Refinement and fracture mechanisms of as-cast QT700-6 alloy by alloying method

• Published in: China foundry Vol. 14; no. 1; pp. 16 - 21
• Main Authors: Gao, Min-qiang, Qu, Ying-dong, Li, Rong-de, Jiang, Ke, You, Jun-hua
• Format: Journal Article
• Language: English
• Published: Singapore Springer Singapore 2017; Foundry Journal Agency; School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
• Subjects: Alloys; as-cast QT700-6 alloy; microstructure; graphite; refinement; pearlite lamellar spacing; Carbides; Electron microscopy; Engineering; Furnaces; Grain boundaries; Graphite; Machines; Manufacturing; Materials Engineering; Mechanical properties; Metallic Materials; Methods; Nickel (Metal); Nodular iron; Processes; Research & Development; Specialty metals industry; pearlite lamellar spacing; refinement; TG143.5; microstructure; graphite; as-cast QT700-6 alloy
• ISSN: 1672-6421; 2365-9459
• DOI: 10.1007/s41230-017-5098-2

The as-cast QT700-6 alloy was synthesized with addition of a certain amount of copper, nickel, niobium and stannum elements by alloying method in a medium frequency induction furnace, aiming at improving its strength and toughness. Microstructures of the as-cast QT700-6 alloy were observed using a scanning-electron microscope （SEM） and the mechanical properties were investigated using a universal tensile test machine. Results indicate that the ratio of pearlite/ferrite is about 9：1 and the graphite size is less than 40 tJm in diameter in the as-cast QT700-6 alloy. The predominant refinement mechanism is attributed to the formation of niobium carbides, which increases the heterogeneous nucleus and hinders the growth of graphite. Meanwhile, niobium carbides also exist around the grain boundaries, which improve the strength of the ductile iron. The tensile strength and elongation of the as-cast QT700-6 alloy reach over 700 MPa and 6%, respectively, when the addition amount of niobium is 0.8%. The addition of copper and nickel elements contributed to the decrease of eutectoid transformation temperature, resulting in the decrease of pearlite lamellar spacing （about 248 rim）, which is also beneficial to enhancing the tensile strength. The main fracture mechanism is cleavage fracture with the appearance of a small amount of dimples.