Technical Information 13

Elkem ASA, Silicon Division

Postal address: Office address: Telephone: Revision No. 2P.O.Box 5211 Majorstua Hoffsveien 65 B 47 22 45 01 00 14.03.1997N-0303 Oslo Oslo TelefaxNorway 47 22 45 01 52

Compacted Graphite Iron

Compacted graphite irons (CG-irons) are a range of cast irons having mechanical andphysical properties intermediate between those of flake graphite and nodular graphite castirons. They are of interest to engineers because of their useful combination of strength,thermal conductivity and other properties. They have good founding characteristics and theirproduction requires controls similar to those applied in the manufacture of ductile ironcastings.

This Information Sheet summarizes a successful production route for CG-iron that has beendeveloped by Elkem, as well as examples of structures and properties for such materials.Treatment is based on a high rare earth containing magnesium-ferrosilicon alloy designedspecially for compacted graphite iron production. No introduction of such elements as titaniumor nitrogen is necessary with the present alloying concept.

Recommended composition of treatment alloy :

Si 44 - 48 %Mg 5.0 - 6.0 %Ca 1.8 - 2.3 %RE 5.0 - 7.0 %Al max. 1.0 %Balance iron

Recommendations for Compacted Graphite Iron Production

Base Iron CompositionBase iron composition should preferentially be hypereutectic with a carbon equivalent ofabout C.E. = 4.3 - 4.5. Impurities should be kept low. Suggested base iron composition is:

C 3.7 - 3.9 %Si 1.5 - 2.0 %S 0.005 - 0.010 %P max. 0.03 %

Alloy AdditionThe addition rate of the alloy described above has to be adjusted according to base metalcomposition and treatment process for each individual foundry. Base metal sulphur level andtreatment temperature are important parameter that strongly affect the required alloy additionrate. Experiences with the base iron composition given above have shown best results foraddition rates between :

0.3 - 0.4 weight percent addition of alloy specified above

The addition method may be an ordinary ladle treatment process as for ductile iron production(sandwich or tundish ladle).

Inoculant AdditionIt is recommended to add a high efficiency inoculant for production of CG-irons. Barium andStrontium containing ferrosilicon inoculants are found to give good results. Inoculant additionrate should be similar to that for ductile irons, i.e. between 0.3 and 0.8 wt percentage. Thehigher levels giving a fine distribution of compacted graphite throughout castings and lesssensitivity to carbide (cementite) formation.

MicrostructuresThe Figures below shows examples of microstructures obtained for a CG-iron castingproduced from a high RE-containing MgFeSi-alloy. Structures in both thin (5 mm) and thick(35 mm) section sizes are given to show the limited section sensitivity obtained.Homogeneous CG-structures throughout all section sizes are a characteristic featureobtained by the high RE-containing alloy in comparison to an ordinary lower RE-containingMgFeSi-alloys.

(a) (b)Figure 1: Microstructure of compacted graphite iron produced from a high RE-containing MgFeSi-alloy, (a) 5 mm section size, (b) 35 mm section size (100X).

CG-Iron Composition

Example of final CG-iron chemical composition obtained from the given parameters:

% C % Si % Mn % P % S % Ti % La % Ce % Mg

3.8 2.5 0.20 0.02 0.007 0.012 0.008 0.015 0.010

Mechanical PropertiesMechanical properties of CG-iron will of course be affected by the ferrite/pearlite-ratio, but fora predominantly ferritic grade properties may be (from 25 mm diameter separately cast testbar) :

Yield strength

(MPa)

Tensilestrength

(MPa)

Elongation

(%)

310 - 340 340 - 400 3 - 6

For more information, see the Elkem Product Data Sheet “CompactMag”.