At present, the elongation indexes of high strength and high plasticity pearliticproduced by domestic foundry enterprises are not unified, including qt700-7, qt700-10, qt800-5, QT800-6 and qt900-5. Pearlite Nodular Cast Iron with high strength and high plasticity was first proposed for the production of nodular cast iron crankshaft. The reason of fracture of nodular cast iron crankshaft is caused by insufficient fatigue strength. To improve the bending fatigue strength of crankshaft, it is necessary to improve the fatigue strength of nodular cast iron material and crankshaft structure, such as shot blasting and fillet rolling.
It can be seen from the figure that when the tensile strength is less than 700MPa, the bending fatigue strength of ductile iron increases with the increase of tensile strength, which is basically linear; The bending fatigue strength of QT700-2, qt800-2 and qt900-2 (notchless) is 280 ~ 300 MPa. In order to further improve the fatigue strength, it is necessary to improve the ductile iron strength and plasticity. Because the higher the elongation of the material, the easier it is to absorb the energy of fatigue crack growth, so that its growth speed is slowed down, so as to improve the fatigue limit of the material. ADI fatigue strength of different brands illustrates this point. ADI with grade 1 050 has the highest fatigue strength. The data provided by No.2 foundry of Dongfeng commercial vehicle forging company also shows that under the condition of the same tensile strength, the comprehensive properties of ductile iron with high elongation are better than those with low elongation, and the advantages of notched fatigue limit and fracture toughness are more obvious, which indicates that the elongation has a greater impact on high sensitive materials.
The above content fully shows that in order to obtain high fatigue strength, ductile iron should not only improve the tensile strength, but also have a certain plastic fit, that is to say, the bending fatigue strength of ductile iron depends on the tensile strength and the corresponding elongation of the material, but mainly on the tensile strength.
The relationship between bending fatigue strength and tensile strength, elongation of ductile iron has been reported in many literatures. Professor Liu Guanghua of Nanjing University of science and technology cited the domestic and foreign references about the relationship between fatigue strength and fatigue life of nodular cast iron and tensile strength and elongation in the paper “evaluation of application prospect of nodular cast iron crankshaft”
σ- 1 — fatigue strength / MPa;
σ B — tensile strength / MPa;
δ—— Elongation /%.
N — fatigue life（ × 104）；
M — bending moment / N · M.
Among them, formula (1) represents the relationship between fatigue strength and tensile strength, elongation of ductile iron, and formula (2) represents the relationship between fatigue life and tensile strength, elongation of ductile iron crankshaft. It can be seen from formula (1) and formula (2) that increasing the tensile strength and elongation at the same time is conducive to improving the fatigue strength and fatigue life. However, these are still experimental data and theoretical calculations, and the exact results can not be given yet. It is necessary to collect more data and carry out more tests before making the group standard of high strength and high plasticity ductile iron.