CAST IRON INOCULATION - Elkem

More documents

Recommendations

Info

$ageing of refractory castables; which interactions exist and ... - Elkem$

6. Control of Inoculation Although nuclei cannot be observed directly at solidification temperatures, they have an effect on some properties which can be measured by: • recording cooling curves • measuring depth of chill in chill wedges • counting the number of eutectic cells • counting the number of graphite nodules 6.1 Cooling Curves Cooling curves record the changes in temperature with time as a consequence of a change of energy within the system. A deviation from normal cooling indicates the occurrence of a source of heat such as the heat of crystallization released by a precipitating phase. The location of the inversion points on the generally S-shaped cooling curve in the region of eutectic crystallization indicates the tendency of the melt to solidify “grey” or “white”. A high level of nucleation promotes a higher arrest temperature which, by avoiding the white eutectic, will result in less risk of carbide formation. Conversely, when the inversion point is at a low level on the cooling curve, there will be a tendency for cementite to precipitate instead of graphite giving a “white” structure. An increased cooling rate, as found in thin sections, will increase the degree of undercooling that must be balanced by an increased number of active nuclei to avoid the formation of white iron. In the iron-carbon system there is only a 7 ºC interval between “grey” solidification and sufficient undercooling to cause “white” solidification. In Figure 7 the cooling curve for an uni noculated reference melt is compared with a curve from a melt inoculated with 0.25% inoculant addition. The uninoculated melt shows inversion at 1145 ºC whereas inversion occurs at 1162 ºC for the inoculated melt. This means that the uninoculated melt is undercooled by 20 ºC and the inoculated melt by 3 ºC, which gives “white” and “grey” solidification, respectively. Figure 7: Solidfication curves for uninoculated ductile iron (a), and inoculated ductile cast iron (b) (30 mm section size). 6.2 Chill Testing The traditional method to determine the tendency of a melt to solidify “grey” or “white” is by examining chill wedges. The larger the zone of white iron, the fewer the number of nuclei that were active in initiating a “grey” solidification. Figure 8 shows chill wedges from a foundry which had an average 11.2 mm of chill for a period of two week on uninoculated cupola iron. By adding 0.2% FeSi (85% Si), the average chill depth was reduced and with 0.125% Superseed ® inoculant addition, the chill depth was reduced even further. 6.3 Eutectic Cell Count The number of eutectic cells in grey iron can be determined on etched microspecimens. If an effective inoculant has been added to the melt, there will be a large number of active nuclei to promote graphite precipitation at low undercooling during solidification. This will be represented on the micro-specimen by a high cell count for grey iron and a high nodule count for ductile iron. Table 5 shows the result of cell counts after inoculation. The eutectic cell number increases as the inoculant addition to the base melt is increased. Other factors, such as over-inoculation leading to shrinkage proprensity, will influence the optimum inoculant addition.
Table 5: Eutectic cell count (30 mm round bars). Figure 8: Cupola melted grey iron; no inoculation (left), inoculated with 0.2% FeSi85% (centre), and inoculated with 0.125% Superseed ® inoculant (right). 7. Fading of Inoculation 7.1 Principle Effects The effects of inoculation are at a maximum immediately after the addition of the inoculant. The rate of inoculant fading, which depends upon the composition of the inoculant and the condition of the iron to which it is added, may be very rapid and much of the inoculating effect may be lost in the first few minutes after the addition. The principal effects of fading are: • to cause greater undercooling to take place during eutectic solidification and to lead to a greater tendency to chilling in grey and ductile cast irons, particularly in thin sections; • to reduce the number of nodules formed in ductile iron and to cause a deterioration in their shape. If suffic ient ly severe, the deterioration in shape may affect the mechanical properties of the casting; • to reduce the number of eutectic cells growing in flake graphite irons result ing in a less uniform size distribution of graphite in the casting and a reduction in mechanical properties. There are some well established facts concerning fading which are of practical significance: • all inoculants fade; • there is no period after inoculation during which fading does not occur. To obtain the maximum effect, metal should be cast as soon as possible after the addition of inoculant; • some inoculants fade more slowly than others; • inoculating effects vary according to inoculant composition. It is desirable that foundries should carry out tests to determine which is the most suit able inoculant for their purpose.
Page 1 and 2: CAST IRON INOCULAtion THE TECHNOLOG
Page 3 and 4: The microstructure of an iron casti
Page 5 and 6: 5.2 Constituents of an inoculant Mo
Page 7: 5.5 Specification of Inoculants The
Page 11 and 12: 9. Inoculation and Shrinkage The so

CAST IRON INOCULATION - Elkem

Create successful ePaper yourself

Delete template?

Save as template?