Test method for effect of niobium on eutectic temperature and as cast microstructure of ductile iron

Cast pig iron, ferrosilicon, ferromanganese, ferromolybdenum, pure copper and ferroniobium were used as raw materials. According to the target chemical composition (mass fraction,%) of 3.8-4.0 C, 2.7-3.2 Si, 2.3-2.8 Mn, 0.4-0.6 Mo, 0.4-0.7 Cu and 0.2-0.8 Nb, they were melted in medium frequency induction furnace at 1 450-1 500 ℃, The pouring temperature is (1 400 ℃) ± 20) ℃。 The spheroidizing agent is FeSi mg10re7, and the addition amount is 2.0%. First, the spheroidizing agent is broken into small pieces and put into the bottom of the treatment package, then the inoculant and perlite slag collector are covered on the spheroidizing agent, and 1 / 2 ~ 2 / 3 of the molten iron is flushed into it. At the end of the boiling of the molten iron, the remaining molten iron is flushed into it, and then the slag collector is added to remove the slag thoroughly. 75sife inoculant was used, and the addition amount was 1.5%. During spheroidizing treatment, half of inoculant is covered on the spheroidizing agent, and the other half is put in when hot metal is added. Before discharging, the chemical composition is detected. After all the charge is melted, a proper amount of molten iron is poured into  45 mm × 4 mm, and the chemical composition was analyzed by spectrometer.

After slag removal, the molten iron is injected into the mold (resin sand casting) and poured into the Y-shaped test block as shown in the figure. The arrow in the figure indicates the sampling position of the metallographic sample. The solidification cooling curve of molten iron was determined by thermal analysis method. After spheroidizing inoculation treatment, appropriate amount of molten iron was poured into the sample cup. Under the specific heat dissipation condition of the sample cup, the solidification temperature curve of molten iron in the sample cup was collected and recorded by thermal analyzer. Then the eutectic transformation temperature of ductile iron was obtained by deriving the curve with origin8.5 software.

Metallographic samples were cut from Y-shaped test block, polished and observed by optical microscope. The microstructure (graphite phase, spheroidization rate and matrix structure) of nodular cast iron was quantitatively analyzed by Image Pro Plus software.