DROSS IN DUCTILE IRON CASTINGS.

[ibf]

HELLO !

"DROSS" (Magnesium oxides and sulfides)is an undesirable inclusion in ductile iron castings. However, some foundries claim that it is not detrimental to the mechanical properties (which I DO NOT agree though). My questions are:

1. Does "dross" act as a crack - like indication on a rotating - bending fatigue test ? Any data would be very welcome ?

2. Have you experienced any kind failure due to "dross" in any type of application ?

Thank you all.

 

[arunmrao]

Dross defects occur in ductile irons due to improper removal before pouring.
There are traps/filters in gating,down sprue etc which do not let this enter the mold cavity. These get exposed during machining and the castings are rejected.
Dross like any other inclusion causes a discontinuity in matrix,hence can be considered as a stress raiser.

I am trying to locate some reference from Karsay's book on Ductile iron. Hope I locate it soon.

 

[arunmrao]

Here is an explanation on effect of dross inclusions.

http://www.ductile.org/didata/Section3/3part1.htm#Effect of Metal Cleanliness

Effect of Metal Cleanliness

Under bending and torsional fatigue conditions in which the cyclic stresses reach a maximum at the component surface, fatigue strength is reduced by the presence of inclusions, dross, and other surface defects which act as crack initiation sites. Figure 3.30 shows that increasing the volume fraction of non-metallic inclusions significantly decreases fatigue strength. The influence of non-metallic inclusions on fatigue strength increases as matrix hardness increases. The increasing use of Ductile Iron components with as-cast surfaces places an increased importance on the elimination of surface defects for applications requiring optimum fatigue strength.

The reduction of dross-related surface defects through the use of filters in the mold filling system can result in a 25 per cent increase in fatigue life, as shown in Figure 3.31. The use of good foundry practices, including minimizing residual Mg content, careful deslagging of ladles, good gating and pouring practices, the use of filters in the gating system and the reduction of the effects of flake-forming elements in both the metal and molding materials, can result in fatigue strengths for ascast surfaces that are within 5 per cent of those obtained on components with machined surfaces.

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[ibf]

Thank you very much arunmrao.