Magnesium is the main graphitization element, and the spheroidizing ability of rare earth is only one third of that of magnesium. Therefore, the chemical composition of rare earth magnesium nodular cast iron should be dominated by magnesium and supplemented by rare earth.
The use of rare earth in nodular iron is a process innovation with Chinese characteristics, which has made an important contribution to the production of nodular iron. The main role of rare earth in nodular iron: rare earth can promote the spheroidizing effect of magnesium alloy (spheroidizing rate and ball roundness). Its effect on preventing spherical graphite distortion in wall thick nodular iron parts has been paid attention to, which is also one of the main reasons why rare earth is contained in spheroidizing agents at home and abroad.
Some elements in castings can destroy and hinder the spheroidization of graphite. These elements are the so-called spheroidizing interference elements. The interference elements are divided into two categories. One is the consumption of spheroidizing element type interference elements, which form MGS, MgO, MgSe, RE2O3, re2s3, re2te3, etc. with magnesium and rare earth, so as to reduce the spheroidizing elements and destroy the formation of spheroidal graphite; The other is intergranular segregation interference elements, including tin, antimony, arsenic, copper, titanium and aluminum. During eutectic crystallization, these elements are enriched at the grain boundary, which promotes the formation of deformed dendritic graphite in the later stage of eutectic. The greater the atomic weight of spheroidizing interference elements, the stronger the interference effect. Now, many studies have found the critical content of interference elements in cast iron. When the content of these elements is less than the critical content, It cannot form distorted graphite.
In cast iron with interference elements, the addition of rare earth can eliminate its interference effect. Some research reports pointed out that the sum of interference elements in cast iron should be less than 0.10%, that is, z = Ti + Cr + sb + V + as + Pb + Zn +… < 0.10%. Some studies have pointed out that as long as 0.005-0.04% Ce is added to neutralize Al, Sb, Ti, Pb and Bi in molten iron, for example, as long as 0.005-0.007%, 0.014%, 0.15% and 0.008% Ce are added to neutralize Ti, Pb, Sb and Al, respectively.
The destructive effect of interfering elements is greater when the casting wall thickness and cooling speed are slow.
The interfering elements also affect the matrix of nodular iron. Te and B strongly promote the formation of white mouth, Cr, as, Sn, Sb, Pb and Bi stabilize pearlite, and Al and Zr promote ferrite.
It is worth noting that some compound spheroidizing agents of spheroidizing elements and interfering elements are being developed to improve the treatment effect of large section ductile iron and the roundness of graphite balls.
(1) metamorphism: it is mainly manifested in the transformation of flake graphite into spherical graphite. Graphite spheroidization can reduce stress concentration, refine as cast structure, improve the shape and distribution of non-metallic inclusions, and is conducive to the improvement of material properties. Therefore, rare earth nodular iron has higher mechanical properties than gray iron, and its seismic resistance, wear resistance and cutting performance are better than steel.
(2) purification effect: molten iron often contains harmful impurities such as oxygen and sulfur, which will cause pores, cracks and slag inclusion in the casting, reducing the strength, toughness and plasticity of the material. Rare earth elements have strong binding ability with sulfur and oxygen to form refractory compounds, which can play the role of desulfurization and deaeration in molten iron. At the same time, rare earth can also eliminate the adverse effects of harmful elements such as Pb, Zn and Bi in molten iron.
(3) improve casting performance: adding rare earth into molten iron can significantly improve the fluidity of molten iron and reduce casting defects such as segregation and hot crack.
(4) reduce the loss of magnesium.
The appropriate residual magnesium content (mg residual%) and residual rare earth residue (re residual%) in nodular cast iron are important factors to ensure the quality of nodular cast iron. After the test, the residual magnesium content (mg residual%) and residual rare earth residue (re residual%) in nodular cast iron and the ratio of Mg residual% to re residual% shall be controlled within the following range:
Medium and small crankshafts: Mg residue: 0.04-0.06%; Re residue: 0.025-0.045%.
Large crankshaft: Mg residue: 0.04-0.07%; Re residue: 0.015-0.025%. Mg residue%: re residue% ≥ 1.
The main components of spheroidizing agent are as follows:
Spheroidizing agent for medium and small crankshaft: Mg: 7-9%; RE：4－6%。
Spheroidizing agent for large crankshaft: Mg: 8-11%; RE：0.8－1.6%。