Graphite Flotation 1

[kpedde01]

I need some technical help; I have a situation where I need to take decision on a casting with graphite flotation. So I would like to know if you have some information (an equation or something) to correlate mechanical properties and graphite flotation (from micro-structure), so that I can predict the drop in mechanical properties (roughly). I am looking for some kind of equation or empirical formula (if any) to correlate between micro-structure of the graphite flotation area to mechanical properties. The graphite flotation area was not cleaned even after machining. Or at least if you can share some literature(links) it would be or great help.

 

[arunmrao]

You are probably referring to Kish graphite or Type C graphite found in hyper eutectic cast irons. The primary graphite formed during solidification is large and tends to float upwards due to lower density. The remaining liquid metal is then lowered in carbon content. Machining of such castings leaves voids and results in por finish. The mechanical properties of this cast iron is also reported to be low.

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[MOB1]

I think that kpeddie01 may be referring to the flotation of graphite nodules in large nodular (or spheroidal graphite) iron castings. This is particularly common in large castings used for gear manufacture for ore grinding mills. Because of its prevalence the gears are often machined such that more metal is removed from the cope face to remove the maximum amount of metal containing excess nodules.
In my experience this phenomenon does not cause a loss of mechanical properties and hence the fatigue resistance should not be affected.
The effect is routinely observed by NDE inspectors who find that ultrasonic signals are highly attenuated in the zone containing excess nodules.
Finding literature describing the effect may prove difficult.

 

[kpedde01]

Hi MOB1,

Thanks for the answer but its not for gear, its for wind systems. And in the past we have experienced some failures due to graphite flotation in the components. But I was trying to see if there are any equations to correlate the graphite flotation to mechanical properties.

 

[TripleG1983]

Graphite flotation does not effect tensile poperties. It can reduce fatigue resistance marginally but i doubt it would cause a failure there maybe other factors that have caused this. If it has then your design criteria is too tight. This should be reviewed and fully investigated and understood. The main problem with graphite flotation is a poor surface finish.

You can predict whether graphite flotation has occurred in nodular irons by using an equation. With proper process control graphite flotation should not occur. It is not possible to use an equation to correlate graphite flotation to mechanical properties due to the metallurgy of the alloy.

 

[MeltLabGuy]

Carbon flotation in ductile iron is caused when the foundry allows the Carbon Equilivant to exceed the ability of the magnesium to suppress graphite formation in the liquid. My experience suggests that limit is about 4.55 - 4.60 with moderate levels of magnesium 0.030 to 0.035%. Spectrometers can vary some in their readings on carbon, and silicon, so the actual limit may be different for different foundries. I work with high definition thermal analysis, and you can actually see the graphite liquidus forming when this happens.