CAST IRON INOCULATION - Elkem

More documents

Recommendations

Info

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

7.2 Coarsening of Inclusions It has previously been discussed that graphite nucleation occurs from nonmetallic inclusions in the melt. A significant coarsening of these inclusions occurs within the time interval between inoculation and solidification of the cast iron. This coarsening of inclusions will result in a reduction in the inclusion number density, consequently reducing the graphite nucleation frequency. Hence, the fading of inoculation can be explained by this coarsening of the inclusion population with time. Due to the coarsening, the total number of possible nucleation sites for graphite during solidification is reduced. Figure 9 shows a plot of the number density of inclusions in cast iron as a function of holding time after inocu lation. 7.3 Effects of Various Inclusions Inoculants lose their ability to reduce chill and nucleate graphite if the metal is held for extended periods before casting. However, inoculants have different fading characteristics. The barium-based Barinoc ® inoculant produces a high initial number of nucleation sites throughout the holding period, thus making it an excellent inoculant for ladle treatments. Foundrisil ® inoculant is an effective chill reducer for both low and high sulphur grey iron as well as ductile iron. Another effective inoculant that maintains the inoculation effect is the strontium-containing Superseed ® inoculant. Figure 10 shows the fading characteristics of some inoculants in cast iron. Figure 9: Coarsening behaviour of inclusions in liquid cast iron during holding. Figure 10: Fading characteristics for various inoculants in cast iron. 8. Inoculation and Cast Iron Properties 8.1 Inoculation and Strength Inoculation increases the number of eutectic cells (or nodules) which leads to a finer structure of the iron, and in particular, this will cause an increase in tensile strength in hypoeutectic irons. Figure 11 shows the increase in tensile strength by adding an inoculant. 8.2 Inoculation and Machinability Inoculation increases the number of potent nuclei that will promote graphite nucleation at low undercooling. Improved machinability is achieved by inoculation suppressing the formation of hard un-machinable white iron structures. Inoculation also reduces section sensitivity. While uninoculated irons will show a wide variation in hardness, inoculated grey or nodular cast irons will show more consistent hardness values over a wide range of sections, Figure 12. Figure 12: The wall thickness sensitivity of (Brinell) hardness can be reduced by an inoculant (partly calculated from Rockwell -B* and -C**). Figure 11: Increasing inoculant additions improve tensile strength. The final analyses of these trial melts are identical after inoculation.
9. Inoculation and Shrinkage The solidification of grey iron is characterized by the formation of a skin type array of eutectic cells at the mould/metal interface, followed by the development of eutectic cells ahead of the advancing solidification front. Newly formed graphite compensates partly or fully for the liquid iron contraction, provided it precipitates within a relatively rigid “skin”, which is charac teristic of uninoculated grey iron. How ever, if the mode of solidification is changed, the good shrinkage characteristics can be jeopardized, especially if a rigid skin cannot be developed at the mould/metal interface leaving the mould directly exposed to ferrostatic pressure. Eventually, the mould may yield under the ferrostatic pressure from the remaining liquid, and the increased volume of the mould cavity becomes too high for compensation by graphite precipitation at the end of solidification. Some shrinkage may occur as a result of excessive dilation of the mould although mould geometry will have an influence. Unfortunately, inoculation changes the mode of solidification in such a way that the rigidity of the “skin” is decreased. Inoculant additions should not become excessive to avoid shrinkage and yet the addition should be adequate to ensure “grey” solidification. Test specimens, Figure 13, show that for an equivalent chill depth, the eutectic cell count will be lower when using Superseed ® inoculant in place of foundry grade ferrosilicon. The lower cell count reduces the ferrostatic pressure on the mould and improves the tendency to avoid shrinkage defects. Since the eutectic cell count for nodular cast iron is much higher than for grey iron, one would expect a greater shrinkage tendency, and it is interesting to see that the solidification pattern is in fact similar to over-inoculated grey iron. Ultraseed ® inoculant has proven highly successful in providing fresh nucleation sites to ductile irons of long holding time where the base iron or magnesium treated iron have been held for prolonged times before addition of the post inoculant. Such long hold times are well known to reduce the overall capabilities of the iron prior to inoculation resulting in so-called “dead” iron. Ultraseed ® inoculant will thus reinstall good nucleation effectiveness from reactions with its sulphur and oxygen content forming new nucleation sites. Due to the powerful effects of Ultraseed ® inoculant on raising nodule count and improving chill protection, it has been found that the tendency to shrinkage formation is also reduced with this inoculant. Especially, the type of shrinkage that often occurs as small porosities in hot-spot sections of the complex castings; appear to be effec tively reduced or even eliminated by Ultraseed ® inoculant. Figure 14 shows an example of microshrinkage porosity that has been minimized by the use of Ultraseed ® inoculant. Figure 13: Comparison of the eutectic cell count in 5 mm sections at about equal chill depth (from BCIRA). Figure 14: Example of micro-shrinkage prorosity in ductile iron part that has been minimized by Ultraseed ® inoculant (left), compared to manganese-zirconium containing inoculant (right).
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 and 8: 5.5 Specification of Inoculants The
Page 9: Table 5: Eutectic cell count (30 mm

CAST IRON INOCULATION - Elkem

Create successful ePaper yourself

Delete template?

Save as template?