by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2007
high degree of nodularity. At low nodule numbers, the larger the graphite nodules the greater is the tendency for reduced nodularity. Unfortunately the effects of inoculation treatments are transient and they become reduced with time when the inoculated metal is held in the ladle before castings are poured. This is called "fading". As illustrated in Fig.1(c), fading in flake irons leads to a greater tendency to form chill, and to increases in eutectic cell size thus decreasing tensile strength. The effects of fading of three common inoculant materials in grey and ductile irons can be seen in Figure 4. It is seen that for both types of iron the rates of fading are highest immediately after inoculation. In ductile iron the reduction in nodule number with time will tend to promote less ferrite and more pearlite in the matrix. During the holding of treated metal intended for ductile iron production the problem of fading is even more serious since both Mg loss due to volatilisation and inoculant fade occur. If metal is held too long, without any back up treatment, this will result in unacceptable castings due to the presence of sub-nodular graphite. To reduce the effects of fading castings should be poured as soon as possible after inoculation treatment but often in production situations this cannot always be achieved. The problems of inoculant fade in flake and in ductile irons, the development of reduced fade inoculants, and of late treatments via pouring stream or in the mould techniques have therefore been and continue to be major areas for R&D, and for treatment technology improvements [4-7]. INOCULANT MATERIALS Foundries are often confused by the extensive range of both inoculants and nodularising agents that are available from the ferroalloy producers. Examples of some typical compositions are listed in Table 1. Most inoculants are based on ferrosilicons containing about 70-75% Silicon, or on ferrosilicon - graphite mixtures. In flake irons the normal levels of inoculant ladle addition raise the silicon content by about 0.2%, whereas in ductile irons larger additions are used, raising Si level by around 0.5%. Inoculant grades containing around 45-50% Si are also used where pick up of Si must be limited. Research into understanding the effects of inoculation, and into the development of more potent ferrosilicon compositions, has been continuing since the early 1960's. Important observations from some of this work [4-10] can be summarized as follows: • The effect of silicon on eutectic graphite nucleation and chill reduction is much more marked if the silicon is added as an inoculant than if it were just added to the furnace charge. • The relationship between graphite nucleation and chill reduction is not simple one in that inoculants giving the finest eutectic cells (high cell counts) do not always give the greatest chill reduction. Figure 3: Presence of eutectic carbides in thin section ductile iron.x750 Figure 4: Fading of inoculation effect on eutectic cell counts in grey iron and nodule number in ductile iron [5,7] Figure 2: Undercooled graphite formed in grey iron as a result of low melt nucleation (x1000) METAL Casting Technologies March 2007 17