| Cu precipitation strengthened steel has been used widely in different applications such as ship structure, offshore platform, vehicle structure and aviation etc., due to the better balance on the mechanical properties like toughness, strength and weldability. Cu precipitation strengthened steels with different Ni content were heated at 900 ?C,then aged in different time durations under different temperature parameters. The effect of Ni content, aged temperature and aged time on microhardness and microstructure of Cu precipitation strengthened steel were studied through microhardness testing, the optical microscope, SEM and TEM observation and analysising. The main experimental content are as follows:The content of Ni in steel is an important parameter for controlling the microhardness of Cu steel. When the Ni content is increased from 1% to 3.5%, the hardness of the material is increased from 236 HV to 263 HV; while the Ni content increased, the ratio of bainite and martensite in steel is lower, and the refined the martensite is obtained as well; With the increase of Ni content, the amount of precipitates in the Cu containing steel is increasing, and the size of precipitates is refined.Aging treatment of steel with Ni content of 3 % was carried out at different temperatures for 2 h. Results shows that: The microhardness of the steel increases firstly and then decreases by the aging temperature increasing, and then this microhardness is peaked at 343 HV with aging temperature of 450 ?C, lastly the microhardness increased at 700 ?C.The martensite in the steel is gradually decomposed into polygonal ferrite transformation with the aging temperature increasing and a large nμmber of granular bainite and M-A islands are obtained at 700 ?C; when the aging temperature is lower than 450 ?C, the precipitates start to nucleate at this stage, and reach the peak value of 350 /μm2 when the aging temperature is at 450 ?C. And with the temperature go beyond 450 ?C, the precipitates are in the coarsening stage, the size of the precipitates reach the peak value of 20.1 nm with temperature of 600 ?C. During the above stated process, the morphology of the precipitates changes from spherical to ellipsoid, short rod structure with aging temperature increasing.Aging treatment of steel with Ni content of 2.5% was carried out for different aging time durations at 650 ?C. And the research results indicated, with aging duration increasing, the microhardness of the steel increases first, reaches a peak value of 256 HV at 2 h and then decreases. With the prolonging of aging time, the martensite gradually decomposed into polygonal ferrite; when the aging time is less than 2 h, the precipitates are in the nucleation stage; with the aging time duration is over 2 h, the precipitates are coarsening in the process by a relatively low coarsening rate. The crystal structure of precipitates has been changed with the aging time duration, the crystallographic structure of precipitates has been bcc structure of the B2 phase until the aging time reached 100 h, was transformed into ε-Cu FCC phase. |
