hence the improved fatigue resistance.

The downside is that the skin is thin, and not very even.

Kind regards
David

[911pcars]

This is briefly what I know.

Nitriding is similar to Tennifer heat treatment.

Tuftriding is similar to Nitriding, but the process involves lower bake temperatures that result in less part distortion and thus, parts needing less straightening.

All treatments harden a very thin top layer of the steel for a longer wearing bearing surface.

I couldn't find a source for these treatments in the US a few months ago. EPA regs. must be fairly restrictive.

Sherwood

[jluetjen]

Thanks.

[chris_seven]

It is important to understand the main reason for these treatments.

Nitriding produces very high residual compressive stresses in the surface of a the material being treated. This surface compression makes it more difficult to initiate a fatigue crack in the most highly stressed region of the crank.

It is important to know that you should only nitride steels that have a specific alloy composition to allow suitable nitrides to develop. This ussually means a material with 2-3% Aluminium such as EN40B (UK spec and sorry I don't know the US equivalent).

If you nitride unsuitable steels you will produce a surface high in iron nitrides which are brittle and will tend to spall. This will quickly form a nice grinding paste and completely ruin the component.

Tuftriding can be carried out on most steels and is much more forgiving but also less effective.

Nitrided layers are relatively thick compared to tuftriding.

It is also important to know that Nitriding produces a 'white layer' on the surface of the component and this must be removed, usually by grinding.

I would leave about 0.01" and grind and micropolish. The white layer is iron nitride which will flake if left in place.

Tuftriding is only a few thou thick and also produces a 'white layer' BUT this is the hard surface that provides the resdual compressive stress and should not be removed. The surface growth produced by Tuftriding is minimal.

Journal hardness is a by-product of these processes. The real reason for using them is to produce improvements in fatigue life.

The both take palce at about 550degC and it is important to replace crank bungs after this process has been carried out.

I don't think 911 cranks are manufactured from a steel that can be nitrided so Tuftriding is the best way forward.

[Henry Schmidt]

We have been using Burlington Eng to harden our cranks with truly remarkable results. The process is called Melonite and is a Salt Bath Nitriding System.
Here is a little information.
BTW: we have never had a crank fail after using this process.

http://www.burlingtoneng.com/melonite.html

The MELONITE Process Improves Component Wear and Hardness of Steel Properties

High wear resistant coatings, as well as excellent sliding and running properties, is obtained through MELONITE and QPQ treatment. The service life of steel tools and parts is extended. Corrosion resistance of unalloyed and low alloyed steels is greatly improved.

The MELONITE and QPQ process increases fatigue strength about 100% on notched components made from unalloyed steel parts and about 30-80% on parts made of alloyed steels. The hardness is maintained up to about 930 deg. F and extends the surface life of steel tools and components exposed to heat.

Melonite Process Component Reg
Wear Resistant Coatings With Economic Advantages

Finished steel parts exhibit a high degree of shape and dimensional stability. Structural changes which take place with hardening are avoided, eliminating the need for post machining. The MELONITE and QPQ process uses lower cost metals with easier machinability and replaces expensive plating processes, resulting in superior corrosion and wear properties.
Diffuses Nitrogen and Carbon into the Surface

During the MELONITE process, which takes place between 900 deg. F and 1075 deg. F, the metal surface is enriched with nitrogen and carbon. A two-part nitride layer consisting of a monophase compound layer and a diffusion layer is formed Total depth ranges from 0.008-0.040", depending on the composition of the base material and treating time. Hardness in the compound layer ranges from approximately HV 700 on alloyed steels to about HV 1600 on high chromium steels.



wear resistance

(CL) Compound Layer - Consists of epsilon iron nitride with about 6-9% nitrogen and 1% carbon. The thickness for most applications is around 0.0004-0.0008". It improves:

* Corrosion Resistance
* Scuffing Resistance
* Hot Strength
* Wear Resistance
* Running Behavior

(DL) Diffusion Layer - Contains nitrogen, either dissolved in the iron lattice and/or precipitated as very fine nitrides. Low alloy steels give thicker layers with lower hardness. Higher alloys give greater hardness with thinner layers. It improves:

* Rotating Fatigue Strength
* Pressure Loadability
* Rolling Fatigue Strength

Melonite and QPQ greatly improves the wear properties of thin-section stampings without distortion.
Improvement of Tribological Properties Through Nitrocarburizing
Structure, Hardness and Depth of the Nitrocarburized Layer

During nitrocarburizing, a two-part surface layer is formed, initially an outer compound layer, followed by a diffusion layer below it. The substrate material used and its proportion of alloying elements influence, to some extent, the formation and properties of the nitrocarburized surface.
Compound Layer

The nitrogen-rich inter-metallic compound layer mainly contains iron-carbonitrides and, depending on the type and proportion of alloying elements in the base material, special nitrides.

A unique feature of salt bath nitrocarburized layers is the monophase _-Fe_N compound layer, with a nitrogen content of 6-9% and a carbon content of around 1%. Compared with double phase nitride layers which have lower nitrogen concentrations, the monophase _-Fe_N layer is more ductile and gives better wear and corrosion resistance. In metallographic analysis the compound layer is clearly definable fron the diffusion layer as a lightly etched layer. A porous area develops in the outer zone of the compound layer. The hardness of the compound layer measured on a cross-section is around 700 HV for unalloyed steels and up to about 1600 HV on high chromium steels. Treatment durations of 1-2 hours usually yield compound layers about 10-20 _m thick (0.0004 - 0.0008"). The higher the alloy content, the thinner the layer for the same treatment cycle. Fig. 2 shows the relationship of layer thickness to treatment time with nitrocarburizing temperature of 580 deg. C

(1057 deg. F).



Thickness of compound layes obtained on various materials as a function of nitrocarburizing duration
Diffusion Layer

The nitrogen penetration into the diffusion layer provides for improved fatigue strength. Depending on the initial structure and composition of the core material, the nitrogen in the diffusion layer is dissolved in the iron lattice and/or precipitated as very fine nitrides.



Influence of chromium on diffusion layer hardness and total nitration depth in various 0.40-0.45% carbon steels

With unalloyed steels, the nitrogen is dissolved in the iron lattice. Due to the diminishing solubility of nitrogen in iron during slow cooling, _'-Fe4N nitrides are precipitated in the outer region of the diffusion layer, some in form of needles, which are visible in the structure under the microscope. If cooling is done quickly, the nitrogen remains in super-saturated solution. With alloyed steels which contain nitride-forming elements, the formation of stable nitrides or carbonitrides takes place in the diffusion layer independent of the cooling speed. With increasing alloy content of the steel, the diffusion layer is thinner for identical nitrocarburizing parameters. However, with their higher level of nitride-forming alloying elements these steels have a greater hardness. Fig. 3 illustrates the influence of chromium on the hardness and depth of the diffusion layer in steels with a carbon content of 0.40 - 0.45% after 90 minutes treatment at 580 deg. C (1075 deg. F). Total nitrocarburizing depth shown in Fig. 4 is the distance to the point where the hardness of the nitride layer is equal to the core hardness. After a 90 minute treatment the total nitrided depth is about 1.0 mm (0.040") on unalloyed steel, but barely 0.2 mm (0.008") on a 12% Cr steel. (See Fig. 4.)




Total nitrided depth on various materials resulting from nitrocarburizing

Fig. 7 shows the coefficient of friction both under dry conditions and after lubrication with SAE 30 oil, measured by an Amsler machine. All samples were lapped to a roughness of R_ = 1_m after their respective surface treatments and before testing. Without lubrication the nitrocarburized QP had the lowest coefficient of friction, being less than half of that of the hard chrome or case hardened surfaces. The lowest friction level occurred when nitrocarburized QPQ is lubricated. It is 3-4 times lower than that achieved with the chrome or martensitic surfaces.



Coefficient of friction values for various surface layers, with and without lubrication.
SNC = salt bath nitrocarburized

These results show the direct effect of increased oxidation as it relates to friction on the surface of the nitrocarburized samples. The QPQ sample, with its extra post-oxidation step, has a much higher friction value than the QP specimen, which had part of its original oxidation in the compound layer removed by lapping. However, with this variant, due to the fine microporosity in the QPQ sample which causes the lubrication to adhere better to the surface, this option gives the lowest friction value.

If a uniform running behavior is required the QP process is appropriate. Lubrication has only a slight influence on the coefficient of friction because the oxide layer of the outer surface was removed during the polishing operation.

It has been determined that, unlike with chrome surfaces, the coefficient of friction of nitrocarburized QP and QPQ treated surfaces remains constant, even at varying sliding speeds.

The intermetallic stricture of the compound layer, which contains epsilon iron nitride formed during nitrocarburizing, is extremely resistant to adhesive wear and scuffing. Fig. 8 shows the scuffing loads of gears made from various materials (6). It was established by applying increasing pressure to the flank tooth until galling occurred. Austenitic steel containing 18% chromium and 8% nickel had the lowest resistance to galling, however, after nitrocarburizing its resistance was raised almost five-fold. The performance with SAE 5134 was about tripled. Even SAE 5116, which had already been carburized, more than doubled the scuffing load it could withstand through the compound layer built by the nitrocarburizing treatment.






Scuffing load limit of gears.
SNC = salt bath nitrocarburized

[chris_seven]

Yep, basically Tuftriding but I am sure that there have been some process improvements over the years.

See

“The Tufftride® (also known as Melonite® and Tenifer®) nitrocarburizing process belongs to HEF Durferrit, a German company. The English version of their website is at http://www.durferrit.com/englisch/index.html
About the only info on this site is that “This nitrocarburizing process has undergone continuous development with regard to its regenerability and ecology,” i.e., control parameters may have changed.