Control of other elements in ductile iron

(1) Composition control and selection of as Cast Ferritic Nodular Iron: Micro hypereutectic composition, in which the carbon is slightly higher, but there is no graphite floating, the silicon content is slightly lower, the silicon content of inoculant should be less than 3%, the lower the manganese, the better, Mn < 0.04%, sulfur and phosphorus should be low, so that s ≤ 0.02% and P ≤ 0.02%, because silicon can improve the structure and corresponding plasticity of nodular iron, and Si = 3.0-3.5% can obtain all ferrite structures. Some studies have pointed out that when Si = 2.6-2.8%, cast iron has the highest elongation and impact toughness, but the micro segregation of silicon in iron is more serious with the increase of phosphorus content, which has an adverse impact on mechanical properties. For low sulfur content, low magnesium and low rare earth spheroidizing agent can be selected for spheroidization, and the generation of “black spot” defects can be reduced. The “black spot” is mainly the aggregation of magnesium, cerium sulfide and oxide, In addition, low silicon spheroidizing agent should also be used to ensure multiple inoculation.

(2) For pearlitic ductile iron, the manganese in cast iron can be increased to 0.8-1.0% during production. If some castings are used as wear-resistant crankshaft, the manganese can be increased to 1.2-1.35% to produce as cast pearlite. When the content of elemental copper is more than 1.8%, it hinders the spheroidization of graphite, but promotes the complete pearlization of matrix. Generally, the content of copper in nodular iron should be less than 1.5%. Tin is a strong pearlitic element, and its effect on hardness is greater than that of copper and manganese, but when Sn ≥ 1.0%, it distorts graphite, so its content should be limited to less than 0.08%. Considering that copper can increase the content of pearlite, refine pearlite and strengthen ferrite in pearlite, so as to increase the strength and hardness of nodular iron. The graphitization ability of copper is 1 / 5 of silicon, which can reduce the white mouth tendency of nodular iron and avoid free cementite in castings, a small amount of red copper is added to the ingredients.