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Metallurgical defects


  • pg 1
									                                                                                                             Common Metallurgical
                                                                                                             Defects in Ductile Iron
 Compacted Graphite                                          Exploded Graphite                                                 Chunky Graphite                                                  Spiky Graphite

   Potential Causes;                                       Cause;·                                                        Cause;·                                                         Potential causes;
     Low residual magnesium and/or rare earth from           Excess rare earth additions, particularly when high            Excess rare earth additions, particularly when high             Very small amounts of lead which have not been
     poor nodularisation practice, high temperatures or      purity charges are used. Normally found in thick               purity charges are used. Normally found in thick                neutralised by rare earths result in spiky graphite.
     long holding time.                                      section castings or at higher carbon equivalents.              section castings or at higher carbon equivalents.              This has a catastrophic effect on mechanical
     Excess sulphur in the base iron not balanced by                                                                                                                                        properties.
     sufficient magnesium.                                 Note that once the rare earth addition is reduced, it may      Note that once the rare earth addition is reduced, it may         Bismuth, titanium and antimony can give similar
                                                           take time to dilute the residual rare earth in return metal,   take time to dilute the residual rare earth in return metal,      structures but can also be neutralised with rare
                                                           particularly in induction melted iron.                         particularly in induction melted iron.                            earths.

                                                              Flake Graphite
   Graphite Flotation                                        Surface Structure                                              Nodule Alignment                                                           Carbides

         Potential causes;                                 Cause;                                                          Potential causes;                                             Potential causes;
           High carbon equivalent.                            Excess sulphur build-up in moulding sand. This                 Low carbon equivalent·                                        Low carbon equivalent.
           Excess pouring temperature.                       causes reversion to flake as the magnesium in the               Under inoculation causing growth of large                     Excess magnesium and/or rare earth.
           Slow cooling rate in thicker sections.            iron reacts with the sulphur. The use of higher                 dendrites with nodules aligned between arms of                Carbide promoting elements such as Mn,Cr,V,Mo.
           Insufficient inoculation.                         magnesium / rare earth in the nodulariser or a cerium           the dendrite.                                                 Insufficient inoculation.
                                                             containing inoculant can overcome this.                         High pouring temperature.                                     Rapid cooling rate.

    Irregular Graphite                                            Slag Inclusions                                                        Shrinkage                                                             Gas

  Potential causes;                                            Potential causes;                                             Potential causes;                                           Gas holes are a totally separate subject and can be
    High holding temperature.                                    Inadequate slag control from pouring system.                  Insufficient mould strength causing dilation.             influenced by melting procedures, metal handling,
    Long holding time which can result in "dead" irons.          Lack of slag traps or filter.                                 Inadequate feed metal available.                          refractory conditions,mould moisture,sand
    Poor inoculation or excessive fading of                      Low pouring temperature.                                      Poor gating design.                                       permeability,core resins,hot spots near cores,slag
    inoculation.                                                 Excess additions of slag forming materials.                   Excess magnesium.                                         handling,metal temperature,base metal
  Graphite shape may be improved by a late addition of a         Turbulent mould filling.                                      Low carbon equivalent.                                    composition,mould venting, Ti and Al contents amongst
  powerful speciality inoculant.                                                                                               Under inoculation or over inoculation.                    other sources.
                                                                                                                                                                                         Specialist advise should be sought in cases of gas once
                                                                                                                                                                                         the above have been investigated.
                                                                                                                                                                                         The photomicrograph given here shows dendrite arms
                                                                                                                                                                                         growing inside a hydrogen pinhole.

Elkem produces a complete range of nodularisers and inoculants for the treatment of ductile
iron. MgFeSi alloys are available for applications such as sandwich treatments, tundish                                                                               Revised September 2004 © Copyright Elkem ASA
cover ladles and flow through systems. They are graded so as to be suited for different ladle sizes.
Products for in-the-mould processes are also available.

Ultraseed® inoculant is designed for pure Mg produced ductile iron, irons of low oxygen and
sulphur levels and dead iron which has been held for extended periods of time. This
inoculant will give high nodule counts in ductile iron which is normally difficult to inoculate
and will help to prevent micro-shrinkage.

This is one example of the new generation of Elkem inoculants.

Please refer to your local Elkem representative for further information on the range of Elkem
products for grey, compacted and ductile cast irons.

Ultraseed® is a registered trademark of Elkem.                                                                                                                                    www.foundry.elkem.com

Elkem ASA, Foundry Products Division, Hoffsveien 65B, PO Box 5211 Majorstua, N-0303 Oslo, Norway. Tel: +47 22 45 01 00 Fax: +47 22 45 01 52

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