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Grey Cast Iron Question: Crack Propogation

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Grey Cast Iron Question: Crack Propogation

 
Old 02-11-2019, 08:31 AM
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Default Grey Cast Iron Question: Crack Propogation

I have no real experience with cast irons, thus the search for someone with some specific knowledge.

In a situation that is NOT subject to vibration: Grey cast iron (flake graphite): ASTM 126 Class A, which is a rather low end tensile grade, or ASTM A48 Class 30 (a little stronger).

If a surface crack in formed due to over-stress, is it likely to propagate through the body of the part over time / temperature cycling?

One theory is that the graphite inclusions make grey full of crack nucleation points whether there is an actual crack or not, thus a crack does not change the fracture characteristics appreciably. Opposing theory is that; of course a specific crack will propagate due to additional stress concentration.

Scholarly documentation welcome, as well as experiences.

Last edited by DNMakinson; 02-11-2019 at 08:52 AM. Reason: Incorrectly called out A43 instead of A48
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Old 02-11-2019, 11:31 AM
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Originally Posted by DNMakinson View Post
I have no real experience with cast irons, thus the search for someone with some specific knowledge.

In a situation that is NOT subject to vibration: Grey cast iron (flake graphite): ASTM 126 Class A, which is a rather low end tensile grade, or ASTM A48 Class 30 (a little stronger).

If a surface crack in formed due to over-stress, is it likely to propagate through the body of the part over time / temperature cycling?

One theory is that the graphite inclusions make grey full of crack nucleation points whether there is an actual crack or not, thus a crack does not change the fracture characteristics appreciably. Opposing theory is that; of course a specific crack will propagate due to additional stress concentration.

Scholarly documentation welcome, as well as experiences.
Full disclosure, I'm not a metallurgist...

But, my experience is a crack will follow the stress path once it starts until a. there is no more stress (unlikely) or b. no more material to crack (more likely). In situations where a repair is allowed it's been customary to repair by drilling a hole at the end of the crack which typically will stop the crack from going further (disperses the stresses). This is typical in CI steel mill rolls that get a lot of heat cycling. Once they get a crack started it typically keeps going.
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Old 02-11-2019, 12:12 PM
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Originally Posted by DNMakinson View Post
I have no real experience with cast irons, thus the search for someone with some specific knowledge.

In a situation that is NOT subject to vibration: Grey cast iron (flake graphite): ASTM 126 Class A, which is a rather low end tensile grade, or ASTM A48 Class 30 (a little stronger).

If a surface crack in formed due to over-stress, is it likely to propagate through the body of the part over time / temperature cycling?

One theory is that the graphite inclusions make grey full of crack nucleation points whether there is an actual crack or not, thus a crack does not change the fracture characteristics appreciably. Opposing theory is that; of course a specific crack will propagate due to additional stress concentration.

Scholarly documentation welcome, as well as experiences.
Requisite not-my-area disclaimer aside, it seems to me that accurately quantifying lifetime for the part will be a bit of a duesy given how sensitive thermo-mechanical fatigue crack growth in cast iron appears to be with respect to precise temperature for the loading cycles and morphology of the pearlite and graphite matrices throughout. Not that it can't be done, but having relatively good data about the cycling/material doesn't seem like it would be trivial from my skimming. A lot of published work I found was a bit more focused on propagation starting from microcracks, but I dug up a couple that were a bit more general which might have a helpful reference or two.
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Old 02-11-2019, 01:41 PM
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It has been about a million years since I worked in an iron foundry or took materials science in school, but my recollection is that plain grey iron is pretty bad for resistance of fatigue crack propagation. The foundry I worked in had specialty nickel alloys for just this purpose- I think the one was call NiResist?
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