Why Inconel?

[MagnaMagicBtu]

So I got some precups out of this 7.3 IDI diesel engine I'm rebuilding. These cups to my knowledge are made out of Inconel, a very,very hard abrassion resistant and corrsion resistant alloy. They are hard as hell but also very brittle. I don't understand why they used a brittle metal for a high heat and high pressure application, I think a softer metal like mild-steel would do better as it will expand slightly rather than cracking.

 

It also is left unharmed by hydrochloric acid and Lye.

 

So the question is: Why a very hard brittle metal rather than than a softer metal in a high pressure situation?

 

I think even Titanium would perform better than Inconel as it will not crack when subjected to high stress.

 

They go into this Indirect Injected diesel engine, where air is forced into a pre-chamber (precup) and fuel is injected into the precup and the explosion uses up all the o2 in the precup so it travels down into the main chamber where the rest of the o2 is via the angled passage of the precup.

 

Here are some nice pictures to see what a "Precup" is. They are also very expensive, costing $70-100 a piece.

 

 

So the question is: Why a very hard brittle metal than a softer metal in a high pressure situation? would expanding slightly be better than cracking?

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited March 3, 2014 by MagnaMagicBtu

[swansont]

Not really an expert in this area of physics but a soft material will deform. Is that what you want here? It seems to me that something with a tight tolerance requires a harder material, and hardness and brittleness go together.

[Endy0816]

Yeah, creep resistance is most likely reason. There are stainless steel ones out there too though. Likely just depends on the application and material costs.

 

Been awhile since I looked Inconel up. I'm thinking it was for a similar reason too.

[John Cuthber]

How hot do these things get?

Inconel is still strong at 1000 C (and, I suspect, less brittle than it is at normal temperatures).

Mild steel at that temperature has lost a lot of its strength.

[Enthalpy]

Inconel and other nickel alloys serve at temperatures where steel stops, including stainless steel. The reasons:

- Corrosion

- Creep.

 

Nickel alloys typically serve to +700°C or little more, depending on the stress. A combustion chamber may have walls at +800°C while turbine blades prefer +700°C. Long and expensive research develops better alloys, because this is the fundamental limit to gas turbines. Single-crystal nickel alloy is present-day tech, and turbine blades are actively cooled, with air passing through their hollow centre.

 

Steel would stop much earlier, like +500°C in favourable cases. Creeping starts far before the melting temperature.

Titanium stops before temperatures where nickel serves. Already soft, and worse, it can catch fire. But Al-Ti, about 50/50, has been studied.

 

Molybdenum would be the next step beyond nickel; cobalt is uncommon; then youd have niobium, possibly tantalum and hafnium, and tungsten. Ceramics as much as possible, like ZrO₂ stabilized with Y₂O₃, and carbon-carbon.

 

Nickel alloys can be somewhat brittle at cold, but not at the service temperature. It's a worry when machining, and during temperature cycles. I'd suspect that the environment, like coking, embrittles the nickel parts in a Diesel engine.

[Enthalpy]

At least the elongation at break tells that Inconel is resilient at room temperature. I didn't try by myself.

 

As for titanium, it's embrittled by oxygen, nitrogen, hydrogen... I observed it when welding with insufficient argon protection, using hardware meant for aluminium, and it's impressive. Even at a lower temperature in a piston engine, embrittlement over hours may well preclude titanium there.

 

Anyway, 70$ a part is not the cost of the Inconel. Nickel costs presently 7$/Lb at the LME, and titanium 8$/Lb.