hardening of 4340 for ultimate strength 2

[kristoffon]

Hello, I'm designing a flywheel for storage of 7 MJ. Currently I'm achieving that spinning at 14500 rpm a 500mm diameter (20 inch) x 120mm (4.7 inch) 4340 cylinder.

I'm assuming I can harden it to 2020 MPa (293000 ksi) ultimate strength according to this matweb datasheet:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=b5fe87c8cdde4431b62ad990d4f2042c

FEA shows I'm working with a factor of safety of 2.6.

My questions:

- Is that ultimate strenght value realistically achievable?
- Is my factor of safety adequate?

Thank you.

 

[metengr]

kristoffon;
You should be designing to the yield strength and/or shear strength properties in tension with a suitable factor of safety against plastic deformation, versus using ultimate tensile strength.

I would say 2.5 for a design safety factor for flywheel application (rotating application) is suitable.

Will these flywheels be in concrete vaults underground?

 

[kristoffon]

metengr,

Yes, actually I'm desiging against yeld strength, I only copied ultimate strength as an example.

My idea for containment would be encasing it in a 25mm (1 inch) thick steel cylinder, above ground, a single hole in one side for the axis connecting to an eddy current clutch for energy discharge. Would you consider that containment inadequate?

Thanks.

 

[metengr]

kristoffon;
Yes. I wish I could provide more technical information but I was involved with a recent evaluation of a flywheel energy storage system.

 

[tbuelna]

kristoffon,

Using 4340 alloy steel hardened to 293ksi Ftu (Rc 54) is probably overly optimistic. Take a look at the published data for aircraft quality vacuum melt 4340 (AMS 6414) here:

http://latrobesteel.com/technical_datasheets.cfm

This manufacturer gives a recommendation for oil quench and temper at 400degF to achieve max tensile properties of 276ksi Ftu/222ksi Fty/11% elong.

As for a FS of 2.6 on your analysis, the appropriate FS should be that defined by whatever design or regulatory standard is covering your flywheel's installation. And does your analysis show a positive margin based on that FS? A FS of 2.6 is fairly conservative if your analysis is done carefully. A thorough stress analysis of a high speed flywheel structure must take into account all of the combined dynamic loads, along with the appropriate Kt factors.

Hope that helps.
Terry

 

[kristoffon]

Thank you tbuelna I also thought it was too good to be true.

 

[Tmoose]

"My idea for containment would be encasing it in a 25mm (1 inch) thick steel cylinder, above ground,..........."

probably not

http://www.testdevices.com/white_papers_pdf/spinSafe_art_TD2w.pdf

http://www.testdevices.com/white_papers_pdf/accident_art_TD5w.pdf

Dan T

 

[WKTaylor]

Tmoose's post is a valuable reminder that estimates of damage potential are usually waaaaayyyy off... and that containment of a high energy catastrophic failure can be very difficult.

Aero engine manufacturers have a heavy load to bear when it comes to "containment" of rotating disc & blades to prevent catastrophic airframe damage. Debris is not permitted to penetrate fuselages nor wing/empennage structure. Debris can spew-out forward or aft (if it has a high probability of not striking the airframe)... so only partial containment is necessary. The worst offenders are the large fan blades and discs. Imbalance loading after a failure must also be dealt with... along with the potential for fire/explosion, searing-hot-air leaks and electrical arcing/sparking. These problems are NOT trivial and require more than intuition and R-O-Ts to solve.

I recently ran across the following graphic demonstrations of tire inflation safety cages for aircraft tire/wheel Assys. The problem of containment is just as severe and non-intuitive... with the complicating factor of the instantaneous expansion of the contained pressurized air [nitrogen preferred]. The videos on the following website over-inflate aircraft tires deliberately, to test various protective cage designs. You think this is a simple problem, yet existing cage designs failed miserably [IE: spectacularly/catastrophically]. Click on the corresponding image for a detailed short video.

http://www.alberthaviation.com/TireCageVideos.htm

Regards, Wil Taylor

 

[SMF1964]

High strength usually goes hand-in-hand with lower toughness. Before you build this, I would strongly suggest that a fracture mechanics analysis is needed to determine the critical flaw size for the material/strength conditions. Heat treating your materials to that high a hardness (Rc 54) will also make them hghly susceptible to stress-corrosion cracking and/or hydrogen embrittlement failures, even under mild humidity conditions.

I also agree with Tmoose's statement that your protections system will "probably not" be adequate.

The truck tire videos illustrate the amount of energy contained within a volume of compressed air. I suspect that the energy contained within your flywheel to be similarly significant. If your flywheel breaks off a piece from the edge of your wheel, how much energy is that piece containing that is now traveling at 14500 rpm x 3.1416*20 inches (>800 miles per hour if I do my math correctly)? If your wheel breaks into pieces completely ... 1" of steel plate will not contain it.

 

[metengr]

As I mentioned above you should have a concrete vault for containment in addition to using a steel housing for vacuum. You certainly are not going to operate a high speed flywheel in air with windage. There is much more to flywheel energy storage systems in terms of what is discussed above.

 

[kristoffon]

Gentlemen thanks again for the advice.

I'm not planning on using vacuum because I don't want to store the energy for any amount of time. Charge it and use it when full, then charge it again.

I figure I can build the flywheel with integrated motor and eletromagnetic clutch for less than half what a 400 hp motor with PLC control will cost me.

It's a hobby project. It's for a glider launch winch. Power is needed for 20 seconds only.

As for containment I might just overbuild it -- if I could be sure it would be safe. 10 inch concrete shell with 1 inch steel wall perhaps? It does have to be above ground so that we can move it.

If I have to destroy a flywheel to test the containment then it won't be cheaper anymore, though.

 

[TemperedMartensite]

You may want to investigate a maraging type of steel, such as C300, C350. I would have much more confidence in those alloys.

 

[kristoffon]

Thanks TempMart for the tip but maraging steels are very hard to find in my area in large sizes and I wish to stick to readily available materials like 4340...

 

[TemperedMartensite]

Ahhh yes... now I see, you are making a 20" disc. That certainly limits your choices.

 

[Metalguy]

Me, I'd just use a big-block Chevy.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein