Cast Structure and Structure of High Silicon Molybdenum Ductile Iron for Automotive Exhaust Manifold

The figure shows the original as-cast structure of high silicon-molybdenum ductile iron. From Figure a, it can be seen that the spheroidizing effect of this cast alloy cast iron is better, a large amount of graphite is evenly distributed on the matrix and grain boundary, and the graphite presents mainly spherical shape, sometimes polygonal or worm-like, with high spherical graphite roundness, which meets the engineering application standards.From the observation diagram b, it is found that the diameter of graphite is about 10-25 um, and there are about 3-6 graphites in the area of 100 um x 100 um.At the same time, the microstructure of high silicon molybdenum ductile iron mainly consists of white matrix, spherical graphite and gray granular material.Graphite and gray particulate matter are mosaic on the matrix and aggregate at the grain boundary, and gray particulate matter is the main aggregation point of many grains.These spherical graphite and grey particulate matter hinder dislocation movement to some extent during deformation and are beneficial to improving high temperature properties of cast iron.

In order to further clarify the structure and composition of as-cast materials, XRD phase analysis was carried out. The test results show that there are only five different diffraction peaks in the range of 20 to 100 degrees when 2Theta angle changes. Among them, there is one strong diffraction peak when 2Theta equals 44 degrees and the other four are relatively weak.The PDF comparison shows that the structure consists of ferrite and ductile iron. Although the high silicon molybdenum ductile iron also contains 4.7% Si, 1.1% Mo and 0.6% Mn, n o other substances are found i n the diffraction results.In conjunction with the previous figure, it is shown that the white matrix with more distribution in the structure of the material is ferrite.

In order to further determine the as-cast structure of high silicon-molybdenum ductile iron, the as-cast structure of high silicon-molybdenum ductile iron was analyzed in depth with scanning electron microscope and energy spectrometer, with emphasis on the unknown parts of the structure and the structure of high silicon-molybdenum ductile iron observed with SEM, the results are shown in the figure.It can be seen from the diagram that in addition to ferrite matrix and spherical graphite, high silicon molybdenum ductile iron also has some particulates and wormlike substances as shown in Figure a, skeletal substances as shown in Figure b, and a few dark grey particles as shown in Figures A and B.The particles and wormlike substances shown in Fig. a are white with small color difference, so it can be preliminarily determined that they are the same substance.

To further prove the composition of these substances, the unknown parts of the sample in the EDA diagram are used to quantitatively explain the structure of the substances. The EDA results of each part are shown in the figure.In Fig. a, the atomic ratio of Fe to Si is about 10:1, which indicates that the element Si is partially incorporated into the ferrite matrix, so that the content of Si in cast iron can sometimes be high, up to 5%.The results of Fig. b show that the carbon-rich graphite can be judged as graphite by its shape, in which the amount of doped Fe and Si is very small and its ratio is close to 10:1. It indicates that the surrounding graphite is surrounded by ferrite, which is consistent with the research results of G. Nicoletto and other scholars.

It can be seen from Fig. C and Fig. d that both Si and Mo are prone to segregation during solidification. Si tends to appear at the edge of graphite while Mo tends to form carbides with carbon.From Fig. e to Fig. l, skeletal and surrounding substances at grain boundaries are carefully analyzed. Fig. e shows that there is no carbon in this material composition, mainly some Fe and Si, with a ratio of more than 10:1. During this period, in addition to the part of Si incorporated into Fe, there are also a few FeS I compounds.The skeletal substance mainly consists of C, Si, Fe, Mo and other elements, and the content of Mo mainly concentrates in the range of 10%-20%. Through analysis, it can be considered that the skeletal compound is a molybdenum carbide.It is reported that the type of carbon of this molybdenum is M6, which is a molybdenum carbide.According to literature reports, this molybdenum carbon type is M6C6C type structure, easy to occur around the grain boundary or graphite of Fe-C eutectic.

The high melting point and stable structure of Mo-rich carbides contribute to the stability of structure and the improvement of properties to a certain extent.However, its distribution is not uniform, segregation is easy, the connection between the matrix is broken, and its hard and brittle characteristics worsen the mechanical processing and mechanical properties of the material, so its content should be properly controlled.