Chunky graphite

 

This defect mainly occurs in thick-walled castings made of nodular graphite cast iron, and there predominantly in slowly cooling zones (e.g. underneath exothermic feed devices, s. exothermic risers, or in riser necks). This defect may occur regardless of the molding and/or casting process. The mechanical properties, particularly ductility of the material are significantly affected by chunky graphite.

The defect is characterized by local, cellular arrangement of very finely distributed graphite with short, rounded flake, i.e. with a very high particle count that could be referred to as compact. The main part of the rest of the structure is roughly spheroid with regular occurrence of clusters of this local material degeneration in the thermal center of the cast parts. In cutting or fracture surfaces, the zones containing chunky graphite are visible as dark spots (not to be mistaken for dross). Another method to make this defect visible is metallographic grinding.

Equal to vermicular graphite, this graphite formation must be classified in the range between flake and nodular graphite. The coral-shaped composition that can only be distinguished in a micrograph as isolated individual graphite particles, can be made visible by means of deep etching (Figures 1 to 3).

 

 

 

 

 

Formation of chunky graphite is promoted with increasing silicon, copper, nickel, calcium, and Cer contents; elements such as lead, antimony, arsenic, bismuth, tin, and boron have a contrary effect.

Chunky graphite was predominantly found in parts where the raw material had very low contents of trace elements and Cer mix metals (Cer-MM) were used nonetheless.

The influence of nickel is so extensive that with austenitic nodular graphite cast ironchunky graphite occurs even in thin cross sections. Increased contents of carbon equivalents, such as CE > 4.1 %, seems to promote formation of chunky graphite, even without the addition of Cer-MM.

With silicon contents of less than 2.0 %, less chunky graphite was found. Calcium promotes formation of chunky graphite if it is added to the melting process at a later stage in the form of calcium-containing ferro-silicon. Therefore, addition of silicon for adjustment of the silicon content ought to be made in the solid charge before melting.

The spherulite counts in zones with chunky graphite is always rather low, obviously as a consequence of the low cooling rate in thick-walled parts despite effective inoculation treatment (s. Inoculation). With regard to formation conditions, various mechanisms, some contradicting each other, are discussed:

 

  • Local disturbance or destruction of the austenite layer encasing the primarily formed spherulites through thermal turbulences in the residual melt;
  • Bursting of large, coarse spherulites as a consequence of massive internal stresses;
  • Extensive branching of the graphite caused by segregation-related decrease of the boundary surface energy (mainly through calcium).

 

 

The encasing austenite layer is ascribed great importance in the formation of chunky graphite, with the layer being comprised of broken spherulites. If the austenite shell is to weak, it is possible for the spherulites to be destroyed and subsequently enter the inter-dendritic cavities.

Metallographic examination has revealed that both larger areas of very fine chunky graphite and smaller zones with coarser structures are present in the same casting. The major proportion of chunky graphite is present around the dendrite branches (s. Dendrite), i.e. in the inter-dendritic cavities.

It is particularly interesting to see that from a specific size the graphite spheres are broken up into spherical sectors, particularly in casting areas with very low solidification (Fig. 4). It is easily conceivable that such spheres disintegrate or brake if they are moved during solidification of the melt. If such spheres are present in the residual melt, they may be crushed to small pieces depending on the flow distances and the solidification duration. REM images confirm the thesis that chunky graphite particle are neither small spheres nor flakes but spherical sectors (Fig. 5).

 

 

There is no doubt that chunky graphite is generate through partly coupled growth of graphite and austenite and with regard to its morphology it is similar to vermicular graphite so that the two graphite states can be mistaken for one another. The main difference is that chunky graphite is much finer and only occurs in restricted zones.

Measures for prevention (acc. to S. Hasse, FT&E):

1. Chemical composition

 

  • In thick-walled castings, the carbon equivalent (CE = % C + 1/3 % Si) ought to be < 4.1 %;
  • If possible, silicon contents must be reduced (2.0 to max. 2.2 %); if nickel is used in the casting, silicon must be further reduced by 0.2 to 0.4 %;
  • In Cer-free austenitic nodular graphite cast iron, chunky graphite may be avoided if the iron complies with the following conditions:
    % C + 0.2 % Si + 0.06 % Ni < 4.4 %

 

 

2. Addition of specific elements

 

  • With pure charge materials, do not use Cer (Ce-MM);
  • The only one of the elements to suppress formation of chunky graphite (Pb, Cu, As, Te, Bi, Sb, Sn, B etc.) that is suitable without limitations is antimony. Additions of antimony in a range between 0.002 to 0.005 % hardly have any influence on the mechanical properties, i.e. change in structure.

 

 

3. Allow for directed solidification

 

  • Use cooling ingots and/or molding materials with great heat wetting capacities (zirconium sand or chromite sands).

 

 

4. Inoculation

 

  • Use Sb- or Bi-containing inoculation agents;
  • Add maximum 0.2 % of the inoculum;
  • Perform inoculation as late as possible in the process (mold inoculation or late stream inoculation); this helps prevent reduction of nodule counts with increasing wall thickness and resulting graphite degeneracy;
  • If possible, use calcium-free ferro-silicon.

     

    • Fig. 1: Cutting surface of a casting with chunky graphite areas, that can be clearly distinguished as dark spots, even with the unaided eye.
    • Fig. 2:  Metallographic grinding through the zone shown in Fig. 1, magnification to 100:1
    • Fig. 3: Deep etching of fine chunky graphite (source: FT&amp;E)
    • Fig. 4: Deep etching, disintegrated or broken graphite sphere
    • Fig. 5:  Exposure of chunky graphite surfaces
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