The metallographic sample is made by standard metallographic technology. The concrete methods are: grinding the observation surface on the sandpaper of different sizes one by one → polishing the observation surface to no scratch → flushing and drying the surface with anhydrous alcohol → corrosion surface of 4% nitric acid alcohol for about 10s → washing the corrosion residue on the polishing surface with distilled water → washing and drying the surface with anhydrous alcohol → sample preservation. The as cast and heat treated ductile iron samples were observed by Olympus and 4xc PC optical microscope.
Quantitative metallographic analysis
According to gb/t 9441-2009 metallographic inspection standard of nodular iron, quantitative metallographic analysis was carried out on the spheroidizing rate, graphite ball number, graphite average size, graphite precipitation, ferrite content and pearlite content of nodular iron by means of the measurement functions of area, diameter, area percentage and count in image Proplus 6.0. At the same time, the formula of spheroidization rate is as follows:
N1.0, n0.8, n0.6, N0.3 and N0 respectively represent the number of graphite particles with graphite area ratio of ≥ 0.81, 0.80-0.61, 0.60-0.41, 0.4-0.21 and ≤ 0.20.
The graphite morphology, matrix structure and tensile fracture morphology of ADI were observed and analyzed by jsm-7500f field emission scanning electron microscope.
The fine structure of acicular ferrite and high carbon austenite in the microstructure of ductile iron matrix was observed by jem-3010 high resolution transmission electron microscope made by Japanese electron. Before testing, the sample preparation shall be carried out strictly according to the requirements of transmission test sample. The specific methods are as follows: cutting the sample into thin sheet with thickness of about 1mm → grinding the thickness of the sheet to 40 μ M-60 μ m with sandpaper → mechanical cutting into several small Φ 3mm thin plates → chemical thinning of the thin sheet by double jet electrolytic thinners (electrolyte is 7% perchlorate ethanol solution, the voltage of electrolytic thinning is 50V, the sensitivity value is 100, and the temperature is controlled below -10 ℃) → jfc-1600 The ion thinning of the sample was carried out by ion sputtering instrument (the thinning voltage is 4.5kev, the angle is 6 °, the speed change is 3.5 RPM) → the sample is preserved.
The ADI phase composition obtained by different isothermal quenching temperature was analyzed by xrd-7000 X-ray diffractometer and jade 6.5 software.
The types and distribution of elements in the phases of ADI matrix were analyzed by the combination of Oxford x-max50 energy spectrometer (EDS) and scanning electron microscope. Among them, the diameter of the energy spectrum beam spot is 1 μ m, the test voltage is 15kV, the test current is 3 × 10-10a, and the working distance of the energy spectrum is 8mm.
The carbon content of the micro areas in as cast and heat treated (austenite + water quenched) ductile iron samples was measured by jxa-8100 (4CH) type electron probe (EPMA) made by Japanese electronics. The measuring point was located between two adjacent graphite balls, with six measuring points. The outer part of the graphite ball was two points, and the middle four points were arranged in a straight line. The diameter of probe beam spot is 1 μ m, the test voltage is 15kV and the test current is 10na. Before the measurement, the elements P, C, Si, Mg, Al, Fe, Ni, Fe S2, Mn and Cu were corrected by using standard samples KTI opo4 (potassium Titanophosphate), SiC, MgO, Al2O3, single Fe, Ni, Fe, Fe S2, Mn and Cu respectively. The correction method was ZAF.