[Weissach911]

Originally Posted by Flieger

If 12% is eutectic, then wouldn't 10% Si be hypoeutectic, not hypereutectic?

The KS "Alusil" barrels were meant to be the wear surface and were run with hard, iron coated pistons. The MAHLE "Nikasil" barrels were meant to be the hard surface with their thin Nickel-Silicon-Carbide coating on the Aluminum barrel and were run with a normal Aluminum piston and relatively soft rings.

The MAHLE alloy for pistons and cylinders is a higher Silicon content than the JE alloy, which is the one more stable at high temperature but a little softer and with more thermal expansion. I do not remember the numbers but there was a thread on here about materials for making new, CNC machined heads that discussed alloys. chris_seven was the main contributor.

Sorry 10% was a typo - should have been 17% - which is hyper -- 10% would indeed be hypo.

4032-T6 is very similar to the Mahle Alloy and the Silicon does reduce the expansion by a significant amount but also reduces the melting point and hence - in general- the softening point.

One important property of materials used at elevated temperatures - an aluminium at piston crown temperatures is just such an application - is the -equi-cohesive temperature. This the point at which grain boundaries in the material start to slide over each other and allow deformation.

2618 is better than 4032 in this respect. The pistons that used to be used in Mercedes F1 engines had a Beryllium content - since banned and this alloy is not only stronger, stiffer has better thermal conductivity but is also more resistant to creep - sorry to drift off the subject.

Originally Posted by Flieger

And a mean tensile stress reduces the fatigue endurance limit. So I would want to have the minimum preload (at any temperature) to be just enough to overcome the cylinder pressures and keep everything from fretting.

Or were you referring to the Aluminum fatigue? Well, Aluminums do not exhibit a fatigue endurance limit and even if they are in compression the polycrystalline nature will mean that there will always be some grains oriented in a bad direction that will be loaded in shear or tension which can start a crack even with cyclical compressive loads.

I agree that preload should be a minimum and I think I said in an early post that anything that increased the pull out force was undesirable.

I don't think fatigue in the studs us a major concern but studs will generally be designed to have enough preload to avoid fatigue loading.

I am not sure what you mean about the 'bad' orientation of grains in a Aluminium based material. Aluminium is face centred cubic which means it has 12 slip systems available to allow deformation so its its behaviour is generally quite isotropic.

Most metals that exhibit fatigue endurance limits have 'interstitial' alloying elements. These interstitial atoms block the movement of dislocations and inhibit the formation of the slip band intrusions and extrusions that usually initiate fatigue cracks. Interstitial atoms such as carbon are quite small.

Aluminium atoms are already quite small and the vast majority of alloying elements are subsitutional and these don't much change fatigue behaviour other than to increase strength.

The precipitation hardening alloys also don't really produce fatigue endurance limits as precipitate particles are again relatively large.

Stell at room temperature is Body Centred Cubic and there is a large octahedral interstital site which can be occupied by a Carbon atom and this is generally responsible for the endurance limit.

If a 'bulk' material is loaded in compression the orientation of individual grains will not lead to the production of fatigue cracks. As you say you will always need a traction vector to generate a crack. If you load a material in hydrostatic compression it will not fail even if it is polycrystalline as are the majority of day to day metals (Gas Turbine blades are single crystal to eliminate grain boundary sliding). I think you will only see resolved shear stresses at surfaces where buckling or other geometric instabilities occur.

I do think, however, that at around 200degC Aluminium/Silicon alloys can exhibit fatigue softening due to variations in tensile loads. The more highly loaded the stud the more likely this is to occur. This mechanism would almost certainly cause a loss of preload.

Originally Posted by ivath

Found this information about Dilavar:

Dilavar (Ni 13 (Nb NiMnCrV X 68 12 5) - Material No. 1.3937)
It is a nickel alloy.

I also found this text on the Lnengineering website:

On 911 models, certain engines came fitted with Dilivar head studs, which were designed to match the expansion of the cylinders, but also suffered from hydrogen embrittlement, which is a condition that results from the accumulation of hydrogen gas in the atomic structure of the metal.

If hydrogen embrittlement is the cause, and not SCC, it can explain that the bolts fail at the engine stand. In my previous job the same thing happened with new bolts on oil equipment, even before it left the factory.

I don't think you can call this a Nickel Alloy. The convention is to name an aaloy after the main alloy constituent and 13% Nickel is almost ceratinly described as a Stainless Steel. The fact the studs are virtually non-magnetic is a good guide to the fact that the crystal structure is Austenitic in nature. Stainless Cutlery Steels, which are ferritic are about 13% Nickel. The other elements present help to stabalise the Austenite phase to remain at room temperature.

Materials such as Inconel 718 which are similar to ARP's Age625 are around 55% Nickel and are traditinally known as Nickel Alloys.

The Hydrogen embrittlement theory is interesting.

Hydrogen embrittlement is a reasonably common phenomenon but there must be a souce of hyrogen for this to be a problem.

In oil/gas environments knows as 'sweet and sour' gas there is always hydrogen present in pipelines and this hydrogen can and does diffuse into plant and equipment and casues significant problems. The hydrogen normally needs to be in an atomic form to diffuse into a metal and is normally the result of a reaction rather than in gaseous from.

Electroplating high strength bolts can also produce atomic surface hydrogen if plating is not extremely well controlled and it is common practice to de-embrittle plated bolts particulalry in a areospace environment.

If Dilavar is a precipitation hardening Nickel Alloy then its susptibility would be much reduced compared to conventional high strength steels.

Nickel based alloys of this type are the material of choice for most 'Hydrogen' applications.

It is also true to say that many of the high strength precipitation hardened stainless steels can be suseptible to Hydrogen embrittlement but would need to operate in an environment wher hydrogen was present.

A salt water environment could help to induce this type of problem but parts would need to immered for significant amounts of time for this to become an issue.

There are two ways to reduce the sensitvity of Preciptitation Hardened Stainless Steels to hydrogen embrittlement.

The first is to heat treat them to an over-aged condition which lightly reduces strength. but is beneficial.

The second is to add Niobium to the alloy composition.

It is interesting that Niobium is present in Dilavar.

I think it is unlikely that the studs pick up hydrogen in their operating environment and if they did then conventional steel would suffer in the same manenr as would 17-4PH which could be worse.

It may be possible for Hydrogen to have been introduced into Dilavar at some manufacturing stage - such as during acid pickling but it does seem unlikely and would have been relatively easy to resolve and eliminate.

If a bolt or fastener has a prior defect such as a hydrogen crack the loading it to a specific conditon that does not cause failure will mean that it is in a stable condition and providing the stress intensity at the crack tip doen't increase there will be no mechanism to cause a brittle failure.

The propogate a crack in an unstable manner the crack tip has to be provided with additional energy - the stress intensity must increase.

The only explanation can be that the fastener has a prior defect and that it has been initially loaded to a level just below the critical stress and then left.

If there is then some expansion or contraction taking place due to temperature change the KIC of the material could be exceeded and fialure would occur in a brittle manner.

This would only be true for very high strength materials and the 'cracks' would have to quite large.

My conclusions in general are that I would tend to use standard steel studs in standard engines. I would be tempted to use Dilavar or A286 in Magnesium cases with Nikasil barrels and would use Supertec Studs in high performance motors.