EFFECT OF INDUCTION TEMPERING ON CARBIDE PRECIPITATION BEHAVIOR AND TOUGHNESS OF A 1000 MPa GRADE HIGH STRENGTH LOW ALLOY STEEL

By comparing induction tempering with conventional tempering,the effect of induction reheating tempering on carbide precipitation behavior and toughness of a 1000 MPa grade high strength low alloy steel was investigated.Microstructures of the steel in different heat treatment stages were characterized using SEM and TEM(with EDS),mechanical properties inclusive of Vickers hardness and toughness were tested.The results showed that microstructure of quenched samples consisted of lath martensite and lower bainite,needle like carbides were observed in lower bainitic lath.With tempering temperature increasing from 400 ℃ to 550 ℃,the shape of carbides located within the bainitic lath gradually changed from needle like to short rod like type.Carbides were fine and well distributed using induction tempering.When the tempering temperature was 550 ℃,the long axis length of short rod like carbides located within the bainitic lath by conventional reheating tempering was 200 nm,whereas the long axis length of short rod like carbides located within the bainitic lath by induction reheating tempering was about 60 nm.When tempering by conventional reheating,carbides mainly precipitated along martensite lath boundaries,while carbides were more dispersed in the matrix lath by induction reheating,the size of these dispersed carbides was less than 100 nm when tempering temperature was 550 ℃.As a result,a superior of mechanical properties with 344 HV and Charpy impact energy of 133 J at-20 ℃ was obtained with induction reheating tempering at550 ℃.