Which material with high stiffness and good corrosion resistance?

[Fred]

We have chosen 316L for a piston of Ø 0.8mm but it seems that the mechanical properties are too low for the application = bending pbs

The dimensions cannot be modified==> we are seraching for materials with good corrosion properties, higher young modulus and if possible correct machining response.

Every advice will be welcome

 

[stanweld]

I think that you may be looking for a material with a higher YS and YS/TS ratio. If so you may wish to look at Alloy 2205 duplex S.S.

 

[EdStainless]

You will not find alloys much stiffer than Fe based alloys. Higher yield strength is prob your best bet.
The 2205 should be a good bet as long as you don't get it hot, nothing over 600F.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Fred]

First, many thanks for your response....

I would prefer a material with a higher young modulus because I think the yield stress of our 316L is already good (Ø 0.8mm)

Could you please tell me where I can find mechanical caracteristics of 2205 to compare with.

 

[stanweld]

2205: TS 90ksi min. YS 65 ksi min.
UNS S32550: TS 110 ksi min. YS 80 ksi min
316L: TS 75ksi min. YS 25 ksi min.

A very modest increase in Modulus but large increase in strength can be achieved with Inconel Alloy 718 at appreciable added cost.

If you need greater Modulus , alloy groups outside ferrous or nickel based will be required - at very high cost.

 

[Maui]

The modulus of elasticity for ferrous based alloys usually falls between about 28 to 32 million psi. If you are convinced that a higher modulus material is required for your application, then you need to consider a different alloy system.


Maui

 

[Carburize]

Does it need to be metallic?

 

[Fred]

It doesn't need to be metallic but obviously the price will be determinant. Moreover we can not choice a material too brittle.

Concerning the young modulus, where can I find a comparative table of a wide variety of alloys?

 

[stanweld]

Fred,
To better help us to help you, what are the design parameters for this piston? What is the corrosive environment?

ASM Handbooks have much info on Young's Modulus for a variety of materials. Manufacturers like Carpenter have considerable information for the alloys, which they manufacture, on their web sites.

 

[sreid]

Tungston is almoat twice as stiff as steel.

 

[IRstuff]

Tunsten can be dissolved in sodium hydroxide.

TTFN

 

[btrueblood]

Well, if we're gonna make "cost no object" ... Iridium is a high-modulus material with good corrosion resistance (it is a "platinum group" metal). It's also extremely dense and costs several hundred US $ per ounce. Makes a wonderful catalyst for hydrazine decomposition amongst other reactions...

 

[stanweld]

btrueblood,
For really high $$$ and high Modulus and corrosion resistance, why not Osmium?

 

[sreid]

stanweld,

But for the ultimate $$$$$ and highest modulus and ultimate corrosion resistance-Diamond.

Actually a CVD diamond or diamond like coating would give another high modulus material an almost chemically inert surface-perhaps not unrealistic for a 0.8 mm piston.

 

[NickE]

Also wouldnt a CVD diamond coating increase the modulus of a part much like sandwich construction in composites?

(I have some cantilever beam test samples that are much stiffer than comparable steel beams due to a TiN coating.)

 

[Fred]

Is it possible to evaluate the modulus increase of a layer of 2-3 µm. Could you precise your example.

An other problem could be the bad behaviour of such a hard coating against possible shocks.

 

[NickE]

Yes you can theroetically (IIRC.. I believe TVP and I discussed this a long while ago) use the thickness and modulus of the thin film and then use the shear modulus of the core matl to calculate teh bending stiffness. Obviously as the skins or core get thicker so does the beam.

I havent done anything quantitative with this info, there was some push originally to investigate fatigue life performance of TiN stiffened thin beams, but production work has pushed that to the side.

nick

 

[EdStainless]

Fred, there are a couple of ways to go with this.

If you have parts bending (yielding) that is because the strength is too low, not the modulus. There are a lot of good options at much higher strengths. You have selected about the softest iron based alloy around.

If the parts are flexing too much (but not yielding) then higher modulus will help. The highest stiffness engeneering materials that I have worked with are tungsten carbides and ceramics (SiC).

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[metman]

Molybdenum has about 3x the modulus of steel as I recall but what is the corrosive environment?