Figure 1 shows SEM morphology of pearlite in different parts of lzqt600-3profile. The pearlite clusters in the figure have obvious lamellar structure and are arranged in the same direction. The pearlite lamellae spacing is the smallest at the edge and the largest in the center.
The pearlite lamellar spacing observed by human naked eye is only qualitative description. In order to obtain more accurate data, the pearlite lamellar spacing of different parts is measured by Image J software, and the average value is obtained after multiple statistics. The results are shown in Fig. 2. According to the data in the figure, the pearlite lamellar spacing measured by software increases gradually from the edge to the center, and the value increases from 274 nm to 286 nm at 1 / 2R to 316 nm at the center, which is consistent with the result of human visual evaluation. The phenomenon of increasing pearlite lamellar spacing from edge to core is due to the different cooling rates of hot metal in the three parts, which leads to the different diffusion of atoms in the alloy. The larger the cooling rate is, the more unfavorable the diffusion of C atoms is, so the pearlite lamellar spacing from the edge to the center increases gradually.
Through artificial judgment and Image J software, the number of graphite spheres, shape factor, diameter distribution, pearlite content and lamellar spacing of lzqt600-3 ductile iron profile edge, 1 / 2R and center are qualitatively and quantitatively described. The following conclusions are drawn: from the edge to the center of the profile, the number of graphite spheres decreases from 318 / mm2 to 303 / mm2, and the equivalent diameter increases from 29.86 to 31.03 The shape factor of graphite ball decreased from 0.724 to 0.660. From the edge to the center of the matrix, the ferrite grain size increases and the amount of pearlite increases, and the lamellar spacing increases from 274 nm to 316 nm.
The reason for the difference of microstructure in three parts of lzqt600-3 ductile iron profile is due to the use of horizontal continuousin the profile manufacturing process. In this manufacturing process, the treated molten iron is poured into the holding furnace and solidified into a solidified shell with certain strength under the action of horizontal water-cooled graphite sleeve crystallizer, and then the profile is drawn and formed under the action of traction machine. The edge of the drawn profile is formed by cooling and solidifying the molten iron flowing through the inner side of the graphite sleeve. At this time, the rate is low and the nucleation rate is low. This is because the cooling mode of hot metal condensation in this part is air convection outside the mold or cooling by secondary spray mist. Therefore, from the edge to the center, the number of graphite balls decreases, the ball diameter increases, and the microstructure becomes coarse. In addition, the hot metal formed at 1 / 2R and the core stays in the mold for a long time, so that the graphite balls and grains have enough time to further grow up. Therefore, the number of graphite balls in these two parts is small, the diameter is large and the structure is coarse.
The increase of pearlite content and lamellar spacing from edge to core in lzqt600-3 ductile iron profile is due to the high cooling rate at the edge and low cooling rate at the core, which leads to the difference in the transformation time from austenite to pearlite. When the cooling rate at the edge is large, the time for molten iron to undergo solidification is short, and austenite is not enough to fully transform into pearlite. On the contrary, the cooling rate in the core is small, and the time for molten iron to undergo solidification is long, and austenite is more fully transformed into pearlite, so the number of pearlite in the core is the most. At the same time, the diffusion of each atom in the alloy is also affected by the cooling rate. When the cooling rate is high, the C atom is not easy to diffuse. On the contrary, when the cooling rate is small, the C atom is easy to diffuse, so the pearlite lamellar spacing in the center is the largest.