Control of carbon and silicon in molten iron of nodular cast iron piston ring

1. Carbon content

The carbon in the nodular cast iron piston ring does not damage the mechanical properties like the gray cast iron ring. As long as there is no graphite floating, the choice of high carbon is beneficial: ① high carbon and many graphite crystal nuclei can refine the graphite ball and improve the roundness of the ball; ② High carbon and large graphitization expansion make the piston ring more compact and reduce shrinkage; ③ High carbon makes nodular cast iron closer to eutectic composition, which is conducive to the improvement of fluidity.

However, too much carbon, that is, the amount of carbon exceeding the eutectic point, will lead to flowering graphite and worsen the morphology. In serious cases, graphite floats, the fluidity of molten iron decreases, and the porosity defects on the upper surface of the casting ring increase. Depending on the V / O value of the ring sample, the carbon content should be controlled at 3.65% ~ 3.8%.

2. Silicon content

Silicon is a strong graphitization element. The graphitization ability of silicon entering the molten iron from the inoculant is stronger than that of the original silicon in the molten iron. Because the cooling rate of piston ring in the mold is very fast, it is possible to achieve cementite < 5% in the as cast matrix structure only when the number of graphite balls (> 30000 spheres / cm2) specified in Goetze technical conditions is reached. Therefore, inoculation is very important. The inoculant with strong inoculation effect as described above should be selected, but the dosage should not exceed 0.6% to avoid flowering graphite. In order to have sufficient graphitization ability, depending on the V / O ratio of the ring sample, the total silicon should be 2.4% ~ 2.8%, deducting the silicon brought in by the inoculant and spheroidizing agent.

At present, ZHY casting has successfully applied medium frequency furnace and electric arc furnace to produce single double piece nodular cast iron piston rings for cars, “xf157”, “6105” and other main engines. Its metallographic structure (spheroidization qualification rate, graphite ball number qualification rate and ball diameter) has met the same requirements abroad, and the casting scrap rate is also very low, The final ductile iron piston ring has good mechanical properties and meets the needs of users.