Effect of normalizing temperature on mechanical properties of nodular cast iron samples

The tensile specimens of room temperature nodular cast iron as cast and treated at different normalizing temperatures were tested at room temperature by electronic universal testing machine. The tensile strength and elongation of as cast nodular cast iron are 688mpa and 8.0%. After normalizing, the tensile strength and elongation of nodular cast iron change. When the normalizing temperature is 870 ℃, that is, about 30 ~ 50 ℃ above the upper limit of eutectic transformation temperature, the tensile strength is 759 MPa and the elongation is 5.4%, in which the tensile strength increases by 10% and the elongation decreases by 20% compared with the as cast. However, when the normalizing temperature is 930 ℃, the tensile strength remains basically unchanged, while the elongation increases significantly to 9.5%. Zhao Xiaolong and others also found that with the disappearance of free cementite in steel structure, its tensile strength has a downward trend, and its elongation has a certain increase. This may be due to the decomposition of a small amount of cementite, while the matrix and graphite ball have hardly changed.

SEM and BSE diagrams of tensile fracture morphology of as cast and normalized nodular cast iron are shown in the figure. The fracture causes of materials are various, which are not only related to the properties of materials, but also related to the shape, size and external conditions. In terms of macro fracture, fracture is generally divided into ductile fracture and brittle fracture. It can be seen from the figure that under the as cast and 930 ℃ normalizing conditions, it is found that the fracture surface of the sample is uneven and fibrous, and the fracture edge presents gray shear lip shape, which belongs to ductile fracture. The fracture surface of the specimens normalized at 870 ℃ and 900 ℃ shows herringbone pattern stripes. The fracture surface is flat and gray, belonging to brittle fracture. From the microscopic samples, there are cleavage small planes and river patterns spreading around at the fracture of 870 ℃ and 900 ℃ normalized samples, which show the characteristics of quasi cleavage brittle fracture. However, as cast and 930 ℃ normalized specimens have less dissociation planes and shear tear ridges, and there are more dimples, which show ductile fracture.