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Metallurgy- Can you directly compare the physical properties of ductile cast irons?

ManualEd

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Possibly OT? I wasn't sure if this belonged here, or in the Metrology section.

I'm trying to replicate small parts from an obsolete cooling fan that are made from 80-55-06 ductile cast iron.
Partly because cheap, partly to minimize vibration.
Aluminum blades bolt to the cast iron.

The customer is installing VFD's on fans that are designed to run continuously, to save power by turning the fans off when not needed.
OEM VFD proposals from the 1990's stated that most parts will have to be changed every 10000 spin up cycles due to the fatigue life of the parts, which matches what I've seen as pretty typical design specs on fans.

I'm considering replacing these near-net 80-55-06 castings with an 110-70-11, or 130-90-09 cast austempered ductile iron block, then machine the shape out of the block to minimize the effect of the casting scale and imperfections. (the austempering may be done after rough machining if necessary)

My question is: Can you directly compare the physical properties of different ductile cast irons?
If the tensile, yield, and elongation all greatly exceed the original material, are there any properties of the 130-90-09 austempered cast iron that may be worse than the 80-55-06 ductile iron?

I also have the same question for aluminum: Are there any properties of a 7075-T6 alloy that may lead to a lower fatigue life than a 5052 cast fan blade?

Thanks!
 
you may be overthinking the cast iron dampens vibration part. Balancing the blades may be a better option.
The entire assembly gets checked dynamically balanced a few times a year if it needs it.

I don't want to change too much about the assembly without knowing if the cast iron serves any purpose other than it was a cheap way to make a semi-complex form back in the 80's without every garage having a CNC.
 
Damping Capacity is the measure to compare between materials. Automotive research is probably where most of the data is.
 
I would want to be careful working on a machine with known fatigue-failure problems, unless I was sure there was a very well-managed maintenance program.
Castings are usually used to get near-net shape at low cost. You could probably get WORSE fatigue life out of a part CNC-milled from wrought material if you neglect its anisotropy.
Where does it fail in fatigue? Are the limiting stresses from unbalance, flutter, acceleration torque??
 
My question is: Can you directly compare the physical properties of different ductile cast irons?
If the tensile, yield, and elongation all greatly exceed the original material, are there any properties of the 130-90-09 austempered cast iron that may be worse than the 80-55-06 ductile iron?
The number you're looking for is fracture toughness, which can be tested using the Charpy v-notch test.

There are two parts to the equation, however. Fracture toughness and the starting size of the material's largest defect/inclusion. The idea here is that the defect grows over time through stress/destress cycles. The higher the fracture toughness, the slower it grows. Eventually, the defect reaches a critical size, causing the part to fail.

Since it's difficult to predict where this defect might be located within the material, engineers play it safe and assume a worst case scenario where it's in the area of highest stress concentration.

As for the starting defect size, that's based on the material composition and how it's made. Cast iron is air melted, so defects are going to be relatively large, regardless of grade. Smaller defects can be found in steels that are VIM (vacuum induction melted) and VAR (vacuum arc remelted).

Actually there's also a third part to the equation, which is highest stress concentration. If this part was designed in the 90s, it could likely be improved in geometry using FEA analysis. Ultimately it's up to the engineers to redesign this part, using a combination of FEA and material change, maybe 4340 VAR.

As for the aluminum blades, probably much less of a concern because the area of highest stress concentration in a solid spinning object (as opposed to a ring) is the hub. That said, an improvement to the shape of the blades using more advanced software could help the fan deliver higher air flow at lower RPM, which in turn could also reduce the max loading on the hub.

It's not inconceivable that a hub could be designed with virtually infinite life, relative to the life expectancy of the rest of the fan. However, past a certain point, e.g. 20-30 years, the bean counters get involved and figure the time value of money negates the cost justification of a product with infinite life.
 
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As orange said, there’s 2 kinds of analysis, chemical, when making CI it’s optical emission spectrometers, combustion analysis ( carbon and sulphur LECO) XRF
Notably foundry’s don’t sell by analysis, neither do steel makers, they sell by mechanical properties, UTS elongation hardness low temp impact strength ( izoid or charpy) even grain size etc etc trick is to try to replicate what you originally had, unless there is an overwhelming reason to change, most hubs I’ve seen were steel, worked ok at least all the big fans like 15’ diameter ID fans, they were encased in concrete and had plant condition monitoring for vibration, hubs never failed, but more than a few blades did, quite a bang, blade vibration and fatigue was the killer so there were special balancing guys from the fan maker who spent a fair while swapping blades, fileing bits off and all sorts above my head,
Interestingly we had to ring the power plant energy controller before starting a fan as the bugger would trip the switch yard, apparently Mwatts of electricity to run, amazing , clever guys
Mark
 








 
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