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. |
Originally Posted by DNMakinson
(Post 1522326)
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. 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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Originally Posted by DNMakinson
(Post 1522326)
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. |
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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