Normalizing treatment can improve the strength of nodular cast iron crankshaft. The purpose of normalizing nodular cast iron is to refine the matrix structure. The technological process is to reheat the nodular iron casting to the austenitizing temperature of 50 ° C, convert the ferrite and pearlite in the matrix structure into austenite, and some spherical graphite is dissolved in austenite. After air cooling after heat preservation, austenite is transformed into finer pearlite, so as to improve the strength of the casting.
Since the invention of nodular cast iron, it has developed rapidly. Its tensile strength is significantly higher than that of gray cast iron, and even increased to 700 ~ 900MPa, which is close to or even beyond the level of carbon steel. Compared with forged steel crankshaft, nodular cast iron crankshaft not only has the advantages of simple manufacture and low cost, but also has the excellent characteristics of wear resistance, shock absorption and low sensitivity to cracks. Therefore, nodular cast iron has the possibility to replace forged steel crankshaft. Normalizing treatment can significantly improve the plasticity and toughness of nodular cast iron. Data [423 show that under laboratory conditions, the properties of Y-shaped test block of normalized nodular cast iron can approach the requirements of qt900-5. This shows that normalizing treatment may make it develop into plastic material, which is different from ordinary nodular cast iron. The wide and successful application of normalized nodular cast iron crankshaft fully proves the development potential of nodular cast iron. Since then, it has attracted countries to deeply develop new nodular cast iron materials.
The nodular cast iron crankshaft should be tempered after normalizing to improve toughness and eliminate internal stress. The tempering temperature is 550, – 600 ℃, and the tempering temperature is less than 550. C. The second kind of temper brittleness may occur due to the segregation of impurity elements; Tempering temperature is higher than 600. C. It is possible to reduce the strength and hardness too much.