Effect of quenching and tempering on microstructure and hardness of QTNi4MoV ductile iron prepared by continuous-casting
Microstructure and hardness changes of QTNi4MoV ductile iron during quenching and tempering were studied by means of optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and hardness tester.The results show that after graphitizing annealing treatment at 1323 K for 4 h,the microstructure of the QTNi4MoV ductile iron consists of graphite balls,retained austenite and acicular bainite,with a hardness of about 40 HRC.After holding at 1223 K for 1 h and oil quenching,the microstructure of the QTNi4MoV ductile iron is composed of graphite balls,martensite and retained austenite,with a hardness of 48.5 HRC.After quenching,the microstructure of the QTNi4MoV ductile iron has good tempering resistance,and when the tempering temperature reaches 773 K,the quenched martensite begins to decompose.After primary and secondary tempering at 773 K for 3 h,the microstructure of the QTNi4MoV ductile iron is composed of graphite,tempered martensite and a small amount of retained austenite,the hardness values are 47.5 and 46 HRC,respectively,and the tensile strength is higher than 900 MPa,meeting the mechanical performance requirements for the manufacturing materials of aluminum alloy die-casting molds during service.