By controlling chemical composition, optimizing and improving the spheroidizing incubation process, the graphite grade and size of QT450-10 can be optimized (spheroidizing grade: 2, graphite ball size: 6-8, spheroidizing rate: > 95%), and the influence of graphite on low temperature impact toughness of ductile iron can be reduced.The heat treatment process is normalized, the sample is heated to 920 ~C for 2 hours, then cooled to 600 ~C with the furnace, and then air-cooled to make the matrix structure full ferrite. The low-temperature impact toughness of ductile iron is significantly improved. The figure shows the metallographic structure after heat treatment.
The mechanical properties of tensile test bar and impact test block were tested and the data obtained were obtained.
The mean values of the influence of Si and Ni on the tensile strength and low-temperature impact properties are obtained from Table 3 by means of orthogonal test and analysis of the factor indexes. The variation curves of the influence of Si and Ni on the test indexes (tensile strength and low-temperature impact properties) can be obtained by the relationship between the two process factors (6 level factors) and the mean values.
It can be seen that the tensile strength increases with the increase of Si content, because the solid solution of Si strengthens the ferrite matrix and improves the strength of ferrite.However, the energy absorbed by low temperature shock at -40 C decreases significantly when the content of Si exceeds 2.35%.At this time, the pearlite of matrix structure is less than 5%, even if all ferrite matrix structures can be obtained, it is difficult to obtain good low-temperature impact properties at high silicon.In order to meet the tensile strength and impact value of QT1450-10, the impact value can be reduced appropriately, the Si content can be increased, the strength can be improved, and the material performance requirements can be met. Therefore, the Si content is 2.3%-2.35%.4%, then carry out test verification.
The influence of the content of N I on the tensile strength and impact properties at -40 C increases with the content of N I.
The tensile strength increases as Ni solution strengthens the ferrite matrix, increasing the strength of the ferrite.Conversely, with the increase of N I content, the impact value of ductile iron block at -40 C increases continuously, which is in accordance with the theory that the content of N I is beneficial to improving the impact toughness at low temperature.It can be seen from Fig. 5 that the content of Ni in the range of 0.4%~0.5% is the optimum to ensure the tensile strength and meet the impact value requirement of -40 C.
A group of data with the best comprehensive performance can be obtained from the orthogonal test: when wS i=2.2%-2.35%, wNi=0.3%-0.4%, the tensile strength is 455 MPa and the energy absorbed by impact at low temperature is 7.5J.
By studying the relationship between chemical composition, matrix structure and mechanical properties of ductile iron, adopting reasonable chemical composition design, adding a certain amount of Ni element to strengthen, guaranteeing good spheroidization and incubation effect, strict charge control and proper heat treatment process, the mechanical properties of precision casting QT450-10 with tensile strength (>450MPa) and impact energy (>5J) at low temperature (-40 C) can be achieved.