Process control of ductile iron castings

The matrix structure of spheroidal graphite cast iron has a very significant impact on its impact properties. It is the basic premise of low temperature and high toughness spheroidal graphite cast iron to ensure that the spheroidal graphite cast iron is an all-ferrite matrix. The spheroidization rate above 90% is a necessary condition to ensure the low temperature impact value. The cracks of nodular cast iron extend along the boundary of graphite spheroid. The more round the spheroid is, the more effective it is to avoid cracks. Therefore, the higher the spheroidization rate is, the more conducive to the improvement of dynamic load properties such as impact performance and fatigue strength. In addition, the increase of the number of graphite spheres in ferritic ductile iron can also inhibit the formation trend of carbides to a certain extent, which is conducive to improving the low-temperature impact toughness. Therefore, a reasonable spheroidization and inoculation process is needed to obtain high spheroidization rate, fine and large number of graphite spheres.

(1) Raw materials. Select high-quality raw materials to improve the quality of molten iron, such as pure pig iron and high-quality scrap steel, and ensure continuous feeding.

(2) Smelting. Use medium frequency electric furnace to smelt molten iron, take samples at 1420~1440 ℃ for detection, adjust the composition in front of the furnace, try to avoid repeatedly adjusting the chemical composition, and prevent the oxidation of molten iron and the inclusion of mixed lining materials. The oxidation slag inclusion in molten iron is greatly reduced by high temperature standing, and the superheat temperature is controlled at 1470~1490 ℃.

(3) Spheroidizing treatment. The spheroidizing agent shall be embedded in the spheroidizing bag to ensure a reasonable and accurate amount of addition, to avoid poor spheroidizing caused by insufficient addition of spheroidizing agent, and to avoid excessive addition of spheroidizing agent to increase scum.

(4) Inoculation treatment. Two-stage inoculation is adopted. Inoculate once with the flow during tapping, and then conduct instant inoculation during pouring. The time from inoculation to the beginning of pouring shall not exceed 15 min to avoid spheroidization and inoculation decay.

(5) Pouring. During the pouring process, the liquid level in the pouring cup is 3~5 times of the diameter of the sprue to avoid siphon slagging, and the pouring temperature is controlled at 1 340~1 360 ℃.