Technical Information 6

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

Fading of Inoculation

Inoculants for cast irons produce their effects by creating a marked increase in the number ofnucleation sites suitable for graphite growth during solidification of the eutectic. Thesebecome apparent as an increase in the number of eutectic cells in grey irons and in thenumber of graphite nodules in ductile irons. This results in a reduction in undercooling thusminimizing the risk of forming iron carbides or "chill" particularly in thin, rapidly cooledsections.

The effects of inoculation are at a maximum immediately after making the addition and theyfade with time. The rate of fading depends on :

(1) the inoculant composition

(2) the type of iron to which it is added

(3) temperature

(4) surface energies

(5) diffusion rates

Fading may be very rapid with much of the effect lost within the first few minutes after addition.Fading of inoculation can be explained by the coalescing and re-solution of the nucleipopulation which causes the total number of potential nucleation sites to be reduced (i.e.growth or coarsening of nuclei particles according to the so-called Ostwald Ripening effect).This behaviour is in close agreement with experimental observations of fading as illustrated inthe following figures.

Figure 1: Reduction in nuclei population (micro-inclusions) with time (left), and fading characteristics ofinoculation (right).

Principal Results of Fading:

(1) To cause greater undercooling to take place during eutectic solidification thus leading toan increased tendency to chilling in grey and ductile irons particularly in thin section.

(2) To reduce the numbers of eutectic cells or graphite nodules in the structure and to cause adeterioration in graphite form. Severe fading can promote the formation of undercooledgraphite with associated ferrite in grey irons and significant amounts of non-nodulargraphite in ductile irons, both of which can adversely affect the properties of the iron.

Some Well Established Facts Concerning Inoculant Fade:

4 The effects of all inoculants fade with time.

4 There is no period after inoculation during which zero fading occurs. To obtainmaximum effect the metal must be cast as soon as possible after inoculation, theultimate being addition of the inoculant to the pouring stream.

4 The effects of some inoculants fade more slowly than others depending on theircomposition and conditions of use.

4 An inoculant which gives a high eutectic cell number is not necessarily the mosteffective in reducing chill. In grey iron, Strontium-based inoculants are recognisedto give the best combination of a coarse cell structure, low shrinkage tendency, andvery low chill level.

4 Under any particular set of conditions it is not possible to predict the fadingcharacteristics of an inoculant from its composition. Foundries therefore shouldcarry out tests to determine which is the most suitable inoculant for their purpose.These tests should be made under careful control to avoid the spurious effects offactors such as metal temperature, inoculant storage, etc.