Material Question

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mac_campshure
Posts: 50
Joined: Tue Jan 14, 2003 9:34 am

Material Question

Post by mac_campshure »

What is spearoidal graphite iron casting.
thanks mac_______
jpfalt
Posts: 982
Joined: Sun Jan 05, 2003 12:55 pm

Re: Material Question

Post by jpfalt »

Spheroidal graphite cast iron is one which has either been heat treated or alloyed to have the graphite form into spheres rather than flat flakes.

Typical gray cast iron has around 4% carbon. Only less than 1% can actually stay dissolved in the ironwhen it's in the annealed condition, the rest separates out as iron carbide or as graphite. In gray cast iron, the graphite forms into thin sheets separated by thin layers of low carbon iron.

Two methods are used to cause the graphite to form into spheres.

The first is to rapidly solidify and quench the cast iron from liquid to solid. As quenched, all of the carbon is dissolved in a very hard iron carbide form called white cast iron. It is very hard and very brittle. The white cast iron is then tempered and the carbon comes out of solution and forms irregular shaped globs of graphite surrounded by relatively large areas of low carbon iron. This is called maleable iron and has much higher strength and much higher ductility than gray cast iron.

Another approach for heat treatment is to take the gray cast iron and hold it at high temperature to cause the iron to go back into solution and slowly cool it so that the carbon comes slowly out of solution to form larger globs of graphite. It is not as effective in getting rid of all the graphite flakes and is less ductile than maleable iron.

The other approach is to add something to the liquid iron to make it grow globs of graphite quickly. the material is magnesium and it is stirred into the iron when the iron is transferred from the melting furnace to a pouring ladle. When cooled in the mold, the magnesium attracts the carbon into spherical globs of graphite. The resulting iron, called ductile iron, has the strength and ductility of maleable iron without the difficulty of quenching and heat treatment.
mac_campshure
Posts: 50
Joined: Tue Jan 14, 2003 9:34 am

Re: Material Question

Post by mac_campshure »

Thanks for the clear explanation.
I take it this is a strong casting and can be made lighter with an alloy.
thanks mac_________
jpfalt
Posts: 982
Joined: Sun Jan 05, 2003 12:55 pm

Re: Material Question

Post by jpfalt »

All of the methods produce parts of about the same weight. The magnesium added to make ductile iron is just a fraction of a percent and causes no significant change in casting weight.

The strength of the final part for maleable and ductile is about the same as a low carbon steel, although I know that the strength of ductile can be increased with alloying material, manganese, nickel, chrome, etc. I used to work in an iron foundry and we had some ductile iron parts for lift trucks that were above 80,000 psi tensile due to alloying of the melt.
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