Research On The Preparation Of High Strength-toughness Ductile Iron Sleeves By Vacuum Molding Process

Ductile iron with the toughness of steel and the casting of gray iron are widely used in automobiles and ships components. However, ordinary ductile iron such as QT400-18 and QT700-2 are unable to obtain high strength and toughness, which limits the scope of application of ductile iron, and it is essential to further improve strength and toughness of ductile iron. But improving the strength and toughness of ductile iron simultaneously are one of the problems in the field of materials science. In general, when the strength is enhanced,the toughness must be accompanied by a decrease. For these reasons, high strength and toughness ductile iron were studied through alloying method, and as-cast QT700-6 alloy was successfully prepared. To further enhance its strength and toughness, the low temperature heat treatment for as-cast QT700-6 alloy was conducted.SEM was used to analyze the as-cast QT700-6 alloy, and results shown that its ratio of pearlite and ferrite was about 9:1. Graphite ball diameter was less than 40μm, and its refinement mechanism was forming carbide due to the chemical reaction between niobium and carbon elements in the liquid phase, which increased the heterogeneous nucleation.Alloying elements inhibited graphite growing up. Pearlite lamellar spacing was 248 nm,which was sorbite body that hindered the movement of the dislocation, and it improved effectively the tensile strength of ductile iron. The fracture mode was cleavage fracture, and crack formation location was the interface between the graphite and pearlite. Graphites broke away at a lower stress and cracks expanded gradually with a further increase stress. When the stress was greater than fracture strength of pearlite, the fracture occurred. As-cast QT700-6 alloy impact fracture behavior was caused by G-m interface. G-m interface component structure and G-m interface stress state and the strength had an effect on fracture behavior of as-cast QT700-6 alloy. The main microcosmic fracture of as-cast QT700-6 alloy brittle fracture was transgranular cleavage fracture.Low temperature annealing heat treatment for as-cast QT700-6 alloy was conducted,and results shown that the flocculent graphite ball which was not ideal graphite ball changed to rounded graphite ball in the as-cast microstructure after 0.5h low temperature annealing heat treatment, which could slightly improve the level of the ball and increase the average diameter of the graphite. A number of point graphites appeared in the matrix closed to idealgraphite balls. The change of graphite morphology contributed to improve the strength and toughness of ductile iron. At the same time, the average of pearlite lamellar spacing was larger than as-cast QT700-6 alloy, and the granular pearlite appeared in the microstructure,which was related to the decomposition of cementite. The number of cleavage steps of specimens after heat treatment was slightly less than as-cast specimens in the tensile fracture,but the number of dimples was greater than as-cast specimens, which could improve the toughness. By comparing the surface roughness of heat treatment specimens with as-cast specimens, the former was greater than the latter.